Level 1: Likely reliable evidence
Cardiac troponin is an important marker of cardiac injury and necrosis that has become increasingly useful in the diagnosis of acute MI. While older-generation troponin assays could require serial measurements over several hours to provide informative results, newer high-sensitivity troponin tests have shown promise in reducing the time to diagnosis. A recent cohort study derived and validated a clinical algorithm for early diagnosis of acute MI using a high-sensitivity cardiac troponin T (hs-cTnT) assay in the emergency department (Arch Intern Med. 2012;172:1211-1218).
A total of 872 patients (median age 64 years) presenting with chest pain were randomly divided into derivation and validation cohorts (436 patients each). Patients with ST-segment elevation myocardial infarction or terminal kidney failure requiring dialysis were excluded prior to randomization. All patients had hs-cTnT assays at presentation and at one hour. Patients also had complete clinical assessment including history, physical examination, 12-lead electrocardiography, continuous electrocardiography monitoring, pulse oximetry, standard blood tests, and chest radiography and additional hs-cTnT assays at two, three, and six hours after presentation. The reference standard for acute MI diagnosis was adjudication by two independent cardiologists using all available medical records to 60-day follow-up.
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A two-part clinical algorithm was developed in the derivation cohort to diagnose or rule out acute MI using the baseline and one-hour troponin tests. Acute MI is ruled out if the baseline hs-cTnT level is <12 ng/L AND the absolute change in hs-cTnT level within one hour is <3 ng/L. Acute MI is diagnosed if the baseline hs-cTnT level is ≥52 ng/L or the absolute change in hs-cTnT level within one hour is ≥5 ng/L. Patients meeting neither set of criteria fall into the “observation zone,” requiring continued observation and additional testing.
The incidence of acute MI in the validation cohort was 17%. The algorithm classified 77% of the group: acute MI was diagnosed in 17% and ruled out in 60%. The remaining 101 patients fell into the observation zone (eight of these patients received a final diagnosis of acute MI). For diagnosing acute MI, the algorithm had 97% specificity and 84% positive predictive value. For ruling out MI, it had 100% sensitivity and 100% negative predictive value. In overall analysis of both cohorts, 30-day survival was 99.8% in patients with MI ruled out, 98.6% in patients in the observation zone, and 95.3% in patients with MI ruled in(P<0.001 for trend).
Alan Ehrlich, MD, is a deputy editor for DynaMed, in Ipswich, Mass., and assistant clinical professor in Family Medicine at the University of Massachusetts Medical School in Worcester.
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