What are the diagnostic criteria for Non-ST-Elevation Myocardial Infarction (NSTEMI)?

Diagnostic Criteria for NSTEMI

NSTEMI is diagnosed when there is evidence of myocardial necrosis demonstrated by elevated cardiac troponin (with a rise and/or fall pattern, at least one value above the 99th percentile upper reference limit) in the appropriate clinical context of acute myocardial ischemia, WITHOUT persistent ST-segment elevation on ECG. 1, 2

Core Diagnostic Requirements

The diagnosis requires ALL of the following components 1:

1. Elevated Cardiac Biomarkers

• Cardiac troponin (preferably high-sensitivity troponin T or I) showing a rise and/or fall pattern with at least one value above the 99th percentile upper reference limit 1
• Serial measurements are essential: obtain troponin at presentation and repeat at 1-3 hours (if high-sensitivity assays) or 6-12 hours (if standard assays) 3, 4
• A dynamic change of ≥20% between serial measurements strengthens the diagnosis and helps distinguish true NSTEMI from chronic troponin elevation 5

2. Clinical Evidence of Myocardial Ischemia

At least ONE of the following must be present 1:

• Symptoms of myocardial ischemia: chest pain, pressure, tightness, or anginal equivalents (dyspnea, epigastric pain, arm pain) 1, 2
• New or presumed new ischemic ECG changes: ST-segment depression (≥0.5 mm), T-wave inversion (≥2 mm), or transient ST-segment changes 1, 3
• Development of pathological Q waves on ECG 1
• Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality in a pattern consistent with ischemic etiology 1
• Identification of intracoronary thrombus by angiography or autopsy 1

3. Absence of Persistent ST-Segment Elevation

• The 12-lead ECG must NOT show persistent ST-segment elevation (>20 minutes), which would classify the event as STEMI instead 1
• ECG may show ST-segment depression, T-wave inversion, nonspecific changes, or even be completely normal 3

Critical Timing and Measurement Details

Troponin Measurement Protocol

• First blood draw on arrival should include troponin, with results available within 60 minutes 3
• Repeat troponin at 1-3 hours if high-sensitivity assays are used 3
• Repeat at 6-12 hours if standard assays are used or if high-sensitivity results are equivocal 3, 4
• At least two samples collected at least 6 hours apart are needed to definitively rule out myocardial necrosis if using standard assays 1

ECG Requirements

• 12-lead ECG must be obtained and interpreted within 10 minutes of first medical contact 3
• If initial ECG is non-diagnostic but clinical suspicion remains high, serial ECGs at 15-30 minute intervals during the first hour 3
• Continuous ST-segment monitoring is valuable for detecting transient ischemic episodes 3

Key Distinctions

NSTEMI vs. Unstable Angina

• NSTEMI: Elevated cardiac biomarkers present 1, 2
• Unstable Angina: No detectable biomarker elevation (based on samples at least 6 hours apart) 1, 2
• Both may have identical clinical presentations and ECG findings; the distinction is made solely by biomarker presence 1

Type 1 vs. Type 2 MI

• Type 1 MI (most common): Atherosclerotic plaque rupture/erosion with intracoronary thrombus causing decreased myocardial blood flow 1
• Type 2 MI: Myocardial oxygen supply-demand mismatch from other causes (hypotension, tachyarrhythmia, anemia, coronary spasm, dissection) 1, 6

Common Pitfalls to Avoid

• A completely normal ECG does NOT exclude NSTEMI: 1-6% of patients with normal ECGs will have MI 3
• Single troponin measurement is insufficient: Must demonstrate rise/fall pattern to distinguish acute event from chronic elevation 5
• Troponin elevation >5-fold the upper reference limit has >90% positive predictive value for Type 1 MI 5
• Elderly, diabetic, and female patients more commonly present with atypical symptoms and non-diagnostic ECGs 3
• Up to 30% of NSTEMI patients have total coronary occlusion on angiography despite lack of ST-elevation 7