What labs should be ordered to rule out a heart attack in a patient with potential cardiovascular risk factors and a history of cardiovascular disease?

Laboratory Tests to Rule Out Heart Attack

In patients with suspected acute myocardial infarction, immediately obtain cardiac troponin (preferably high-sensitivity or ultrasensitive assay), 12-lead ECG, complete blood count, electrolytes, creatinine with renal function estimation, lipid profile including LDL-C, and glycemic status (HbA1c and/or fasting glucose). 1, 2

Essential Immediate Tests

Cardiac Troponin (Priority Test)

• Cardiac troponin T or I using high-sensitivity or ultrasensitive assays is the preferred biomarker and must be obtained immediately to rule out acute myocardial injury 1, 2
• If the initial troponin is normal and symptoms began within 6 hours, repeat troponin measurement 6-12 hours after symptom onset (not after presentation) 1
• Troponin is more sensitive than CK-MB and is the gold standard for diagnosis 1
• Do not delay reperfusion therapy in STEMI patients while awaiting troponin results 1

12-Lead Electrocardiogram

• Obtain a 12-lead ECG immediately (ideally within 10 minutes of presentation) to differentiate ST-elevation MI from non-ST-elevation presentations 1, 2
• Repeat ECG if patient experiences recurrent chest pain 1
• Multi-lead continuous ST-segment monitoring should be initiated where available 1

Complete Blood Count

• Measure hemoglobin to detect anemia, which can cause or exacerbate myocardial ischemia 1, 2
• Anemia is both a potential cause of ischemia and a prognostic factor 1

Renal Function

• Obtain creatinine with estimated glomerular filtration rate (eGFR) as renal dysfunction increases the likelihood of coronary artery disease and negatively impacts prognosis 1, 2
• GFR also affects dosing of renally cleared medications 1

Electrolytes

• Measure electrolytes to identify metabolic derangements that may affect cardiac function and treatment decisions 2

Lipid Profile

• Obtain lipid profile including LDL-C for cardiovascular risk stratification and to guide long-term management 1, 2

Glycemic Status

• Assess glycemic status with HbA1c and/or fasting plasma glucose 1, 2
• If both are inconclusive, an oral glucose tolerance test is useful 1
• Diabetes is a major risk factor and affects prognosis 1

Additional Tests for Specific Scenarios

Early Markers (Symptoms <6 Hours)

• Consider myoglobin and/or CK-MB mass in patients with very recent symptoms (<6 hours) as early markers of myocardial infarction 1
• These are also useful for detecting re-infarction in patients with recent (<2 weeks) MI, when troponin may remain elevated 1

Thyroid Function

• Assess thyroid function at least once in patients with suspected chronic coronary syndrome, as hyperthyroidism can cause or exacerbate ischemia 1, 2

Inflammatory Markers

• Consider high-sensitivity C-reactive protein (hs-CRP) and/or fibrinogen for additional prognostic information 1, 2

Critical Timing Considerations

The key limitation of cardiac biomarkers is their insensitivity during the first 4-6 hours after symptom onset unless continuous pain has been present for 6-8 hours 1. This is why:

• Troponin should be measured on admission AND repeated 6-12 hours after symptom onset if initially negative 1
• Biomarkers are not useful in the prehospital setting due to this timing issue 1
• In STEMI patients, do not delay reperfusion therapy pending biomarker results 1

Common Pitfalls to Avoid

• Do not rely on a single troponin measurement if symptoms began within 6 hours - serial measurements are essential 1
• Do not use troponin testing in the outpatient setting without concurrent ECG and appropriate clinical context - this can lead to inappropriate management 3
• Elevated troponin indicates myocardial injury but requires clinical correlation with symptoms and ECG findings for MI diagnosis 1
• Troponin may remain elevated for 1-2 weeks after large infarction, complicating detection of re-infarction - use CK-MB or myoglobin instead 1

Risk Stratification Based on Results

• Patients with elevated troponin have significantly worse short- and long-term outcomes and specifically benefit from aggressive treatment including low-molecular-weight heparin and GP IIb/IIIa blockers 1
• The degree of troponin elevation correlates with risk of death 1
• Patients with positive cardiac markers, new ST-segment deviations, ongoing chest pain, or hemodynamic abnormalities require hospital admission and aggressive management 2