Establishment of a cut-off value for the diagnosis of acute myocardial infarction using a new CK-MB activity measurement reagent containing anti-MtCK antibody

CK-MB is an important marker for the diagnosis of acute myocardial infarction (AMI). Since mitochondrial CK (MtCK) is universally present in the blood of healthy individuals, it is known to positively affect the measurement of CK-MB using the immunoinhibition method, causing false-positive results. We performed basic evaluation of ACCURAS AUTO CK-MB MtO, a new reagent containing anti-MtCK antibody that inhibits MtCK activity, and attempted to calculate a cut-off CK-MB level to diagnose AMI. The measurement was performed in samples submitted to the Clinical Laboratory of our center for the measurement of CK-MB. This method was confirmed to have satisfactory basic attributes concerning the reproducibility, linearity, lower detection limit, and effects of interfering substances. When 2886 samples were examined using this and conventional methods, the results of the two methods were correlated in some but not in others. In the samples that showed no correlation, MtCK was demonstrated by isozyme analysis using electrophoresis. The AUC calculated from the ROC curve in AMI patients was 0.912 with this method and 0.861 with the conventional method. The sensitivity and specificity of the new method were higher than those of the conventional method. The cut-off value determined by ROC analysis was 7.7 U/l using the new method and 13.6 U/l using the conventional method, causing an increase in false-positive results compared with the cut off value of 25 U/l widely used for the conventional method to date. However, the cut-off value for the new method that yielded a specificity comparable to 99.1%, which is the specificity of the conventional method using a cut-off value of 25 U/l, was 12 U/l. With a cut-off value of 12 U/l, the sensitivity was improved compared with that employing the conventional method, and both the sensitivity and specificity became comparable to those of the CK-MB mass method. This method is very useful for the accurate measurement of CK-MB activity.