Does a sFlt-1/PlGF ratio result > 655 before 34 weeks' gestation necessitate preterm delivery within 2 days? A retrospective observational study.

Introduction: Preeclampsia is associated with adverse perinatal outcomes, including fetal growth restriction (FGR) and preterm delivery. The maternal serum ratio of soluble fms-like tyrosine kinase receptor-1 (sFlt-1) to placental growth factor (PlGF) can be used to evaluate placental dysfunction in cases of preeclampsia and FGR. A need for delivery within 2 days has been recommended for sFlt-1/PlGF ratios > 655 (normal ratio < 38) measured before 34 weeks' gestation. However, few studies have assessed this recommendation in a real-world setting and there remains a need for further evidence-based guidance on the use of the ratio in delivery timing planning in this situation.

Aim: To assess the need for delivery within 2 days associated with sFlt-1/PlGF ratios > 655 before 34 weeks' gestation.

Methods: A retrospective audit of all sFlt-1/PlGF ratio test results obtained at a single maternity hospital between September 2016 and November 2022. The primary outcome was time to delivery after recording a ratio > 655 in patients with a pregnancy between 20 + 0 and 33 + 6 weeks' gestation. Statistical analysis was performed using IBM SPSS Statistics v29.0.0.0.

Results: During the study period a total of 33 patients with suspected or confirmed preeclampsia and/or FGR recorded sFlt-1/PlGF ratios > 655 before 34 + 0 weeks' gestation. Amongst cases with ratios > 655, median time to delivery was 4 days (IQR 1.0-9.0), with 14 (42.4%) delivering in ≤ 2 days, 8 (24.2%) delivering between 2 and 7 days and 11 (33.3%) delivering after 7 days. A significant inverse correlation was observed between time to delivery and gestational age at the time of ratio testing (r_s = -0.484, p = 0.004).

Discussion: This study provides updated recommendations on the use of the sFlt-1/PlGF ratio in predicting the risk of imminent delivery amongst those with high ratios > 655 measured before 34 weeks' gestation. Our results suggest that the risk of imminent delivery can be stratified based on ratio level and gestational age, which in combination with the results of other clinical assessments, can be used to plan delivery timing and allow for considerations of fetal lung maturing corticosteroid and neuroprotective magnesium sulfate therapies prior to delivery.