An InterlaboratoryStudyof Creatine Kinaseand Creatine Kinase Isoenzymes

Over 400 laboratories participated in an interlaboratory study of creatine kinase (EC 2.7.3.2) in which theywere requested to analyze five lyophilized samples for total creatine kinase and creatine kinase isoenzymes and to complete a questionnaire regarding methodology. The specimens were prepared from an homogenate of human myocardium that had been centrifuged, dialyzed, and added to a matrix containing bovine serum albumin, buffer, and N-acetylcysteine. Stability studies indicated a half-life for the samples of two to seven years at 37 #{176}C. The among-vial coefficient of variation was 1.4% or less. For totalcreatine kinase most laboratories used an NADP+ reduction method monitored at 340 nm (86%) and reported results in U/L (87%) at either 30 #{176}C (42%) or 37 #{176}C (48%). The average of the 37 #{176}C results reported for each sample was within 10% of the results obtained by the Center for Disease Control with the Scandinavian Society’s Recommended Method (Scand. J. C/in. Lab. Invest. 36: 711, 1976), adapted to a centrifugal analyzer. To separate the creatine kinase isoenzymes, 280 laboratories used 26 different methods or kits, including electrophoresis on cellulose acetate (45%) and agarose (39%), ion-exchange chromatography (13%), and selective activation (3%). Twenty-nine different methods or kits were used to quantitate the isoenzymes, with about 21 % of the laboratories using visual examination (ultraviolet irradiation). Quantitative values reported for the MB isoenzyme ranged from within normal limits to 19-fold greater for each sample. A few participants (14%) reported BB isoenzyme in every sample, although most laboratories indicated BB was absent.