As mentioned previously, QC refers to those measures that must be included during each assay in order to verify that the test is working properly. The following items are essential elements of quality control that must be performed during every assay:
- Each run must include one full set of controls
- The controls for each test run must yield results within the limits of the manufacturer's criteria for acceptability and validity of the run.
- All test kits must be used before the expiration date to ensure valid results
- Physical parameters of the test such as incubation time and temperature must be followed to ensure proper performance.
Ordinarily, each test kit has a set of positive and negative control that is to be included in each test run. These controls are considered to be internal controls, while any other controls included in the run are referred to as external controls. Internal controls are essential for QC measures for each run and are intended for use only with the lot number of the corresponding test kit. External controls can be included on a run to monitor consistent performance, lot to lot variation between kits, and to serve as an indicator of assay performance on samples that are borderline reactors.
External controls, or otherwise known as internal quality control (IQC) specimens are used in internal quality control programs, whereby IQC samples are included in serological assays. The IQC samples are then evaluated against Westgard rules, whereby the IQC values are plotted in a Shewhart type chart (this may be in terms of arbitrary units or IQC o.d./Cut-off o.d.). Westgard rules define specific performance limits and are designed to detect both random and systematic errors. Of the six commonly used Westgard rules, three are warning rules and the other three are mandatory rules. The latter, if broken should result in the rejection of the test run.
