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A new method for the determination of creatine kinase-MB activity in the serum is presented. The principle of this method is the direct measurement of the activity of creatine kinase M subunits by inhibiting antibodies. The total test procedure takes 15 min. In the sera of all the 83 patients tested, who have clinically proven myocard infarction, creatine kinase-MB activity can be measured between the 6th and 28th hour after infarction. At the time of maximum total creatine kinase activity the percentage of creatine kinase-MB activity is between 6 and 17%, the mean value being 8%. In cases of emergency this method can be used for the differential diagnosis of elevated total creatine kinase activities of unknown origin.

Creatine kinase MB isoenzyme was measured (using antibody inhibition) in serum of patients with exogenous intoxication, acute pancreatitis, cerebrovascular accidents, meningitis, encephalitis, skeletal muscle disease, shock, postoperative states and after coronary arteriography, cardiac catherisation of cardioversion. CK-MB activity was revealed only in sera of patients with exogenous intoxication (severity III and IV), polymyositis, scleroderma, after operation or after coronary arteriography, cardiac catherisation or cardioversion. As it is not possible to differentiate between CK-MB and CK-BB using inhibiting antibodies against CK-M subunit, CK-isoenzyme activity was determined in parallel, using precipitating antibodies. No CK-BB was found in any case. The determination of CK-MB isoenzyme after blocking of the CK-MB subunit by means of inhibiting antibodies is suitable for clinical diagnosis. The method significantly increases the value of creatine kinase measurement.

The immunological method of determining creatine phosphokinase-MB in the serum of patients with myocardial infarction described here is based on the differential measurements of CK-activities before and after a specific immuno-precipitation of the CK-MB type. The minimum activity of the CK-MB-type which it is possible to determine with this method is 4% of the total activity. In patients with clinically confirmed myocardial infarctions 1-15% (mean 7.8%) of the total activity can be calculated as CK-activity of the MB-type on the first/second day(s) after the infarction. In patients with increased total CK-activity and without verified infarction the CK-MB content does not differ significantly from zero. The differences between the two groups are statistically significant. In patients with myocardial reinfarction the CK-MB-activity is higher than that after the first infarction. The immunological method to determine creatine-phosphokinase isoenzyme MB is of differential-diagnostic value in myocardial infarction.

For the determination of creatine kinase isoenzyme catalytic concentrations in serum two methods based on immunological reactions are presented: One method uses inhibiting antibodies, which selectively block the activity of creatine kinase M subunits ("Inhibition Test"). This test is used for routine measurements of creatine kinase MB catalytic concentration; Another method uses precipitating antibodies, which allows a quantitative differentiation of creatine kinase isoenzymes MM, MB and BB ("Precipitation Test"). This test is used as a control for the Inhibition Test for the possible presence of creatine kinase BB activities in doubtful cases. Procedures, specificity, correlation and application of these methods are discussed.

We have established several mouse hybridoma cell lines which secrete monoclonal antibodies with creatine kinase M subunit inhibiting capacity. Some of these antibodies are useful tools for the determination of the creatine kinase-B subunit activity (CK-MB test). The monoclonal mouse anti-bodies and the conventional polyclonal goat antibodies were compared.

This is a new method for the determination of creatine kinase isoenzyme MB activity in serum. The method uses direct activity measurement of creatine kinase B subunit activity after blocking of CK-M subunit activity by inhibiting antibodies. The test takes no longer than 15 min. The method yields an intra-serial C.V. of 2.0-12.9%, and a C.V. from day to day of 5.5%. The detection limit is 3.4 U/l creatine kinase MB. In the 95 cases with proven myocardial infarction several types of creatine kinase MB activity kinetics could be determined. The percentage of creatine kinase MB of peak CK-total is 6-25%, with a mean of 11.1%. The amount of creatine kinase MB with respect to total CK activity after reinfarction is higher than the amount after initial infarction.