Serum Creatine Phosphokinase: Evaluation of a Commercial Spectrophotometric Method

Abstract

Abstract The clinical and laboratory applications of an assay method for serum creatine phosphokinase (CPK) utilizing a reagent capsule were investigated. Continuous recording of the change in absorbance at 340 mµ, during the assay period gave a slightly sigmoid curve. The time interval between serum addition and the beginning of the linear portion of the absorbance curve varied with the level of enzyme activity. With samples of low activity, up to 8 min. elapsed before the linear phase began. The rate of change in absorbance increased in a linear fashion with respect to increasing CPK concentration, up to a level of about 370 U. When serum samples with activity above this level were diluted and reassayed, and the data appropriately corrected for dilution, significantly increased activity could be demonstrated in the diluted samples. Adenylate kinase from hemolyzed erythrocytes may interfere with measurement of CPK activity in this assay system. This does not present a problem if samples with visible hemolysis are excluded. The range of normal human serum CPK values recommended for this method is 0-40 U./L. at 30°. Abnormal values in patients with injuries or diseases of skeletal muscle were as high as 55,000 U. After acute myocardial infarction, elevated serum CPK levels were detected within 4 hr. after the onset of chest pain. In some of these patients CPK activity did not return to normal for a week or more. Properly used, this method provides a reliable and convenient assay procedure for serum CPK activity.