1. Preparation of QC Samples
Because IA methods tend to be subjected to matrix effects, it is imperative to prepare QCs in the same matrix as the intended sample. Guidelines for pharmaceutical assays recommend that at least three QC concentrations (low, intermediate, and high on the standard curve) be batched with the unknown samples for analysis. This means that QCs are prepared at the same time samples are generated and stored with samples under identical conditions. In some cases, the standard curve range for an IA is not able to cover the entire range of concentrations observed in the unknown samples. When this occurs, QCs concentrations in the range of the observed unknown sample concentrations should be prepared and diluted along with the unknown samples before analysis to evaluate the influence of the dilution process. Parallelism of the high-QC sample should be evaluated.
Sometimes a clinical site may deviate from the study protocol-defined sample-collection procedures. For example, instead of collecting blood into heparin-ized tubes, the sample may be collected into a tube containing another anticoagulant, such as citrate or EDTA. Under these conditions, it is necessary to cross-validate each of the plasma/serum matrices: QC samples in the deviant matrix must be prepared and processed along with the deviant samples to ensure that the method is validated in the new matrix.
In addition to QCs prepared in-house, QCs prepared in another laboratory at the same location or at another location can be used to assess interlaboratory accuracy and precision. For example, as part of the CLIA 88 proficiency test program,
QCs are prepared at a central laboratory and sent to various clinical chemistry laboratories that participate in the program. The mean observed values among the laboratories, not the theoretical values of the QCs, are used for laboratory proficiency evaluation. Criteria are set differently for different tests by CLIA 88 regulations. Also, bioanalytical laboratories involved in a drug-development program may exchange QCs and test samples to ensure lab-to-lab reproducibility in the analyses.
QCs for analytes used in certain high-volume analyses are available from commercial sources. However, unless they are prepared in the exact matrix that is of interest to the analyst, they are of no practical use.
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