The accuracy of calcium analysis in the United States.

The accuracy of the methods commonly used for calcium analysis in clinical laboratories in the United States was studied with the aid of the National Bureau of Standards (NBS). Samples from six large serum pools that had been analyzed by 4,258 clinical laboratories were sent to NBS. At NBS the calcium contents of the samples were determined using isotope dilution and mass spectrometry (ID-MS). The findings show that the bias of the routine methods varies from levels that are insignificant to levels that would have an impact on patient care. The accuracy of the automated methods is comparable to the accuracy of the best manual methods. The accuracy of the proposed calcium reference method is comparable to the accuracy of a number of the methods in current routine use. There is evidence of a widespread lack of standardization at the extremes of the clinical range; this effect is not shown by the results obtained by automated methods. The study underlines the value of developing technology, such as ID-MS, that is capable of providing definitive values. The definitive values can then be used to assess the bias of routine methods on a nationwide scale, using existing programs.