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Abstract

Abstract Blood proteins measured during acute COVID-19 may help predict Long COVID, but models may fail when moved between cohorts and proteomic platforms. We fitted a reference model using age and sex (C0) in Su/INCOV and a protein model that also included FGF2 and PRDX3 (C2). The pair was fixed before outcome access because both proteins had one-to-one assay mappings. Protein values were standardized within each cohort, and Su coefficients were applied to Zurich without outcome-based refitting. Lower Brier scores indicate better prediction. Su contained 204 participants with complete predictors, but only 125 had linked follow-up. The strict analysis included 122 participants (75 events). Its optimism-corrected C2-minus-C0 Brier difference was + 0.00342 (95% interval, − 0.00608 to + 0.02834). Zurich included 113 participants (40 events). The transferred model difference was + 0.01093 (− 0.01721 to + 0.03882), and strict-model transfer gave + 0.00902 (− 0.01809 to + 0.03696). Both models overpredicted Zurich risk. Reviewer-requested intercept updating also yielded an inconclusive difference. Thus, FGF2 and PRDX3 did not reliably improve prediction beyond age and sex. Because protein standardization depends on a target cohort, this pipeline is not ready to predict risk for a patient.