Creatine Kinase MB Isoenzyme and Myocardial Infarction

The proximity of patient, clinician and analyst improves the consultational opportunity for the care of patients suffering from myocardial infarctions. Very early diagnosis of myocardial infarction could be ascertained by bedside measurement of creatine kinase. The simplicity of the test procedure and the immediate availability of results make a significant contribution to the clinical diagnosis. There are many commercial kits for the bedside determination/measurement of creatine kinase (CK) and its MB coenzyme (CK-MB). Upto 40% of the creatine kinase activity of the heart muscle is CK-MB, and no other tissue contains high proportion. Marked increase in the plasma CK activity may only result in heart or skeletal muscle disease, but the plasma CK-MB increase is generally associated with heart muscle damage. The M-subunit of isoenzyme could be a derivative of MM isoenzyme of skeletal muscles or heart muscles and B-subunit could be a derivative of MB or BB forms of isoenzyme. It is well established that following myocardial infarction it may take several hours for CK and CK-MB activity to become normal. A sample of the patient should be analyzed on admission and two further samples should be obtained and analyzed at 6 and 12 hours. Smaller increases can be seen in dermatomyositis and progressive muscular atrophy but normal results are obtained in conditions such as poliomyelitis as the muscular abnormality is secondary to nerve lesions in this disease.

The test procedure uses dry-chemistry reagent slides for analytical measurement. The reagent preparation or blood sample preparation is not required. The color change is measured on a microprocessor-controlled instrument. The CK and CK-MB can be measured separately using specially prepared slides. For measuring CK activity (derivative of B-subunit of isoenzyme) M-subunit is neutralized by antibodies against M-subunit. Adenosine tri-phosphate (ATP) is formed by the action of CK on creatine phosphate and adenosine di-phosphate (ADP). Alpha-glycerophosphate is formed in the presence of glycerol and glycerol kinase. Further by the action of a-glycerophosphate oxidase, hydrogen peroxide is produced from the a-glycerophosphate. Hydrogen peroxide reacts with the peroxidase and oxidizes the colored dye, which is measured spectrophotometrically. Patients admitted within 12 hours of onset of severe chest pain, show elevated levels of CK-MB activity with 98% accuracy as compared to normal or non-infarction control subjects. As there is none or very little activity of CK in red blood cells (RBCs), so RBCs or hemolysis of blood sample does not interfere with the determination of CK-MB. The determination of CK-MB activity has been considered more sensitive investigation than aspartate transaminase or lactate dehydrogenase in myocardial infarction.