Calibration verification is defined as testing materials of known concentration in the same manner as patient specimens. This process assures that the test system accurately measures samples throughout the reportable range. Automated cell counters are exempt from the calibration verification process. Daily testing of two levels of quality control samples on an automated analyzer verifies calibration.
Analyzers employed for other specialties, such as chemistry and immunoassay, are required by CLIA regulations as defined in the CFR to perform calibration verification studies under specified circumstances.
For the purpose of this unit, it is vital to understand the required calibration verification process. Analyzer examples that are not automated cell counters will be incorporated to accomplish this. Specifically, chemistry and immunoassay analyzers will be included in the cited examples.
The purpose of calibration verification is to check the analyzer's calibration by confirming that the test system can provide accurate results across the entire analytical measurement range of the instrument for each analyte.
If the standard calibration process includes a minimum of 3 calibrators, calibration verification is not required as it has already been achieved. If fewer than three calibrators are used for routine calibration, calibration verification is required every six months. Examples of analyzers that require this process are those that perform chemistry and immunoassay testing. Criteria for calibration verification include:
- Measurements are performed every six months.
- Includes a low, mid-point, and high point for the test method
- More frequently if indicated by the manufacturer or the laboratory has determined that the frequency is required more often
- At the time of change in reagent lot number, reagent formulation, or reagent manufacturer
- Major preventative maintenance or replacement of critical instrument parts
- When quality control materials do not perform as expected
Appropriate materials to be used for calibration verification include a minimum of 3 levels which may be obtained from the following sources:
- Calibrators used to calibrate the analytical system but from a different lot
- Commercially available material intended for calibration verification
- Materials of known concentrations
- Proficiency testing samples with known results -- if the material is suitable, e.g., specifically designated by the method manufacturer or no other materials are available.
- Controls with known values -- if the material is suitable; e.g., specifically designated by the method manufacturer, or no other materials are available.
- Previously tested, unaltered patient samples
- Primary or secondary standards or reference materials with matrix characteristics and target values appropriate for the method
Following the testing of the calibration verification materials, results are compared to expected values. The results may be plotted on a graph and compared to the expected results. The plots demonstrate the linearity of the method and verification of spanning the entire analytical measurement range. Images 1 and 2 provide examples of linear and non-linear calibration verification results. Non-linear results may require repeat analysis when evaluated according to the laboratory's established criteria.
All results are documented, reviewed, and compared to the laboratory's established criteria before approval by the laboratory director of delegated personnel.
