What laboratory tests should be ordered for a patient with suspected myocardial infarction (MI)?

Laboratory Tests for Suspected Myocardial Infarction

For patients with suspected myocardial infarction, cardiac troponin (I or T) should be measured at presentation and 3-6 hours after symptom onset as the primary diagnostic test, with additional measurements beyond 6 hours if initial values are normal but clinical suspicion remains high. 1

Primary Laboratory Tests

Cardiac Troponin Testing

• Cardiac-specific troponin (I or T) is the biomarker of choice for diagnosing MI due to its high sensitivity and specificity for myocardial injury 1
• Initial troponin should be measured at presentation, with a second measurement 3-6 hours after symptom onset 1
• Additional troponin measurements should be obtained beyond 6 hours in patients with normal initial values if ECG changes or clinical presentation suggest intermediate or high risk for ACS 1
• A rising and/or falling pattern of troponin with at least one value above the 99th percentile upper reference limit (URL) indicates myocardial injury 1
• High-sensitivity troponin assays allow for more precise quantification of cardiomyocyte injury around the 99th percentile, increasing diagnostic accuracy 2

Troponin Interpretation

• For diagnosis of acute myocardial necrosis, both the peak troponin value and serial changes are important 1:
  • A troponin value above the 99th percentile URL plus evidence of a serial increase or decrease ≥20% (if initial value is elevated) 1
  • For values below or close to the 99th percentile, a change of ≥3 standard deviations indicates acute myocardial necrosis 1
• Troponin values may remain elevated for 7-14 days following MI onset 1

Alternative Biomarkers

• If troponin assays are unavailable, CK-MB (measured by mass assay) is the best alternative 1
• CK-MB should be measured at initial assessment and 6-9 hours later 1
• Gender-specific CK-MB reference values should be used 1
• With contemporary troponin assays available, CK-MB and myoglobin are not useful for diagnosis of ACS 1
• Total CK measurement is not recommended due to lack of specificity 1

Additional Laboratory Tests

The following additional blood tests are recommended in all patients with suspected MI 1:

• Lipid profile including LDL-C
• Complete blood count (including hemoglobin)
• Creatinine with estimation of renal function
• Glycemic status with HbA1c and/or fasting plasma glucose
• Thyroid function (at least once)

Special Considerations

Reinfarction Detection

• For suspected reinfarction, immediate troponin measurement followed by a second sample 3-6 hours later is recommended 1
• Recurrent infarction is diagnosed if there is a ≥20% increase in the second sample value that exceeds the 99th percentile URL 1

Timing Considerations

• If symptom onset time is ambiguous, consider the time of presentation as the time of onset for assessing troponin values 1
• For patients presenting ≥24 hours after symptom onset, a rising/falling pattern is not absolutely required for MI diagnosis 1

High-Sensitivity Troponin Algorithms

• Early rule-out algorithms using high-sensitivity troponin can reduce time required for safe rule-out or rule-in of MI 1, 2
• For patients with chest pain onset ≥3 hours before presentation, very low troponin values (e.g., <5 ng/L for hs-cTnI or <6 ng/L for hs-cTnT) may allow for early rule-out 1

Common Pitfalls

• Elevated troponin is not synonymous with MI - it indicates myocardial injury, which can occur in numerous conditions beyond ischemic heart disease 1, 3
• Conditions that can cause troponin elevation without MI include heart failure, pulmonary embolism, renal failure, myocarditis, and critical illness 1, 3
• Clinicians should be aware of the sensitivity of troponin assays used in their hospitals and the specific cutpoint concentrations for clinical decisions 1
• Failure to obtain serial measurements may lead to missed diagnoses, as a single normal troponin does not exclude MI 1