Commutability Limitations Influence Quality Control Results with Different Reagent Lots

BACKGROUND: Good laboratory practice includes verifying that each new lot of reagents is suitable for use before it is put into service. Noncommutability of quality control (QC) samples with clinical patient samples may preclude their use to verify consistency of results for patient samples between different reagent lots. METHODS: Patient sample results and QC data were obtained from reagent lot change verification records for 18 QC materials, 661 reagent lot changes, 1483 reagent lot change-QC events, 82 analytes, and 7 instrument platforms. The significance of between-lot differences in the results for QC samples compared with those for patient samples was assessed by a modified 2-sample t test adjusted for heterogeneity of QC and patient sample measurement variances. RESULTS: Overall, 40.9% of reagent lot change-QC events had a significant difference (P < 0.05) between results for QC samples compared with results for patient samples between 2 reagent lots. For QC results with differences <1.0 SD interval (83.1% of total), 37.7% were significantly different from the changes observed for patient samples. For QC results with differences ≥1.0 SD interval (16.9% of total), 57.0% were significantly different from those for patient samples. CONCLUSIONS: Occurrence of noncommutable results for QC materials was frequent enough that the QC results could not be used to verify consistency of results for patient samples when changing lots of reagents.