Proficiency testing of enzymes. Charting the way toward standardization.

Laboratories participating in the College of American Pathologists Enzyme Survey (ES) and Comprehensive Chemistry Survey used diverse methods for the same analyte, resulting in a considerable range of values for the commonly performed enzyme measurements. Nevertheless, with the techniques developed for the ES, both the short-term (within-mailing) and long-term (between-mailings) coefficients of variation (CVs) can be determined. The ten-year experience of the ES has shown improvement in the short-term CV; however, long-term stability of enzyme testing requires more effort on the part of the instrument and reagent suppliers and participating laboratories. A reference material with an International Federation of Clinical Chemistry-established aspartate aminotransferase value, National Bureau of Standards RM 8430, is now available and was sent to three large peer groups as part of a special study. Correction of the results to the RM 8430 aspartate aminotransferase value resulted in reducing the range of data from peers using the duPont aca but not from those using the American Monitor KDA or Technicon SMAC. Based on our experience with the ES, goals of 5% for the short-term CV and 10% for the long-term CV are proposed; they are achievable by most laboratories and meet medical needs for biochemical screening. Fixed criteria for the evaluation of enzyme results appear to be appropriate given the way most enzyme data are used clinically.