Observed Clinical and Operational Factors Are Associated with Biomarker Test Utilization in Respiratory Virus RT-PCR Testing: A Real-World Analysis of Inflammatory, Cardiac, and Thrombotic Biomarkers

Background/Objectives: Routinely collected biomarker data are generated through selective testing. We examined associations of observed clinical and operational factors with biomarker availability and explored the incremental discrimination provided by biomarker concentrations for respiratory viral RT-PCR positivity. Methods: This retrospective study included 7726 testing episodes for 13 respiratory viral targets from July 2020 to March 2025. Seven biomarkers were linked using a ±24-h reporting-time window. Models included age, sex, requesting department, operational time, calendar month, and year. Additional analyses included inverse probability weighting (IPW), weight and overlap diagnostics, and five repetitions of five-fold cross-validation. Results: Viral positivity was 12.72%. Biomarker availability ranged from 98.87% for C-reactive protein to 22.87% for B-type natriuretic peptide. Procalcitonin (PCT) result availability was associated with lower odds of viral positivity after adjustment (OR, 0.702; 95% CI, 0.583–0.846). Among episodes with a PCT result, the concentration association was close to the null (OR per doubling, 0.991; 95% CI, 0.953–1.030). The complete-cohort base model had an apparent area under the receiver operating characteristic curve (AUC) of 0.8263. The largest apparent ΔAUC was 0.00418 for D-dimer; its mean out-of-fold ΔAUC was 0.00367. IPW diagnostics showed substantial reductions in effective sample size and sensitivity to extreme weights for some biomarkers. Conclusions: Biomarker availability was associated with the observed clinical and operational factors, whereas concentrations added little discrimination for positivity within the available viral panel. These findings illustrate selection in retrospective laboratory data and do not assess biomarker usefulness for their intended clinical indications.