Accuracy of clinical laboratories studied by comparison with definitive methods.

Samples of lyophilized human serum were circulated to participants in the CAP Chemistry Survey program. The participants measured the concentrations in the samples of calcium, iron, lithium, magnesium, potassium, and sodium. Samples were also sent to the National Board of Standards (NBS), and the concentrations of these seven analytes were measured; definitive methods were used. The participant values and the NBS values were compared, providing a measure of the accuracy of the participating laboratories and of the individual methods used by the participants. There was, in general, close agreement between the NBS values and the participant values. A bias that varied with the concentration of the analyte was a common feature.