Improved specificity of myoglobin plus carbonic anhydrase assay versus that of creatine Kinase-MB for early diagnosis of acute myocardial infarction

Study objective: Carbonic anhydrase III (CA-III) is an enzyme released from skeletal muscle in a fixed ratio with myoglobin during cell injury, but unlike myoglobin it is not found in cardiac muscle. This study compared the clinical utility of serum myoglobin (S-Mgb) in conjunction with the ratio of S-Mgb to CA-III (S-Mgb/CA-III) versus creatine kinase-MB (CK-MB) for the early diagnosis of acute myocardial infarction (AMI). Methods: This prospective observational study set at a university teaching hospital emergency department enrolled 251 consecutive consenting patients who presented with symptoms consistent with cardiac ischemia or infarction of less than 12 hours' duration. Patients with trauma or kidney failure were excluded. Standardized history and physical examination data were recorded, as were the results of serial blood sampling for S-Mgb, CA-III, and CK-MB at 0, 1, and 3 hours after patient presentation. A positive test for the study assays was defined as an S-Mgb concentration of more than 110 ng/mL with an S-Mgb/CA-III of 3.21 or higher by receiver operating characteristic analysis. Data were analyzed with McNemar's chi(2) test for symmetry and confidence intervals (Cls), using the exact method. Results: Thirty (12%) of the 251 patients were found to have AMI by World Health Organization criteria. Mean time from symptom onset to presentation was 3.2 hours. The use of S-Mgb plus S-Mgb/CA-III compared with CK-MB for identification of AMI in patients presenting within 3 hours of symptom onset yielded respective sensitivities of 47.8% versus 17.4% (P=.02); specificities of 98.9% versus 100% (P=NS); positive predictive values of 84.6% (95% Cl, 54.6% to 98.1%) versus 100% (95% CI, 39.8% to 100%); and negative predictive values of 93.5% (95% CI, 90.0% to 96.6%) versus 90.0% (95% Cl, 84.8% to 93.9%). Conclusion: S-Mgb in conjunction with S-Mgb/CA-III was significantly more sensitive than CK-MB yet equally as specific for the early diagnosis of patients with AMI.