Expressing total analytic variance as the sum of the squares of imprecision and inaccuracy, or bias, and applying the Cotlove rule recommended by the 1976 College of American Pathologists Conference on Analytical Goals in Clinical Chemistry, namely, that analytic variance should be less than one fourth of the appropriate biological variance, I derive a rule for maximum allowable imprecision in the context of single-point diagnostic testing that takes into account the bias of the test procedure. This rule may be expressed in terms of a population-based reference range (in particular, the range of test results shown in a group of healthy individuals) and the bias of the test method. The latter is required not to exceed one eighth (0.125) of the reference range. These concepts are applied to eight common analytes for which estimates of the biases of specific methods and of within-laboratory imprecision have been published for large numbers of laboratories participating in recent College of American Pathologists proficiency surveys. Results indicate that some methods widely used in 1978 fail to meet the minimum accuracy criterion, while others show negligible bias. Even neglecting bias, more recent data show that average within-laboratory imprecision is still too high for sodium, chloride, and calcium but acceptable for potassium, glucose, cholesterol, urea, and uric acid.