Evaluation of direct and indirect coupled enzyme assay systems for measurement of creatine kinase activity.

Direct assay of creatine kinase by the method of Siegel and Cohen with the Technicon “SMA 12/60” is compared with two indirect coupled-enzyme assay methods. The direct method was linear to greater than 2000 U/liter, whereas the indirect methods were linear to 250 and 1500 U/liter, with use of serial dilutions of a creatine kinase preparation. Increasing the activity of the auxiliary enzymes, hexokinase and glucose-6-phosphate dehydrogenase, in one of the coupled enzyme assay methods increased its linear range from 250 to 700 U/liter. The observed findings are explained on the basis of the fundamental differences between direct and indirect coupledenzyme assay systems and, within the latter system, the effect of the activitiesof auxiliaryenzymes.