BACKGROUNDCurrently, no laboratory tests exist to stratify for the risk of developing sinusoidal obstruction syndrome (SOS), an early endothelial complication after hematopoietic cell transplantation (HCT). Risk biomarkers of SOS have not been verified in a prospective cohort accounting for differences between practices across institutions. Herein, we aimed to define risk groups for SOS occurrence using 3 proteins: L-ficolin, hyaluronic acid (HA), and stimulation 2 (ST2).METHODSBetween 2017 and 2021, we prospectively accrued 80 pediatric patients across 4 US centers. Biomarkers were tested by ELISA blind to patient groupings and associated with SOS incidence on day 35 after HCT, and overall survival (OS) on day 100 after HCT. Cutpoints were identified using retrospective cohorts and applied to the prospective cohort.RESULTSCombination of the 3 biomarkers measured on day 3 after HCT in the prospective cohort provided 80% (95% CI 55%–100%) sensitivity and 73% (95% CI 62%–83%) specificity for risk of SOS occurrence. Patients with low L-ficolin were 9 times (95% CI 3–32) more likely to develop SOS, while patients with high HA and ST2 were 6.5 (95% CI 1.9–22.0) and 5.5 (95% CI 2.3–13.1) times more likely to develop SOS. These 3 markers also predicted worse day 100 OS — L-ficolin: HR, 10.0 (95% CI 2.2–45.1),P= 0.0002; HA: HR, 4.1 (95% CI 1.0–16.4),P= 0.031; and ST2: HR, 3.9 (95% CI 0.9–16.4),P= 0.04.CONCLUSIONL-ficolin, HA, and ST2 levels measured as early as 3 days after HCT improved risk stratification for SOS occurrence and OS and may guide risk-adapted preemptive therapy.TRIAL REGISTRATIONClinicalTrials.gov NCT03132337.FUNDINGNIH.