In QA/QC, why are positive controls or a test mix included?

• A. To measure urine pH
• B. To verify assay performance across the panel and confirm reagents and instrument responses are functioning correctly.
• C. To determine sample volume
• D. To check patient identity

Answer: (C)

Positive controls or a test mix are included to confirm that the assay can reliably detect the target across the entire panel and that reagents, calibrations, and the instrument are functioning correctly during the run. Using known-positive samples validates the assay’s performance, helps catch issues like reagent degradation, calibration drift, or instrument problems, and ensures the results are trustworthy for patient reporting. The other options describe unrelated tasks: measuring urine pH is a specific analyte measurement, determining sample volume is a procedural aspect, and checking patient identity is an administrative safeguard—none of these verify the analytical performance across the panel.