What is the best test to diagnose acute myocardial infarction (MI)?

Best Test for Diagnosing Acute Myocardial Infarction

The measurement of cardiac troponin (cTn) levels is the best test for diagnosing acute myocardial infarction (AMI), with serial measurements showing a rise and/or fall pattern being essential for accurate diagnosis. 1

Diagnostic Criteria for AMI

The diagnosis of AMI requires:

1. Detection of a rise and/or fall of cardiac troponin values with at least one value above the 99th percentile upper reference limit (URL)
2. At least one of the following:
   • Symptoms of ischemia
   • New significant ST-segment-T wave changes or new LBBB on ECG
   • Development of pathological Q waves on ECG
   • Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality
   • Identification of intracoronary thrombus by angiography or autopsy 1

Optimal Troponin Testing Strategy

• Initial measurement: Obtain cardiac troponin at presentation
• Follow-up measurement: Repeat at 3-6 hours after symptom onset
• Additional measurements: Obtain beyond 6 hours in patients with initially normal troponin but with ECG changes or high clinical suspicion 1

Advantages of High-Sensitivity Troponin Assays

High-sensitivity cardiac troponin (hs-cTn) assays provide:

• Earlier detection of myocardial injury (can detect elevations as early as 2-4 hours after symptom onset)
• Greater diagnostic accuracy compared to conventional assays (AUC 0.96 vs 0.90)
• Particularly valuable in patients presenting within 3 hours of chest pain onset 2
• Ability to detect very small amounts of myocardial damage 1

Interpreting Troponin Results

For diagnosis of AMI:

• An elevated value above the 99th percentile URL is required
• Evidence of a serial increase or decrease ≥20% is required if the initial value is elevated
• For values near or below the 99th percentile, a change of ≥3 standard deviations indicates acute myocardial necrosis 1, 3

Important Caveats and Pitfalls

1. Troponin elevation is not specific to AMI: Elevated troponin can occur in many non-ACS conditions including:

   • Heart failure
   • Renal failure
   • Myocarditis
   • Pulmonary embolism
   • Sepsis
   • Tachyarrhythmias 1, 3

2. Clinical context is crucial: Troponin elevation must be interpreted within the appropriate clinical context of myocardial ischemia 1

3. Chronic elevations: Patients with renal failure or heart failure may have chronically elevated troponin levels, requiring careful interpretation of results 3

4. Outdated markers: With contemporary troponin assays, CK-MB and myoglobin are no longer useful for diagnosis of ACS 1

Algorithm for AMI Diagnosis

1. Measure cardiac-specific troponin (I or T) at presentation
2. Repeat measurement 3-6 hours after symptom onset
3. If both values are below the 99th percentile URL and no rise/fall pattern is observed:
   • If >6 hours since symptom onset and no ECG changes: AMI unlikely
   • If <6 hours since symptom onset or concerning ECG changes: obtain additional troponin measurement
4. If any value exceeds the 99th percentile URL and shows a rise/fall pattern:
   • With clinical evidence of ischemia: Diagnose AMI
   • Without clinical evidence of ischemia: Diagnose myocardial injury 1

In summary, serial cardiac troponin measurements showing a characteristic rise and/or fall pattern, with at least one value above the 99th percentile URL, in the appropriate clinical context of myocardial ischemia, is the best test for diagnosing acute myocardial infarction.