\documentclass[10pt]{article}

\usepackage{fullpage}
\usepackage{setspace}
\usepackage{parskip}
\usepackage{titlesec}
\usepackage[section]{placeins}
\usepackage{xcolor}
\usepackage{breakcites}
\usepackage{lineno}
\usepackage{hyphenat}





\PassOptionsToPackage{hyphens}{url}
\usepackage[colorlinks = true,
            linkcolor = blue,
            urlcolor  = blue,
            citecolor = blue,
            anchorcolor = blue]{hyperref}
\usepackage{etoolbox}
\makeatletter
\patchcmd\@combinedblfloats{\box\@outputbox}{\unvbox\@outputbox}{}{%
  \errmessage{\noexpand\@combinedblfloats could not be patched}%
}%
\makeatother


\usepackage{natbib}




\renewenvironment{abstract}
  {{\bfseries\noindent{\abstractname}\par\nobreak}\footnotesize}
  {\bigskip}

\titlespacing{\section}{0pt}{*3}{*1}
\titlespacing{\subsection}{0pt}{*2}{*0.5}
\titlespacing{\subsubsection}{0pt}{*1.5}{0pt}


\usepackage{authblk}


\usepackage{graphicx}
\usepackage[space]{grffile}
\usepackage{latexsym}
\usepackage{textcomp}
\usepackage{longtable}
\usepackage{tabulary}
\usepackage{booktabs,array,multirow}
\usepackage{amsfonts,amsmath,amssymb}
\providecommand\citet{\cite}
\providecommand\citep{\cite}
\providecommand\citealt{\cite}
% You can conditionalize code for latexml or normal latex using this.
\newif\iflatexml\latexmlfalse
\providecommand{\tightlist}{\setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}%

\AtBeginDocument{\DeclareGraphicsExtensions{.pdf,.PDF,.eps,.EPS,.png,.PNG,.tif,.TIF,.jpg,.JPG,.jpeg,.JPEG}}

\usepackage[utf8]{inputenc}
\usepackage[ngerman,english]{babel}



\usepackage{float}








\begin{document}

\title{Placental Growth Factor in Suspected Preterm Pre-eclampsia: A Review of
the Evidence and Practicalities of Implementation}



\author[1]{Alice Hurrell}%
\author[1]{Alice Beardmore-Gray}%
\author[2]{Kate Duhig}%
\author[2]{Louise Webster}%
\author[1]{Lucy Chappell}%
\author[2]{Andrew Shennan}%
\affil[1]{King's College London}%
\affil[2]{Kings College London}%


\vspace{-1em}



  
  \date{May 29, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
Despite extensive research, the pathophysiology and prevention of
pre-eclampsia remain elusive, diagnosis is challenging, and
pre-eclampsia remains associated with adverse maternal and perinatal
outcomes. Angiogenic biomarkers, including placental growth factor
(PlGF) and soluble fms-like tyrosine kinase 1 (sFlt-1), have been
identified as valuable biomarkers for preterm pre-eclampsia,
accelerating diagnosis and reducing maternal adverse outcomes by risk
stratification, with enhanced surveillance for high-risk women.
PlGF-based testing is increasingly being implemented into clinical
practice in several countries. This review provides healthcare providers
with an understanding of the evidence for PlGF-based testing and
describes the practicalities and challenges to implementation.%
\end{abstract}%



\sloppy


\textbf{Tweetable Abstract}

Placental growth factor in pre-eclampsia: evidence and implementation of
testing

\textbf{Background}

Pre-eclampsia is associated with increased adverse maternal and
perinatal outcomes, as well as substantial costs for healthcare
providers.\textsuperscript{1} The schedule of antenatal care in the
United Kingdom, similar to many other high-income settings, is designed
for early detection of pre-eclampsia to minimise adverse outcomes.
Maternal mortality has dramatically decreased in the United Kingdom over
the last 70 years, likely due to the provision of free antenatal care
and implementation of evidence-based guidelines, alongside adoption of
recommendations from the landmark Confidential Enquiries into Maternal
Deaths.\textsuperscript{2} However, risks to the woman persist, child
morbidity and mortality remain, and pre-eclampsia is the most common
cause of iatrogenic preterm delivery.\textsuperscript{3} The diagnosis
of pre-eclampsia is evolving, particularly on a background of chronic
medical co-morbidities. Proteinuria is not a pre-requisite for
diagnosis, which can be made on the premise of new-onset or worsening of
hypertension in association with neurological, biochemical or
haematological abnormalities or fetal growth
restriction.\textsuperscript{4, 5} Hypertension alone may predict only
20\% of adverse outcomes in pre-eclampsia, and therefore there is a need
for better risk stratification and targeted
surveillance.\textsuperscript{6} Around 10\% of women may present with
suspected pre-eclampsia, often asymptomatic, even in the presence of
severe disease.\textsuperscript{7} At present, women are unnecessarily
admitted to hospital with suspected pre-eclampsia, whilst more severe
cases may go undiagnosed. The uncertain management of this group
presents a considerable workload within maternity care.

Placental growth factor (PlGF) is an angiogenic protein, which is
secreted by the syncytiotrophoblast and promotes placental angiogenesis.
In a healthy pregnancy, PlGF concentrations increase as gestation
advances, reaching a peak at 26 to 30 weeks' gestation, before
decreasing towards term.\textsuperscript{8, 9} Low concentrations of
PlGF precede the clinical onset of pre-eclampsia and abnormalities in
angiogenic factors may predate the clinical syndrome by 10
weeks.\textsuperscript{10} Soluble fms-like tyrosine kinase (sFlt-1) is
a circulating anti-angiogenic protein which adheres to the
receptor-binding domains of PlGF and vascular endothelial growth factor
(VEGF), preventing their interaction with endothelial receptors and
inducing endothelial dysfunction. sFlt-1 concentrations increase towards
term in healthy pregnancies, but are prematurely elevated in women with
pre-eclampsia.\textsuperscript{8} Low PlGF concentrations in
pre-eclampsia may reflect down-regulated expression as well as high
levels of sFlt-1 reducing the bioavailability of
PlGF.\textsuperscript{11} Therefore, low PlGF and high sFlt-1 are
secondary markers of placental dysfunction in pre-eclampsia, in contrast
to hypertension and blood pressure, which are tertiary, downstream
features.\textsuperscript{12} This has focused research on whether
angiogenic biomarkers may aid diagnosis of pre-eclampsia and reduce
adverse outcomes.

The last decade has seen the development of fully automated,
commercially available assays, replacing manual enzyme-linked
immunosorbent assays. This has led to standardised, inexpensive
measurements, with a high turnover and minimal sample handling. Some
PlGF-based tests measure PlGF alone, whereas others quantify sFlt-1 and
PlGF, presenting the results as a ratio. There are currently four
commercially available PlGF-based assays. The thresholds associated with
diagnosis are not interchangeable, as the assays have varying affinity
to PlGF isomers and this has implications for clinical practice and
implementation of PlGF-based testing. The National Institute for Health
and Care Excellence has released specific diagnostics guidance relating
to PlGF-based testing\textsuperscript{13} and has recommended two tests
for routine adoption into the NHS, to be used as rule-out tests for
women with suspected preterm pre-eclampsia. These are the Triage PlGF
test (Quidel) and the Elecsys sFlt-1/PlGF ratio (Roche
Diagnostics).\textsuperscript{5, 13}

\textbf{Triage PlGF Test (Quidel)}

The Triage PlGF test is a single-use, fluorescence immunoassay device,
which is used with the CE-marked Triage MeterPro point-of-care analyser.
Blood must be centrifuged, and plasma extracted before testing. The
analyser can be installed either as a point-of-care test in a clinic or
ward, or in a laboratory, and the assay takes 15 minutes. It detects
PlGF-1 and quantifies concentration in the range of 12 to 3000 pg/ml. It
is recommended by the National Institute for Health and Care Excellence
as a rule-out test for suspected pre-eclampsia between 20 and
34\textsuperscript{+6}-weeks' gestation at a threshold of 100
pg/ml.\textsuperscript{13}

\textbf{Evidence for the Triage PlGF Test:}

The PELICAN study was a prospective, multicentre, observational study
investigating the diagnostic accuracy of PlGF in diagnosing
pre-eclampsia.\textsuperscript{12} This study showed that low PlGF had
high sensitivity (0.96; 95\% CI 0.89 -- 0.99) and high negative
predictive value (NPV 0.98; 0.93 -- 0.995) in diagnosing pre-eclampsia
necessitating delivery within 14 days, in women with suspected
pre-eclampsia before 35 weeks' gestation.\textsuperscript{12} The area
under the receiver operating characteristic curve (AUC) for PlGF was
greater than all other commonly used tests (systolic blood pressure,
diastolic blood pressure, alanine transaminase, urate, dipstick
proteinuria) for making a diagnosis in women presenting with suspected
pre-eclampsia. Low PlGF was classified as
\textless{}5\textsuperscript{th} centile according to predetermined
normal ranges.\textsuperscript{9} However, for implementation in
clinical practice a threshold of \textless{}100pg/ml was identified,
independent of gestation, with test performance similar to the
5\textsuperscript{th} centile and retaining the same high sensitivity
and NPV. When multiples of median were used for analysis, this reduced
the predictive power of low PlGF (unpublished data). The study was
conducted across seven sites in the United Kingdom and recruited 625
women; 346 developed pre-eclampsia, of whom 176 developed pre-eclampsia
prior to 35 weeks' gestation. Therefore, this demonstrates high test
performance across a broad range of clinical settings and population
demographics.

PlGF testing has been investigated in the PETRA trial, a prospective
observational study recruiting 1,112 women from 24 centres across North
America.\textsuperscript{14} This demonstrated that low PlGF
concentration was strongly correlated with time to delivery in women
with suspected pre-eclampsia before 35 weeks' gestation (753 women). The
median time to delivery was 10 days for low PlGF (100pg/ml or lower) and
2 days for very low PlGF (less than 12pg/ml). The sensitivity for
diagnosing pre-eclampsia and delivery within 14 days was 92.5\% and the
NPV was 90.3\%. This lower NPV may reflect the higher prevalence of
pre-eclampsia in the study population, with 71\% of participants (538
women) diagnosed with preterm pre-eclampsia. A secondary analysis of
this study has also been reported.\textsuperscript{15} This found a
significant association between low and very low PlGF and composites of
perinatal and maternal adverse outcomes. The sensitivity and NPV of low
PlGF for adverse neonatal outcomes were 95.8\% and 99.2\%, and for
adverse maternal outcomes 86.8\% and 98.1\%. They conclude that women
with abnormal PlGF are significantly more likely to suffer adverse
neonatal and maternal outcomes, and that PlGF is useful for risk
stratification.

PlGF testing has been evaluated in the PARROT trial, a multi-centre
stepped-wedge cluster-randomised controlled
trial.\textsuperscript{16}The trial design involved random and
sequential transition of maternity units (representing clusters) from
concealed to revealed PlGF testing, alongside a simple clinical
management algorithm. 1023 women with suspected preterm pre-eclampsia
were enrolled from 11 maternity units across the UK. The time to
diagnosis was reduced from 4.1 to 1.9 days, with a significant reduction
in a composite of severe maternal adverse outcomes from 5.4\% to 3.8\%
(aOR 0.32, 95\% CI 0.11 -- 0.96). There was no difference in gestational
age at delivery or perinatal adverse outcomes. The study strengths
include its broad inclusion criteria and a population who were diverse
from an ethnicity and sociodemographic perspective, from multiple sites,
thereby enhancing generalisability of the findings to the real-world
setting. Therefore, this study provided novel evidence that PlGF testing
proves a valuable diagnostic adjunct in suspected pre-eclampsia,
allowing targeted surveillance for those at highest risk of adverse
outcomes.

Initial studies using economic modelling demonstrated that PlGF-based
testing may afford a cost-saving of between \selectlanguage{ngerman}£330 and £1032 per woman
tested.\textsuperscript{17, 18} In a more recent study describing their
cost-effectiveness analysis of the PARROT trial, Duhig and colleagues
found a total cost-saving of £149 per woman, based on £70 per Triage
PlGF test.\textsuperscript{19} Given that there were 646,794 births in
England in 2017 and 10\% of pregnant women have suspected pre-eclampsia,
with 30\% of these presenting prior to 37 weeks' gestation, PlGF testing
could be performed in approximately 38,800 women per year. This
represents a potential cost saving of £2,891,196 each year in England.
These cost-savings are driven by a reduction in outpatient attendances
for those with a normal, low risk result. There was an increased cost
seen associated with inpatient admissions in those at higher risk.
Despite this, overall PlGF testing was found to be cost saving, but the
cost-effectiveness will depend on the cost of the test being performed.
The authors conclude that their more conservative cost-saving is due to
improved clinical risk stratification leading to appropriate
redistribution of resources, rather than an overall reduction of
resource use anticipated by hypothetical analyses.\textsuperscript{19}

\textbf{Elecsys Immunoassay sFlt-1/PlGF ratio (Roche Diagnostics)}

The Elecsys sFlt-1/PlGF ratio is derived by combining the results from
two CE-approved sandwich electrochemiluminescence immunoassays. The
analysis turnaround time is 18 minutes. The test requires a large-scale
laboratory analyser and is not a point-of-care test. The Elecsys sFlt-1
assay has a lower limit of detection of 10pg/ml and a range of 10 --
85,000pg/ml. The Elecsys PlGF assay has a lower limit of detection of
3pg/ml and a range of 3 to 10,000pg/ml. The National Institute for
Health and Care Excellence recommends use as a rule-out test for
suspected pre-eclampsia between 20 and 34\textsuperscript{+6} weeks'
gestation, at a threshold of \selectlanguage{english}[?]~38.\textsuperscript{9}

\textbf{Evidence for the Elecsys Immunoassay sFlt-1/PlGF ratio:}

Rana and colleagues performed a prospective observational study
investigating the association between the Elecsys sFlt-1/PlGF ratio and
adverse outcomes in suspected pre-eclampsia.\textsuperscript{20} They
found that in women presenting before 34 weeks' gestation, sFlt-1/PlGF
ratio out-performed current approaches (systolic blood pressure, alanine
transaminase, creatinine, urate) at predicting adverse outcomes, with an
AUC of 0.89. A cut-off point of 85 was identified, with sensitivity of
72.9\% and specificity of 94.0\% for adverse outcomes, in women
presenting before 34 weeks' gestation. sFlt-1/PlGF ratio was inversely
correlated with time to delivery; delivery occurred within two weeks in
86.0\% of women with a sFlt-1/PlGF ratio above 85. This was a single
centre study, including 616 assessments of suspected pre-eclampsia, of
which 81 were repeat evaluations.

The PROGNOSIS study was a multicentre observational study that analysed
the predictive value of the Elecsys sFlt-1/PlGF ratio in 1050 women,
with 500 women included in a development cohort to determine a ratio
cut-off and 550 women as a validation cohort.\textsuperscript{21} This
demonstrated that a ratio of \selectlanguage{english}[?]~38 had a negative predictive value (NPV)
of 99.3\% (95\% CI 97.9 -- 99.9), with a sensitivity of 80.0 (95\% CI
51.9 -- 95.7) for ruling out pre-eclampsia in less than one week. The
positive predictive value for a diagnosis of pre-eclampsia within four
weeks was 36.7\% (95\% CI 28.4 -- 45.8), with a 66.2\% sensitivity (54.0
-- 77.0). Predictive performance of sFlt-1 and PlGF analysed
individually were not superior to the ratio, with AUC of 78.2\% for PlGF
alone, compared to 88.4\% for sFlt-1/PlGF.\textsuperscript{21} They
conclude that in clinical practice, high NPV is crucial in the
evaluation of suspected pre-eclampsia, as failure to detect imminent
disease could have important consequences for the woman or fetus. 199
women (19\%) developed pre-eclamspia and this lower prevalence may
explain the lower positive predictive value than the PELICAN
study.\textsuperscript{12}

The Elecsys sFlt-1/PlGF ratio has also been evaluated in the INSPIRE
randomised controlled trial.\textsuperscript{22} 370 women with
suspected preterm pre-eclampsia were randomised on an individual level
to revealed or non-revealed PlGF-based testing. Primary endpoint was
hospitalisation within 24 hours of the test. The trial found no
difference in the primary outcome but use of the test increased the
proportion of high-risk patients admitted without influencing overall
admission rate, which may reflect appropriate redistribution of
resources. Overall, 85 women (23\%) developed pre-eclampsia and all of
those developing pre-eclampsia within seven days were admitted to
hospital following the sFlt-1/PlGF test, demonstrating 100\% sensitivity
and 100\% NPV for the defined primary endpoint. They concluded that
larger trials are needed to assess whether the test could be used to
mitigate adverse maternal and perinatal outcomes. The single centre
nature of the trial, with 90\% of participants of white ethnicity,
limits its wider generalizability.

\textbf{Comparison of Triage PlGF Test and Elecsys sFlt-1/PlGF Ratio}

The two PlGF-based tests recommended by the National Institute for
Health and Care Excellence have been subject to direct
comparison.\textsuperscript{23-25} McCarthy and colleagues (2019)
compared the Triage PlGF test, Elecsys sFlt-1/PlGF ratio and the DELFIA
Xpress PlGF 1-2-3 tests in 305 women, of whom 62 developed early-onset
pre-eclampsia.\textsuperscript{23} They found that the AUC were nearly
identical for the Elecsys sFlt-1/PlGF ratio and the Triage PlGF test,
with that for the DELFIA Xpress PlGF 1-2-3 test very similar. This
suggests that the tests are similarly effective, and trade-off between
sensitivity and specificity is dependent on the thresholds, which
currently are slightly different. As more clinical data becomes
available, appropriate and equivalent thresholds for clinical utility
can be derived. It has been suggested that high sensitivity is a more
useful attribute in early detection of pre-eclampsia than specificity
because consideration of benefits, harms and costs indicates a much
greater preference for minimizing false negatives than false positives,
although the ideal would be to avoid both.\textsuperscript{26}

In the COMPARE study,\textsuperscript{23} equivalent clinical thresholds
for PlGF rule-out differed by 50\% and the DELFIA Xpress PlGF 1-2-3 has
a rule-out threshold of 150 pg/ml, whereas the rule-out threshold for
the Triage PlGF test is 100 pg/ml. This illustrates that they are not
identical assays, nor are they interchangeable clinically. The differing
thresholds are likely due to variable PlGF recovery and isomer
cross-reactivity. PlGF is a homo-dimeric glycoprotein which exists in
four isomers, derived from different splicing of primary gene
transcripts.\textsuperscript{27} PlGF-1 and PlGF-2 are the most abundant
forms, and they are secreted in a strongly correlated manner. Isomer
cross-reactivity is variable; one study found 12 to 19\%
cross-reactivity with PlGF-2 and 16 to 23\% cross-reactivity with PlGF-3
for the Roche Elecsys PlGF assay.\textsuperscript{28} This contrasts
with the manufacturer reports; Roche Diagnostics quote less than 8\%
cross-reactivity and Triage quote 9.6\% cross-reactivity with
PlGF-2.\textsuperscript{28}

There have been other interassay comparison studies.\textsuperscript{24,
25, 28} Stepan and colleagues (2016) performed a prospective
multi-centre case-control study of 569 women (178 with confirmed
pre-eclampsia).\textsuperscript{24} They found the Elecsys immunoassay
had a higher specificity and lower sensitivity compared to the Triage
assay at the thresholds selected, but used a cut off of \selectlanguage{english}[?]33 for the
Elecsys sFlt-1/PlGF ratio. As the sensitivity and specificity are highly
dependent on the thresholds selected, it is difficult to draw any
conclusions from these comparative predictive statistics.

A smaller retrospective case-control study in 128 women (44 with
confirmed preeclampsia) found that the Triage assay had a higher
sensitivity than the Elecsys ratio at only a small reduction of
specificity (sensitivity 100\% (95\% CI 86-100) compared to 64\% (95\%
CI 43-82), and specificity 96\% (95\% CI 85-99) compared to 100\% (95\%
CI 93-100)).\textsuperscript{24} This study also clearly demonstrated
that PlGF concentrations were lower when measured on the Triage assay
compared the Elecsys assay. They defined a positive test as PlGF
\textless{}5\textsuperscript{th} centile for a gestational-age dependent
range, and \textgreater{}85 for the Elecsys sFlt-1/PlGF ratio.

In summary, both tests recommended by the National Institute for Health
and Care Excellence have strong test performance with AUC outperforming
currently used tests for diagnosing pre-eclampsia (see Table 1). The
Triage PlGF test has a higher sensitivity when used with the current
rule-out threshold of 100pg/ml, compared to the Elecsys sFlt-1/PlGF
ratio threshold of 38, which has a higher specificity. These and other
tests available on the market appear to be clinically similar in
prediction, and other factors such as cost, and ease of use will dictate
clinical uptake.

\textbf{Practicalities of Implementation}

The choice of which PlGF-based test to implement in a maternity unit is
likely to be dictated by the unique practicalities of each platform,
which are outlined in Box 1.

Despite the clinical and financial advantages of PlGF-based testing,
there have been barriers to adoption encountered in a government-funded
healthcare system such as the UK. These are outlined in Box 2.
Therefore, each maternity unit will need to bring together all
stakeholders in order to choose which PlGF-based test best suits their
needs and setting. It is fundamental that the clinical, laboratory and
financial teams work in partnership, so that every partner understands
the implications of testing.

\textbf{The Future of PlGF}

The National Institute for Health and Care Excellence recommends a
single Triage PlGF test or Elecsys sFlt-1/PlGF ratio as a rule-out test,
concluding that the evidence is currently insufficient to recommend use
as a rule in test.\textsuperscript{13} Furthermore, the guidance is
clear that testing should be used to aid diagnosis of pre-eclampsia, and
not a wider syndrome of placental disease.

The role of repeat PlGF sampling has not been fully investigated and the
impact of repeat testing on maternal and perinatal outcomes is unknown.
This is particularly important in women in whom a clear risk trajectory
or diagnosis is not reached at initial presentation, but ongoing
suspicion of disease remains. One study has recently suggested that
repeat PlGF testing retains high diagnostic accuracy, with a high
sensitivity and NPV.\textsuperscript{29} Another study demonstrated
increasing sFlt-1/PlGF ratios in women who went on to develop
pre-eclampsia or adverse perinatal outcomes.\textsuperscript{30}However,
before repeat testing is recommended, clinical and cost-effectiveness
need to be established, given as an explicit research recommendation in
the diagnostic guideline.\textsuperscript{13} This is being addressed by
the PARROT-2 trial, a multicentre randomised controlled trial of repeat
revealed PlGF-based testing compared to usual care with repeat concealed
testing (ISRCTN85912420).

The role of PlGF in the assessment of pre-eclampsia in multi-fetal
pregnancy is uncertain, as normal ranges and interpretation have not
been defined. There are a few small studies to date, with conflicting
results.\textsuperscript{31-33} The largest of these studied 79 women,
presenting with suspected pre-eclampsia and found elevated sFlt-1/PlGF
ratio was associated with adverse outcomes.\textsuperscript{31} Although
a smaller population of women, use of PlGF-based testing remains an
important challenge due to the higher prevalence of pre-eclampsia in
multi-fetal pregnancy.

PlGF-based testing in the assessment of early-onset fetal growth
restriction warrants further study and there is conflicting evidence.
One study of 213 pregnancies showed that low PlGF had a sensitivity of
98.2\% with a NPV of 99.2\% for placental fetal growth restriction
(confirmed on placental histology).\textsuperscript{34} A large
prospective cohort study of 4,512 women found that addition of the
sFlt-1/PlGF ratio to ultrasonic assessment improved the positive
likelihood ratio for delivering a small-for-gestational-age
infant.\textsuperscript{35} Another study demonstrated that PlGF
outperformed both ultrasound and numerous other
biomarkers.\textsuperscript{36} However, a multicentre study including
592 women demonstrated that PlGF performed no better than estimated
fetal weight \textless{}5\textsuperscript{th} centile in predicting
delivery of a small-for-gestational-age infant.\textsuperscript{37}

Other outstanding issues include the ability of PlGF-based tests to risk
stratify in established disease, as well as the role of PlGF in
low-resource countries. By far the greatest burden of morbidity and
mortality due to pre-eclampsia is borne by women and their infants in
low and middle-income settings. Initial evidence of the impact of
PlGF-based testing in this setting is promising.\textsuperscript{38, 39}

Finally, now that value in diagnosis of pre-eclampsia has been proven,
PlGF and sFlt-1 may prove informative to guide treatments that influence
the outcome or ameliorate development of the
disease.\textsuperscript{40}

\textbf{Summary}

There is a growing body of evidence that PlGF-based testing improves
diagnosis and reduces adverse maternal outcomes in the assessment of
suspected preterm pre-eclampsia, by appropriate risk stratification and
resource redistribution. The Triage PlGF test and Elecsys sFlt-1/PlGF
ratio are both recommended by the National Institute for Health and Care
Excellence but are not interchangeable. Both assays have similar
predictive properties but also unique advantages and disadvantages. The
test to be implemented needs to be carefully chosen for the setting with
involvement from all relevant parties.

\textbf{Disclosure of Interests}

AS has received funds from Perkin Elmer, paid to the university, to
evaluate angiogenic markers. Other authors declare that they have no
competing interests.

\textbf{Contribution to Authorship}

Conceptualisation -- AS; writing -- original draft outline -- AS, LC,
LW, AH; writing -- original draft preparation -- AH; writing -- review
and editing -- AH, ABG, KD, LW, LC, AS. Supervision -- AS. All authors
have given final approval of the version to be submitted.

\textbf{Funding}

The authors received no specific funding for this work.

\textbf{References}

1. Stevens W, Shih T, Incerti D, Ton TGN, Lee HC, Peneva D, Macones GA,
Sibai BM, Jena AB. Short-term costs of preeclampsia to the United States
health care system. \emph{Am J Obstet Gynecol} 2017; \textbf{217} :
237-248.e216.

2. Shennan AH, Green M, Chappell LC. Maternal deaths in the UK:
pre-eclampsia deaths are avoidable. \emph{Lancet} 2017; \textbf{389} :
582-584.

3. Steer P. The epidemiology of preterm labour. \emph{BJOG}
2005;\textbf{112 Suppl 1} : 1-3.

4. Brown MA, Magee LA, Kenny LC, Karumanchi SA, McCarthy FP, Saito S,
Hall DR, Warren CE, Adoyi G, Ishaku S, (ISSHP) ISftSoHiP. Hypertensive
Disorders of Pregnancy: ISSHP Classification, Diagnosis, and Management
Recommendations for International Practice. \emph{Hypertension}
2018;\textbf{72} : 24-43.

5. Excellence NIfC. Hypertension in Pregnancy: diagnosis and
management.~. https://www.nice.org.uk/guidance/ng133, 2019.

6. Zhang J, Klebanoff MA, Roberts JM. Prediction of adverse outcomes by
common definitions of hypertension in pregnancy. \emph{Obstet
Gynecol}2001; \textbf{97} : 261-267.

7. Milne F, Redman C, Walker J, Baker P, Black R, Blincowe J, Cooper C,
Fletcher G, Jokinen M, Moran PA, Nelson-Piercy C, Robson S, Shennan A,
Tuffnell A, Waugh J, Group PI. Assessing the onset of pre-eclampsia in
the hospital day unit: summary of the pre-eclampsia guideline (PRECOG
II). \emph{BMJ} 2009; \textbf{339} : b3129.

8. Levine RJ, Maynard SE, Qian C, Lim KH, England LJ, Yu KF, Schisterman
EF, Thadhani R, Sachs BP, Epstein FH, Sibai BM, Sukhatme VP, Karumanchi
SA. Circulating angiogenic factors and the risk of preeclampsia. \emph{N
Engl J Med} 2004; \textbf{350} : 672-683.

9. Saffer C, Olson G, Boggess KA, Beyerlein R, Eubank C, Sibai BM, Group
NS. Determination of placental growth factor (PlGF) levels in healthy
pregnant women without signs or symptoms of preeclampsia.\emph{Pregnancy
Hypertens} 2013; \textbf{3} : 124-132.

10. Chaiworapongsa T, Romero R, Kim YM, Kim GJ, Kim MR, Espinoza J,
Bujold E, Goncalves L, Gomez R, Edwin S, Mazor M. Plasma soluble
vascular endothelial growth factor receptor-1 concentration is elevated
prior to the clinical diagnosis of pre-eclampsia. \emph{J Matern Fetal
Neonatal Med} 2005; \textbf{17} : 3-18.

11. Chau K, Hennessy A, Makris A. Placental growth factor and
pre-eclampsia. \emph{J Hum Hypertens} 2017; \textbf{31} : 782-786.

12. Chappell LC, Duckworth S, Seed PT, Griffin M, Myers J, Mackillop L,
Simpson N, Waugh J, Anumba D, Kenny LC, Redman CW, Shennan AH.
Diagnostic accuracy of placental growth factor in women with suspected
preeclampsia: a prospective multicenter study. \emph{Circulation}
2013;\textbf{128} : 2121-2131.

13. Excellence NIfC. PlGF-based testing to help diagnose suspected
pre-eclampsia (Triage PlGF test, Elecysys immunoassay sFlt-1/PlGF ratio,
DELFIA Xpress PlGF 1-2-3 test, and BRAHMS sFlt-1 Kryptor/BRAHMS PlGF
plus Kryptor PE ratio)~https://www.nice.org.uk/guidance/dg23, 2016.

14. Barton JR, Woelkers DA, Newman RB, Combs CA, How HY, Boggess KA,
Martin JN, Kupfer K, Sibai BM, Trial PPTbRA. Placental growth factor
predicts time to delivery in women with signs or symptoms of early
preterm preeclampsia: a prospective multicenter study. \emph{Am J Obstet
Gynecol} 2019.

15. Parchem JG, Brock CO, Chen HY, Kalluri R, Barton JR, Sibai BM,
Investigators PTbRATP. Placental Growth Factor and the Risk of Adverse
Neonatal and Maternal Outcomes. \emph{Obstet Gynecol} 2020.

16. Duhig KE, Myers J, Seed PT, Sparkes J, Lowe J, Hunter RM, Shennan
AH, Chappell LC, group Pt. Placental growth factor testing to assess
women with suspected pre-eclampsia: a multicentre, pragmatic,
stepped-wedge cluster-randomised controlled trial. \emph{Lancet}
2019;\textbf{393} : 1807-1818.

17. Duckworth S, Chappell LC, Seed PT, Mackillop L, Shennan AH, Hunter
R. Placental Growth Factor (PlGF) in Women with Suspected Pre-Eclampsia
Prior to 35 Weeks' Gestation: A Budget Impact Analysis. \emph{PLoS
One}2016; \textbf{11} : e0164276.

18. Frampton GK, Jones J, Rose M, Payne L. Placental growth factor
(alone or in combination with soluble fms-like tyrosine kinase 1) as an
aid to the assessment of women with suspected pre-eclampsia: systematic
review and economic analysis. \emph{Health Technol Assess}
2016;\textbf{20} : 1-160.

19. Duhig KE, Seed PT, Myers JE, Bahl R, Bambridge G, Barnfield S,
Ficquet J, Girling JC, Khalil A, Shennan AH, Chappell LC, Hunter RM.
Placental growth factor testing for suspected pre-eclampsia: a
cost-effectiveness analysis. \emph{BJOG} 2019; \textbf{126} : 1390-1398.

20. Rana S, Powe CE, Salahuddin S, Verlohren S, Perschel FH, Levine RJ,
Lim KH, Wenger JB, Thadhani R, Karumanchi SA. Angiogenic factors and the
risk of adverse outcomes in women with suspected
preeclampsia.\emph{Circulation} 2012; \textbf{125} : 911-919.

21. Zeisler H, Hund M, Verlohren S. The sFlt-1:PlGF Ratio in Women with
Suspected Preeclampsia. \emph{N Engl J Med} 2016; \textbf{374} :
1785-1786.

22. Cerdeira AS, O'Sullivan J, Ohuma EO, Harrington D, Szafranski P,
Black R, Mackillop L, Impey L, Greenwood C, James T, Smith I,
Papageorghiou AT, Knight M, Vatish M. Randomized Interventional Study on
Prediction of Preeclampsia/Eclampsia in Women With Suspected
Preeclampsia: INSPIRE. \emph{Hypertension} 2019; \textbf{74} : 983-990.

23. McCarthy FP, Gill C, Seed PT, Bramham K, Chappell LC, Shennan AH.
Comparison of three commercially available placental growth factor-based
tests in women with suspected preterm pre-eclampsia: the COMPARE
study.\emph{Ultrasound Obstet Gynecol} 2019; \textbf{53} : 62-67.

24. Stepan H, Hund M, Gencay M, Denk B, Dinkel C, Kaminski WE, Wieloch
P, Semus B, Meloth T, Droge LA, Verlohren S. A comparison of the
diagnostic utility of the sFlt-1/PlGF ratio versus PlGF alone for the
detection of preeclampsia/HELLP syndrome. \emph{Hypertens
Pregnancy}2016; \textbf{35} : 295-305.

25. Benton SJ, Hu Y, Xie F, Kupfer K, Lee SW, Magee LA, von Dadelszen P.
Angiogenic factors as diagnostic tests for preeclampsia: a performance
comparison between two commercial immunoassays. \emph{Am J Obstet
Gynecol} 2011; \textbf{205} : 469.e461-468.

26. Cnossen JS, ter Riet G, Mol BW, van der Post JA, Leeflang MM, Meads
CA, Hyde C, Khan KS. Are tests for predicting pre-eclampsia good enough
to make screening viable? A review of reviews and critical
appraisal.\emph{Acta Obstet Gynecol Scand} 2009; \textbf{88} : 758-765.

27. Black C, Al-Amin A, Rolnik DL, Kane SC, Stolarek C, White A, da
Silva Costa F, Brennecke S. Midpregnancy testing for soluble fms-like
tyrosine kinase 1 (sFlt-1) and placental growth factor (PlGF): An
inter-assay comparison of three automated immunoassay
platforms.\emph{Placenta} 2019; \textbf{86} : 11-14.

28. Cheng YKY, Poon LCY, Shennan A, Leung TY, Sahota DS.
Inter-manufacturer comparison of automated immunoassays for the
measurement of soluble FMS-like tyrosine kinase-1 and placental growth
factor. \emph{Pregnancy Hypertens} 2019; \textbf{17} : 165-171.

29. Duhig KE, Webster LM, Sharp A, Gill C, Seed PT, Shennan AH, Myers
JE, Chappell LC. Diagnostic accuracy of repeat placental growth factor
measurements in women with suspected preeclampsia: a case series
study.\emph{Acta Obstet Gynecol Scand} 2020.

30. Zeisler H, Llurba E, Chantraine FJ, Vatish M, Staff AC, Sennstrom M,
Olovsson M, Brennecke SP, Stepan H, Allegranza D, Schoedl M, Grill S,
Hund M, Verlohren S. Soluble fms-like tyrosine kinase-1 to placental
growth factor ratio: ruling out pre-eclampsia for up to 4 weeks and
value of retesting. \emph{Ultrasound Obstet Gynecol} 2019; \textbf{53} :
367-375.

31. Rana S, Hacker MR, Modest AM, Salahuddin S, Lim KH, Verlohren S,
Perschel FH, Karumanchi SA. Circulating angiogenic factors and risk of
adverse maternal and perinatal outcomes in twin pregnancies with
suspected preeclampsia. \emph{Hypertension} 2012; \textbf{60} : 451-458.

32. Saleh L, Tahitu SIM, Danser AHJ, van den Meiracker AH, Visser W. The
predictive value of the sFlt-1/PlGF ratio on short-term absence of
preeclampsia and maternal and fetal or neonatal complications in twin
pregnancies. \emph{Pregnancy Hypertens} 2018; \textbf{14} : 222-227.

33. Droge L, Herraiz I, Zeisler H, Schlembach D, Stepan H, Kussel L,
Henrich W, Galindo A, Verlohren S. Maternal serum sFlt-1/PlGF ratio in
twin pregnancies with and without pre-eclampsia in comparison with
singleton pregnancies. \emph{Ultrasound Obstet Gynecol} 2015;\textbf{45}
: 286-293.

34. Benton SJ, McCowan LM, Heazell AE, Grynspan D, Hutcheon JA, Senger
C, Burke O, Chan Y, Harding JE, Yockell-Lelievre J, Hu Y, Chappell LC,
Griffin MJ, Shennan AH, Magee LA, Gruslin A, von Dadelszen P. Placental
growth factor as a marker of fetal growth restriction caused by
placental dysfunction. \emph{Placenta} 2016; \textbf{42} : 1-8.

35. Gaccioli F, Sovio U, Cook E, Hund M, Charnock-Jones DS, Smith GCS.
Screening for fetal growth restriction using ultrasound and the
sFLT1/PlGF ratio in nulliparous women: a prospective cohort
study.\emph{Lancet Child Adolesc Health} 2018; \textbf{2} : 569-581.

36. Griffin M, Seed PT, Duckworth S, North R, Myers J, Mackillop L,
Simpson N, Waugh J, Anumba D, Kenny LC, Redman CWG, Shennan AH, Chappell
LC. Predicting delivery of a small-for-gestational-age infant and
adverse perinatal outcome in women with suspected
pre-eclampsia.\emph{Ultrasound Obstet Gynecol} 2018; \textbf{51} :
387-395.

37. Griffin M, Seed PT, Webster L, Myers J, MacKillop L, Simpson N,
Anumba D, Khalil A, Denbow M, Sau A, Hinshaw K, von Dadelszen P, Benton
S, Girling J, Redman CW, Chappell LC, Shennan AH. Diagnostic accuracy of
placental growth factor and ultrasound parameters to predict the
small-for-gestational-age infant in women presenting with reduced
symphysis-fundus height. \emph{Ultrasound Obstet Gynecol}
2015;\textbf{46} : 182-190.

38. Ukah UV, Mbofana F, Rocha BM, Loquiha O, Mudenyanga C, Usta M, Urso
M, Drebit S, Magee LA, von Dadelszen P. Diagnostic Performance of
Placental Growth Factor in Women With Suspected Preeclampsia Attending
Antenatal Facilities in Maputo, Mozambique. \emph{Hypertension}
2017;\textbf{69} : 469-474.

39. Manriquez Rocha B, Mbofana F, Loquiha O, Mudenyanga C, Ukah UV,
Magee LA, von Dadelszen P. Early diagnosis of preeclampsia using
placental growth factor: An operational pilot study in Maputo,
Mozambique. \emph{Pregnancy Hypertens} 2018; \textbf{11} : 26-31.

40. Thadhani R, Hagmann H, Schaarschmidt W, Roth B, Cingoez T,
Karumanchi SA, Wenger J, Lucchesi KJ, Tamez H, Lindner T, Fridman A,
Thome U, Kribs A, Danner M, Hamacher S, Mallmann P, Stepan H, Benzing T.
Removal of Soluble Fms-Like Tyrosine Kinase-1 by Dextran Sulfate
Apheresis in Preeclampsia. \emph{J Am Soc Nephrol} 2016; \textbf{27} :
903-913.

Table 1: Test performance of PlGF-based tests\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Triage PlGF test\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
Elecsys sFlt-1 /PlGF ratio\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
DELFIA Xpress PlGF 1-2-3 test\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
BRAHMS sFlt-1/ PlGF plus ratio\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Recommended rule-out threshold\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
[?] 100 pg/ml\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
[?] 38\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
[?] 150 pg/ml\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textgreater{}55\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Sensitivity\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
96\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
80\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
87.5\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Negative predictive value\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
98\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
99.3\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
97.2\%\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Suggested rule in threshold\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textless{}12 pg/ml\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textgreater{} 85\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textless{}50 pg/ml\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textgreater{}188\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Relevant study\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
PELICAN\textsuperscript{12} PARROT\textsuperscript{16}\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
PROGNOSIS\textsuperscript{21} INSPIRE\textsuperscript{22}\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
COMPARE\textsuperscript{23}\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Cheng \emph{et al.\textsuperscript{28}}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

Box 1: Practical aspects of implementing PlGF-based testing.\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Barriers} Inconsistent cross-discipline awareness of the impact
of testing Lack of clinical coding for suspected pre-eclampsia,
undermining appreciation of the cost-benefit of testing Difficulty
predicting long-term costs Resistance from laboratory staff in some
units Current testing not available on whole blood\tabularnewline
\midrule
\endhead
\textbf{Facilitators} National guidance now recommending adoption of
testing Momentum of clinical community moving towards use in practice
Funding available to aid implementation in some healthcare settings
(e.g. tests and triage meters provided through the Innovation and
Technology Payment programme, supported by NHS England)\tabularnewline
\bottomrule
\end{longtable}

Box 2: Barriers and facilitators to implementation of PlGF-based testing
in UK healthcare setting\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Barriers} Inconsistent cross-discipline awareness of the impact
of testing Lack of clinical coding for suspected pre-eclampsia,
undermining appreciation of the cost-benefit of testing Difficulty
predicting long-term costs Resistance from laboratory staff in some
units Current testing not available on whole blood\tabularnewline
\midrule
\endhead
\textbf{Facilitators} National guidance now recommending adoption of
testing Momentum of clinical community moving towards use in practice
Funding available to aid implementation in some healthcare settings
(e.g. tests and triage meters provided through the Innovation and
Technology Payment programme, supported by NHS England)\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

