Mapping Preeclampsia Risk Thresholds to Screen-Positive Rates in Routine First-Trimester Screening: A Single-Centre Audit

Background/Objectives: First-trimester screening for preeclampsia (PE) with the Fetal Medicine Foundation (FMF) competing-risks algorithm assigns each pregnancy a numeric risk (1:X), which is compared against a threshold to decide aspirin eligibility. What proportion of a real-world screened population a given numeric threshold actually selects—its screen-positive rate (SPR)—depends on which markers were available and on the population’s underlying risk distribution, independently of how well that threshold detects PE. We audited this relationship directly from routine data from a single centre.