SAFELY RULING OUT DEEP VEIN THROMBOSIS IN PREGNANCY WITH THE LEFt SCORE AND TRIMESTER-ADJUSTED D-DIMER CUTOFFS (the PREG-DVT Study): A PROSPECTIVE COHORT STUDY.

Venous thromboembolism (VTE), which includes deep vein thrombosis and pulmonary embolism (PE), is a leading cause of maternal morbidity and mortality in high-income countries.1 DVT is suspected in pregnancy greater than 10 times more often than it is confirmed.2 The overlap between physiological symptoms of pregnancy (e.g. leg edema and calf, thigh and groin discomfort) and those of DVT often leads to referral for investigation of DVT, especially in later stages of pregnancy. DVT during pregnancy has a strong left leg predominance likely due to left iliac vein compression by the right iliac artery and the gravid uterus.3,4 Unfortunately, the only validated strategies to exclude DVT in pregnant patients require diagnostic imaging as the first step in the diagnostic algorithm. Specifically, compression ultrasonography (CUS), either as a single, whole-leg ultrasound2,5 or as a serial proximal ultrasound6 has been validated to exclude DVT during pregnancy. The LEFt clinical decision score-a simple three-item tool including Left leg symptoms, Edema (≥2 cm calf circumference difference compared to the other leg), and First trimester onset of symptoms-has shown promise in the LEFt derivation study6 and retrospective validation studies.7,8 More recently, the score was tested in a prospective study (LEAD study, OC, ISTH Congress, Paris 2026; submitted for publication), which confirmed its safety in association with D-dimers at the usual cutoff of 500 ng/ml to safely rule out DVT in pregnant patients. However, the diagnostic was quite low as only 7.4% of included patients could have DVT excluded based on this association. When exploring D-dimer at trimester adjusted thresholds of \<500 ng/ml FEUs (1st trimester), \<750 ng/ml FEUs (2nd trimester) and \<1000 ng/ml FEUs (3rd trimester), D-dimer was negative in 142/352 (40.3%) and yielded negative predictive values of 142/142, 100% (95% CI 97.4%-100%). Therefore, we plan a multinational management diagnostic outcome study in preg