Six-hour versus 12-hour protocols for AMI: CK-MB in conjunction with myoglobin

The objective was to test the hypothesis that a protocol using myoglobin and creatine kinase-MB (CK-MB) can rapidly and safely exclude myocardial infarction (MI). The study used a prospective, convenience cohort of ED patients with clinically suspected myocardial ischemia. Myoglobin was measured on presentation, 2 and 6 hours later; CK-MB was measured on presentation, 6, 12, and 18 hours later. Of 519 patients, 76 (15%) had MIs, all of whom "ruled in" within 12 hours using a combination of myoglobin and CK-MB, for a sensitivity of 100% (95% CI, 95% to 100%), specificity of 92% (95% CI, 89% to 94%), LR (+) of 12 (95% CI, 9 to 16), and an LR (-) of 0.03 (95% CI, 0.0 to 0.05). Of the 76 patients with MIs, 73 ruled in with a 6 hour protocol, also using a combination of CK-MB and myoglobin, for a sensitivity of 96% (95% CI, 89% to 99%), specificity of 92% (95% CI, 89% to 94%), LR (+) of 11 (95% CI, 8 to 16), and an LR (-) of 0.04 (95% CI, 0.01 to 0.12). Our results support the hypothesis that, using an abbreviated protocol with CK-MB and myoglobin, MI can be reliably ruled out in ED patients with suspected ischemia.