Main findings
Our current prospective cohort study has confirmed the findings from our
earlier case-control study that PlGF is reduced in trisomy 21 affected
pregnancies. In addition, our study has shown that adding PlGF to the
combined test does not increase the detection rate and that replacing
PAPP-A with PlGF in the combined test has increased rather than reduced
the screen positive rate. Our data would indicate that screening centres
providing trisomy 21 screening and either concurrently performing or
considering adding preeclampsia screening will need to measure both
PAPP-A and PlGF and thus expect no reduction in laboratory reagent costs
if seeking to maintain the existing screening test performance.
The strengths of our study were firstly its prospective design, secondly
the use of both internal and external quality assurance assessment for
all screening markers and thirdly its sample size, which allowed
assessment of the impact on false positive and screen positive rates
between the three risk estimation options. Although the number of
trisomy 21 affected pregnancies was not high, they were sufficient in
number to allow us to highlight that adding PlGF or replacing PAPP-A by
PlGF did not improve the detection rate, especially if the screening
centre is already achieving a high detection rate for the expected false
positive and screen positive rates using the conventional combined test.