Protocol for Ordering Serial Cardiac Enzymes in Suspected Acute Coronary Syndrome
Cardiac-specific troponin should be measured at presentation and 3-6 hours after symptom onset in all patients with suspected acute coronary syndrome to identify a rising and/or falling pattern. 1, 2
Initial Troponin Testing
- Cardiac-specific troponin (troponin I or T) is the preferred biomarker for diagnosis of acute myocardial infarction (AMI) 1
- Initial troponin should be measured at presentation in all patients with symptoms consistent with ACS 1, 2
- A second troponin measurement should be obtained 3-6 hours after symptom onset 1, 2
- If the time of symptom onset is unclear or ambiguous, the time of presentation should be considered the time of onset for assessing troponin values 1, 2
When to Obtain Additional Troponin Measurements
- Additional troponin levels should be obtained beyond 6 hours after symptom onset in patients with normal troponins on serial examination when: 1
- Electrocardiographic changes are present (such as ST-segment depression or T-wave inversion) 1, 2
- Clinical presentation confers an intermediate or high index of suspicion for ACS 1, 2
- High-risk features are present (including age ≥65 years, ≥3 risk factors for CAD, prior coronary stenosis ≥50%, ST deviation on ECG, ≥2 anginal events in prior 24 hours) 1, 2
Interpretation of Results
- Look for a rising and/or falling pattern of troponin values, which is essential for AMI diagnosis 1
- Absolute changes in high-sensitivity troponin have better diagnostic performance than relative changes for AMI diagnosis 3
- The presence and magnitude of troponin elevations are useful for both short-term and long-term prognosis 1, 2
What NOT to Order
- With contemporary troponin assays, creatine kinase myocardial isoenzyme (CK-MB) and myoglobin are not useful for diagnosis of ACS and should not be ordered 1, 2
- Point-of-care troponin testing should not be used in isolation as a primary test in the prehospital setting 1
Special Considerations
- For patients presenting within 6 hours of symptom onset with initially negative troponin, remeasurement between 6-12 hours after symptom onset is recommended 1
- It may be reasonable to remeasure troponin once on day 3 or day 4 in patients with confirmed MI as an index of infarct size 1, 2
- B-type natriuretic peptide (BNP) may be considered for additional prognostic information but not for primary diagnosis 1, 2
Common Pitfalls to Avoid
- Failing to repeat troponin in patients with high-risk features despite initially negative results 2
- Relying solely on troponin without considering the clinical context and ECG findings 2
- Using outdated biomarkers like CK-MB and myoglobin which add no diagnostic value with contemporary troponin assays 1, 2
- Inadequate timing between serial measurements - studies show that a 3-hour interval may not be sufficient for ruling out AMI in all patients 4
- Discharging patients based on a single negative troponin measurement, especially when obtained early after symptom onset 5
Newer Approaches
- High-sensitivity troponin assays may allow for earlier rule-out/rule-in protocols, with some studies supporting a 0-hour/1-hour algorithm with high negative predictive value (99.1%) 6
- When using high-sensitivity assays, an absolute change in troponin at 3 hours may be superior to relative change for AMI diagnosis 3