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 inju ry, but unlike myoglobin it is not found in cardiac muscle. This study compared the clinical utility of serum myoglobin (S-Mgb) in conjuncti on with the ratio of S-Mgb to CA-III (S-Mgb/CA-III) versus creatine ki nase-MB (CK-MB) for the early diagnosis of acute myocardial infarction (AMI). Methods: This prospective observational study set at a univers ity teaching hospital emergency department enrolled 251 consecutive co nsenting 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 physi cal examination data were recorded, as were the results of serial bloo d sampling for S-Mgb, CA-III, and CK-MB at 0, 1, and 3 hours after pat ient 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 we re analyzed with McNemar's chi(2) test for symmetry and confidence int ervals (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 A MI in patients presenting within 3 hours of symptom onset yielded resp ective sensitivities of 47.8% versus 17.4% (P=.02); specificities of 9 8.9% 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 pred ictive 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 wa s significantly more sensitive than CK-MB yet equally as specific for the early diagnosis of patients with AMI.