Endothelial Activation Phenotypes and Interleukin-6 Response After Therapeutic Plasma Exchange in Severe COVID-19-Associated Sepsis: A Retrospective Cohort Study

Background and Objectives: Severe COVID-19 frequently fulfills Sepsis-3 criteria and is characterized by thrombo-inflammation and endothelial injury. We evaluated whether a bedside endothelial activation index (EAI = D-dimer/fibrinogen) identifies biologically distinct phenotypes and relates to interleukin-6 (IL-6) response after therapeutic plasma exchange (TPE), and whether baseline IL-6 predicts a ≥50% IL-6 reduction. Methods: Retrospective single-center ICU cohort of adults with SARS-CoV-2 infection, sepsis-related organ dysfunction, and ≥1 TPE session (n = 51). Patients were stratified by median EAI (low vs. high). Outcomes included peri-procedural biomarker/physiology changes (post–baseline), IL-6 responder status (≥50% reduction), correlations with IL-6 reduction (%), and multivariable predictors of response. Results: Compared with low EAI (n = 25), high EAI (n = 26) had higher baseline D-dimer (6.2 vs. 2.2 µg/mL) and lower fibrinogen (2.9 vs. 7.1 g/L) (both p < 0.001). Low EAI showed larger CRP decreases (ΔCRP −84.0 vs. −2.3 mg/L; p = 0.001) and larger fibrinogen falls (Δ −3.1 vs. −0.4 g/L; p < 0.001), while high EAI had larger D-dimer decreases (Δ −2.5 vs. −0.6 µg/mL; p = 0.004) and a modest SOFA improvement (Δ −0.3 vs. +0.1; p = 0.026). IL-6 responders (n = 20) had higher baseline IL-6 than non-responders (365.2 vs. 47.1 pg/mL; p < 0.001). Baseline IL-6 independently predicted response (per doubling: OR 1.94, 95% CI 1.27–2.95; p = 0.002), while age reduced odds (OR 0.91/year, 95% CI 0.84–0.99; p = 0.032). IL-6 reduction correlated with ΔCRP (ρ = −0.41; p = 0.003) and ΔPaO2/FiO2 (ρ = 0.37; p = 0.01). Conclusions: EAI stratifies distinct thrombo-inflammatory patterns around TPE, while baseline IL-6 is the dominant predictor of achieving large IL-6 reductions. To emphasize the novelty and clarify the study objective, this exploratory analysis used a phenotype-stratified framework to test whether a simple bedside endothelial activation index could enrich biological response assessment to adjunctive TPE. The prespecified primary outcome was achievement of a ≥50% IL-6 reduction after completion of the TPE course; secondary outcomes included peri-procedural biomarker, oxygenation, SOFA, and ICU endpoints.