Clinical and Interpretive

The serial quantitation of serum creatine kinase MB (CKMB) levels, often performed at admission and 8-hours, 16-hours, and 24-hours after admission, has traditionally been used as an aid in the diagnosis of myocardial injury. While CKMB has been replaced by troponin assays in the workup of many patients with acute chest pain, CKMB may be useful if the initial troponin determination is abnormal or if a hospitalized patient has a suspected reinfarction.

There are 3 isoenzymes of creatine kinase (CK)-CKBB, CKMM, and CKMB. The primary source of CKMB is myocardium although it is also found in skeletal muscle. CKMB levels increase with myocardial damage. Extreme elevations of CKMB can also be associated with skeletal muscle cell turnover as in polymyositis and to a lesser degree in rhabdomyolysis. It can also be elevated in cases of carbon monoxide poisoning, crush injuries, pulmonary embolism, hypothyroidism, and muscular dystrophy.

Creatine kinase MB (CKMB) values, which exceed 50% of the total CK, probably reflect unusual beta-subunit synthesis since the heart muscle rarely exceeds 30% CKMB.

The newer sandwich assays for CKMB have eliminated the confusion seen with older methodologies, but may result in the inability to detect some infrequent contributions to total CK (eg, macro and mitochondrial forms).

In patients receiving treatment with high biotin doses (>5 mg/day) there should be an interval of at least 8 hours between the last biotin administration and the collection of the specimen as biotin interferes with the assay.