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

NSTEMI Diagnostic Criteria

NSTEMI is diagnosed when cardiac troponin rises and/or falls with at least one value above the 99th percentile upper reference limit, combined with clinical evidence of myocardial ischemia, in the absence of persistent ST-segment elevation on ECG. 1

Essential Diagnostic Components

The diagnosis requires all three of the following elements to be present simultaneously 1:

• Elevated cardiac biomarkers: High-sensitivity cardiac troponin (hs-cTn) T or I with at least one value above the 99th percentile of the upper reference limit 1
• Clinical evidence of myocardial ischemia: At least one of the following 1:
  • Symptoms of myocardial ischemia (chest pain, pressure, tightness, or pain-equivalent symptoms like dyspnea or epigastric pain) 1, 2
  • New ischemic ECG changes 1
  • Development of pathological Q waves on ECG 1
  • Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality consistent with ischemic etiology 1
  • Intracoronary thrombus detected on angiography or autopsy 1
• Absence of persistent ST-segment elevation: No ST-elevation >20 minutes on 12-lead ECG 1

Biomarker Requirements

Serial troponin measurements are mandatory when initial values are not diagnostic 1:

• Use high-sensitivity cardiac troponin (hs-cTn) as the preferred biomarker 1
• Apply validated algorithms: ESC 0h/1h or 0h/2h protocols with blood sampling at presentation and 1-2 hours later 1
• At least two samples collected at least 6 hours apart are required to definitively rule out myocardial necrosis if using conventional assays 1
• The rise and/or fall pattern of troponin is critical—static elevation alone may indicate chronic myocardial injury rather than acute MI 1

ECG Findings in NSTEMI

While persistent ST-elevation is absent by definition, NSTEMI commonly presents with the following ECG abnormalities 3:

• ST-segment depression ≥0.5 mm (0.05 mV), particularly in multiple leads—this is the hallmark finding and correlates with increased mortality 3
• T-wave inversion ≥2 mm (0.2 mV), especially when symmetrical and deep in precordial leads, suggesting critical LAD stenosis 3
• Transient ST-segment elevation that resolves 1, 3
• Pathological Q waves indicating prior MI 3
• Normal ECG does not exclude NSTEMI—1-6% of patients with normal ECG and chest pain will have MI 3

Critical Distinction: Type 1 vs Type 2 NSTEMI

Type 1 NSTEMI (atherothrombotic) 1, 4:

• Results from atherosclerotic plaque rupture, ulceration, fissure, or erosion with intraluminal thrombus 1
• Decreased myocardial blood flow and/or distal embolization causing myocardial necrosis 1

Type 2 NSTEMI (supply-demand mismatch) 1, 4:

• Myocardial necrosis from conditions other than coronary plaque instability 1
• Requires identification of precipitating condition: hypotension, hypertension, tachyarrhythmias, bradyarrhythmias, anemia, hypoxemia, coronary spasm, spontaneous coronary artery dissection, coronary embolism, or coronary microvascular dysfunction 1, 4
• All three components must be present: elevated troponin, clinical evidence of ischemia, AND absence of acute coronary atherothrombosis 4

Differentiation from Unstable Angina

The sole distinguishing feature between unstable angina and NSTEMI is the presence of detectable cardiac biomarkers 1, 2:

• Unstable angina: No biomarker elevation (troponin remains below 99th percentile), usually transient ECG changes if any 1
• NSTEMI: Elevated biomarkers indicating myocardial necrosis 1
• With increasing sensitivity of troponin assays, biomarker-negative ACS (unstable angina) is becoming increasingly rare 1

Timing and Serial Assessment

Obtain 12-lead ECG within 10 minutes of presentation 1, 3:

• If initial ECG is non-diagnostic but clinical suspicion remains high, repeat ECGs at 15-30 minute intervals during the first hour 3
• Serial ECGs significantly improve diagnostic accuracy, especially when symptoms recur 3
• Continuous rhythm monitoring is recommended until NSTEMI is established or ruled out 1

Common Pitfalls to Avoid

Do not rely on ECG alone—up to 25% of NSTEMI patients may have completely normal initial ECG 3:

• Elderly patients, diabetics, and women more commonly present with atypical symptoms and non-diagnostic ECGs 3
• Left circumflex occlusion can present with non-diagnostic 12-lead ECG 3
• Posterior MI may masquerade as NSTEMI with ST-depression in anterior leads (V1-V3) 3

Consider alternative causes of troponin elevation 1:

• Chronic kidney disease, heart failure, myocarditis, Takotsubo cardiomyopathy, pulmonary embolism 3
• The clinical context is paramount—elevated troponin in isolation is insufficient for ACS diagnosis 1

Do not delay serial troponin measurement—more than 50% of patients with infarction have initially negative troponin values upon presentation 5:

• High-sensitivity assays detect Type 2 MI twice as frequently as standard assays 4
• Initial troponin measurement at presentation and repeat using validated algorithm (0h/1h or 0h/2h) is essential 1