Venous and arterial thrombosis following extracorporeal photopheresis: A descriptive cohort study using a federated electronic health record network

BACKGROUND: In 2018, the FDA reported VTE events in patients undergoing ECP, but these reports lacked a population-level denominator to contextualize the risk against the known baseline thrombotic burden of the underlying conditions. STUDY DESIGN AND METHODS: We conducted a descriptive retrospective cohort study using the TriNetX federated EHR network. Patients undergoing ECP with no thrombosis codes in the preceding 6 months or 1 year were identified. The primary outcome was new VTE within 5 and 30 days of the index procedure. Secondary analyses examined VTE subtypes and stratified risk by indication (GVHD vs. CTCL). RESULTS: Among 3006 patients (1-year thrombosis exclusion), the 30-day VTE risk was 6.0% (95% CI, 5.2%-6.9%) and the 5-day risk was 1.4% (95% CI, 1.0%-1.8%); estimates were consistent across thrombosis exclusion periods. Kaplan-Meier analysis showed gradual event accumulation without periprocedural clustering. VTE risk at 30 days was markedly higher in GVHD patients (8.1%; 95% CI, 6.8%-9.5%) than CTCL patients (3.3%; 95% CI, 2.1%-5.0%). Among GVHD patients, upper extremity DVT, PE, and lower extremity DVT contributed approximately equally. Arterial thrombotic events were infrequent (1.5% at 30 days). CONCLUSION: VTE following ECP occurs at a rate consistent with baseline thrombotic risk in the underlying populations and does not cluster periprocedurally, strongly suggesting that patient-level factors rather than the procedure drive the observed risk. A small contributory procedural effect cannot be excluded without a controlled comparator study.