Immunoprecipitation method for CK-MB analysis re-evaluated: influence of CK-BB and macro-CK on blank activities

We evaluated the analytical performance of the immunoprecipitation technique for quantification of creatine kinase (CK; EC 2.7.3.2) isoenzyme MB, focusing on specimens with increased blank activities. The samples we studied had blank values that were equal to or greater than either 15 U/L or the uncorrected CK-MB value. Of 134 specimens selected, 16 and 21 contained macro CK type 1 and CK-BB, respectively. All samples containing macro CK type 1 gave negative CK-MB values, even though four contained significant quantities of CK-MB. Correcting the blank value for the fluid displacement of the second antibody suspension eliminated all but three of the negative results. However, 45 specimens, negative for CK-MB by electrophoresis, gave CK-MB values ranging from 1 to 21 U/L. Furthermore, 11 of the specimens containing macro-CK type 1 were in this group. Thus, the presence of macro-CK type 1 definitely interferes with the immunoprecipitation technique. Our results indicate the need to re-evaluate the diagnostic accuracy of the immunoprecipitation technique, especially when the un-adjusted blank activities are increased.