Hospital admission should not be deferred, pending investigation.
Laboratory investigations complement clinical and electrocardiographic assessment, especially if the latter is normal or shows a non-specific abnormality or left bundle branch block.
Laboratory tests for myocardial infarction (MI) vary in the onset and duration of abnormality and results may be unavailable, or normal, when a decision on thrombolytic therapy must be made.
Tests available include:
Cardiac Troponin I or Troponin T - which are both very sensitive and specific and are the recommended laboratory tests for the diagnosis of MI.
Serial testing is recommended in order to confirm or exclude a rise or fall in troponin concentration.
Troponin is the recommended test for myocardial necrosis, however there is a marked variance in the diagnostic thresholds among different assays. Care should be taken to ensure decisions are made using method specific thresholds. The test becomes abnormal within 2-12 hours of commencement of pain depending on the sensitivity of the troponin assay, and remains abnormal for 7-10 days.
The diagnosis of MI is based on a rise or fall of troponin with at least one value above the 99th percentile for the population, in the setting of clinical or ECG findings consistent with an MI.
Creatinine, Urea, Electrolytes, Glucose; Full blood count. If shock is present: Blood gas arterial. If Lipids studies are required, blood should be collected within 24 hours, or the studies should be deferred for 8 weeks as cholesterol levels decrease after myocardial infarction. Alternative cardiac marker:
CKMB with CKMB/CK ratio - sensitive and specific, but inferior to troponin and to be used only if troponin is unavailable. It may be of value to diagnose reinfarction.
Myoglobin - sensitive, but non-specific. Peaks within 4 hours of the onset of pain. Not recommended due to lack of specificity for cardiac damage.
Creatine kinase (CK) - sensitive but not cardiac specific; only recommended in conjunction with CKMB.
Aspartate aminotransferase, Lactate dehydrogenase - very low specificity; not recommended.
Lactate dehydrogenase isoenzymes - reasonably sensitive and specific, remains abnormal for up to 7 days after onset of pain; not recommended.
The group of 'cardiac enzymes', Creatine kinase, Aspartate aminotransferase and Lactate dehydrogenase, is unsatisfactory for the reliable diagnosis of myocardial infarction; not recommended.
See Cardiac risk assessment.
See also Chest pain suspected ischaemic heart disease in the Guideline on Pathology testing in the Emergency department: Appendix 2.