Which of the following blood tests is most indicative of cardiac damage?
A. Lactate dehydrogenase
B. Complete blood count
C. Troponin I
D. Creatine kinase
Correct Answer: C. Troponin I
Troponin I levels rise rapidly and are detectable within 1 hour of myocardial injury. Troponin I levels aren’t detectable in people without cardiac injury. The troponin complex consists of three subunits—troponin C, troponin I, and troponin T—and is located on the myofibrillar thin (actin) filament of striated (skeletal and cardiac) muscle. The cardiac isoforms troponin T and I are only expressed in cardiac muscle. Hence, cardiac troponin T (cTnT) and I (cTnI) are more specific than creatine kinase (CK) values for myocardial injury and, because of their high sensitivity, they may even be elevated when CK?MB concentrations are not.
Option A: Lactate dehydrogenase is present in almost all body tissues and not specific to the heart muscle. LDH isoenzymes are useful in diagnosing the cardiac injury. The function of the enzyme is to catalyze the reversible conversion of lactate to pyruvate with the reduction of NAD+ to NADH and vice versa.
Option B: CBC is obtained to review blood counts, and complete chemistry is obtained to review electrolytes. Obtain a complete blood cell (CBC) count if myocardial infarction (MI) is suspected in order to rule out anemia as a cause of decreased oxygen supply and prior to giving thrombolytic agents.
Option D: Because CK levels may rise with a skeletal muscle injury, CK isoenzymes are required to detect cardiac injury. In their current guidelines from 2000, the Joint European Society of Cardiology/American College of Cardiology committee redefined AMI as an elevation of cTn in blood above the 99th centile of a healthy reference population in conjunction with signs or symptoms of ischemia. This did expand the diagnostic capacity to detect micro?MI which was not evident by CK?MB measurements.