Subscription to external proficiency programs must be done by the laboratories; the program periodically sends challenge samples called proficiency test samples (PT). Prepared in the test matrix, the PT samples contain drugs or substances at specific concentrations. As expected in actual specimens, often the substances are potential interfering compounds. While the laboratory does not know the identities of the analytes and their concentration, the proficiency organization does. Instructed to test the PT using the same methods used for specimens, once the laboratory returns the tested results, the proficiency organization grades the results. Blind PT may also be submitted by some organizations. Submitted as a specimen, these samples are graded after results are returned to customers from the laboratory.
PT is sent to all certified laboratories quarterly by a single proficiency organization of the NLCP. A report is received by each laboratory, when they submit their results, which lists their results, the mean and standard deviation of all participating laboratories and their scores. Correct identification of each drug and finding its concentration within ±20% of the mean of all laboratories gives the laboratory a perfect score. Concentrations that are 20 to 50% away from the mean are termed as minor errors; reported concentrations of >50% away from the mean or incorrect identification of the drug is referred as a major error. Though required to take corrective action and return performance improvement documentation to the proficiency organization for major errors, a laboratory must investigate all errors.
Two unique QA features are provided in the external proficiency program that cannot be achieved with an internal program. First, an unbiased organization independently checks the laboratory; second, comparison of results with other laboratories in the industry is made easier. Results show that eventually all participating laboratories produce converging, acceptable, and hopefully more accurate, range of results after the initial period when external proficiency program begins and all laboratories have highly diverging results.