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.