Cardiac Troponin I for Diagnosis of Acute Myocardial Infarction*

Serial plasma concentrations of myoglobin, creatine kinase MB (CK-MB) isoenzyme, and cardiac troponin I (cTnl) were measured in 25 patients with a confirmed diagnosis of acute myocardial infarction (AMI), and 74 patients who were suspected of AMI but were subsequently ruled out for this diagnosis. The cutoff concentration for the cTnl assay was optimally determined to be 2.5 ng/mL. Of the three markers, myoglobin had the highest clinical sensitivity (50 percent) when blood was collected between 0 to 6 h after the onset of chest pain. Assays for all serum markers used had high clinical sensitivity (>93 percent) 6 to 24 h after onset. The CK-MB remained highly sensitive for 48 h, while cTnl was sensitive for up to 72 h. Between 72 and 150 h, cTnl had a clinical sensitivity of 70 percent as compared to 21 percent and 18 percent for myoglobin and CK-MB, respec­ tively. The clinical specificity of cTnl for non-AM I patients was equivalent to CK-MB and significantly higher than for myoglobin. The clinical effi­ ciency of cTnl for all samples was better than either CK-MB or myoglobin, owing mainly to the wider diagnostic window. The specificity of cTnl for 59 patients with chronic renal failure, skeletal muscle trauma and disease was better than all of these markers including cardiac troponin T (cTnT). Results of this study show that cTnl is an effective marker for the retro­ spective diagnosis of AMI, and consideration should be given to its use in place of CK-MB. * Send reprint requests to: Alan H. B. Wu, Ph.D., Department of Pathology and Laboratory Medicine, Hartford Hospital, 80 Seymour Street, Hartford, CT 06102.