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Calibration and Linearity Verification

According to CLIA 88, linearity only needs to be evaluated when a method initially is put into service. However, CAP’s accreditation checklist states that laboratories must verify linear range initially and at least semiannually, or when calibration verification fails to meet the laboratory’s acceptable limits. In practice, CAP’s requirement can be met by performing calibration verification.

CLIA and CAP require that method calibration be verified at least every 6 months or whenever critical instrument parts are replaced, major preventative maintenance is performed, or quality control indicates an unusual shift outside acceptable limits that is not corrected by other means. Calibration verification involves assaying calibration materials representing the lower limit, mid point, and upper limit of the reporting range in the same manner as patient samples. The material to be used is not restricted and can consist of any material with known value including callibrators, quality control samples, patient specimens analyzed by another instrument, or homemade spiked samples. Calibration verification confirms that an assay has remained stable throughout the reportable range of patient test results. It is a check on linearity as well as calibration. The values obtained for calibration verification should be within acceptable limits set by the manufacturer or laboratory. Calibration verification does not need to be performed after reagent changes if the lab demonstrates that changing lot numbers did not affect the reportable range and control values are not adversely affected by the change.

CLIA 88 requires that reportable range be verified only during method evaluation, but CAP requires that it be verified periodically. This requirement can be met in several ways; by doing a linearity study, performing a correlation study that includes low and high samples spanning the manufacturer’s range, serially diluting a high patient sample, running 20 repetitions of zero standard or a very low patient sample to determine the LLD, and determining acceptable range of recovery for low, medium, and high standards.

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