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

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. Methods: Retrospective cohort of 5493 singleton pregnancies with a first-trimester PE risk calculated at 11-14 weeks (2020-2026) in one Romanian fetal-medicine centre. Pregnancies were stratified into five groups according to which markers were available at screening (maternal factors ± biochemistry [PAPP-A] ± uterine artery pulsatility index [UtA-PI] ± mean arterial pressure [MAP]), and the centre-specific SPR at fixed risk thresholds and the risk threshold corresponding to fixed SPR targets of 5%, 10%, 15% and 20% were computed for each group. The operational clinical threshold (1:150 before and 1:100 after the introduction of systematic MAP measurement in October 2023) was benchmarked against this map. Outcome ascertainment was quantified to determine whether these data could support any estimate of detection performance. Results: Among pregnancies screened with the maternal factors + MAP + UtA-PI + PAPP-A combination now used in routine practice (n = 1797), the operational threshold of 1:100 corresponded to an SPR of 16.7%, closer to the 15% than the 10% mapping point (local threshold for SPR = 10%: 1:66); the same numeric threshold corresponded to an SPR of only 11.0% among pregnancies screened with maternal factors and UtA-PI alone (n = 317). The historical reduction in SPR from 25.0% (pre-October 2023, threshold 1:150) to 16.1% (post-October 2023, threshold 1:100) was attributable mainly to the threshold change itself, since a same-era comparison at an identical fixed threshold (1:100) showed little difference in SPR between examinations with and without MAP (16.2% vs. 16.0%). Documented pregnancy outcome was available for only 13.5% of pregnancies (742/5493) and was strongly enriched for complicated pregnancies (documented PE prevalence 5.53% in this subset versus 0.87% in the whole cohort and 0.87-1.2% in published Romanian single-centre series). Consequently, outcome data in this dataset were considered unsuitable for local estimation of detection performance or treatment effect, and no such estimate is offered. Conclusions: The numeric risk threshold used clinically does not correspond to a fixed screen-positive rate across marker combinations in routine practice; local mapping of this relationship is a low-cost, outcome-independent audit that any screening centre can perform on its own data before, and independently of, any attempt to validate detection performance.