External quality assessment or proficiency testing (PT) is performed to ensure the reliability of test results by comparing results to other laboratories that use the same method system and/or to an assigned value.
The CLIA standards for handling proficiency testing specimens are as follows:
- PT samples must be tested with the laboratory's regular patient load.
- PT samples must be tested the same number of times that patients' samples are tested routinely.
- Laboratories participating in PT programs must not engage in interlaboratory comparison of PT sample results.
- Laboratories may not send PT samples to another laboratory for analysis.
- Laboratories must document all steps of processing for PT samples.
- PT is required for only the primary method used for testing of analytes in patients' samples during the period covered by the PT event.
In return for their participation, the laboratory will receive the following information:
- results for each analyte sample;
- mean result for each analyte;
- standard deviation of results by the comparative method;
- number of laboratories using the same method;
- standard deviation index (SDI); and
- lower and upper limits of acceptability of results.
PT results that are between the lower and upper limits of acceptability are considered satisfactory.
External quality control serves several purposes, including:
- evaluating the internal quality control program;
- detecting errors in a lab's methods, including technical errors; and
- providing a comparison of testing methods, which is useful in selecting new methods.