Precision refers to the ability of an assay to produce the same result for a given sample when repeatedly tested over time (reproducibility). There are three levels of precision as it pertains to routine laboratory testing: (1) within-run precision is the measurement of reproducibility when multiple aliquots of a sample are analyzed in parallel during one assay run; (2) between-run precision is the measurement of reproducibil-ity when multiple aliquots of a sample are analyzed in separate assay runs on the same day; and (3) between-day precision is the measurement of reproducibility when multiple aliquots of a sample are analyzed over a period of more than 1 day. Depending on the anticipated testing volume for the assay, a laboratory might or might not be required to define each type of precision.

Given that a large number of sample replicates will be needed to accurately define assay precision, the laboratory should carefully plan the experiments needed to define this parameter. A large volume of the test material should be prepared and then divided into multiple aliquots, which will then be assayed independently. Precision testing should also include more than one lot or batch of test reagents. For qualitative tests, at least two levels of the analyte should be tested (negative and low positive). For quantitative tests, specimens with analytes in the low, moderate, and high ranges of quantitation should be selected for precision verification as well as a negative sample. For assays that are analytical in nature (tests looking at genetic variants or mutations), the number and types of specimens to use for precision studies are less well defined. At the least, samples

Table 3

Commercial Sources of External Standards/Control Materials


Website and telephone number; 888-746-7921; 800-426-0764; 800-638-6597; 800-676-1881


Advanced Biotechnologies American Type Culture

Collection BBI Diagnostics National Institute of

General Medical Sciences; 800-752-3805

Human Genetic Cell

Repository containing the mutations/variants that are found in the majority of the target population should be used to define precision. Guidelines for experiment design, data collection, and data evaluation can be found in the current edition of NCCLS document EP5 (Evaluation of Precision Performance of Clinical Chemistry Devices) (10).

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