Myocardial infarction

Last updated: Friday, 06, August, 2010
Key Information Appropriate Tests

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.

Chemical tests vary in the onset and duration of abnormality and results may be unavailable, or normal, when a decision on thrombolytic therapy must be made.

Serial testing may be necessary in doubtful cases especially when the initial level is within the reference interval.

Tests available include:

Cardiac troponin I or T - very sensitive and specific. 

The test becomes abnormal within 12 hours of commencement of pain, and remains abnormal for about 7 days.  It is the recommended test.

>Creatine kinase (CK) - sensitive but not specific: recommended only in conjunction with CKMB.

CKMB with CKMB/CK ratio - sensitive and specific, but inferior to troponin. It is of value to diagnose reinfarction.

AST, LD - very low specificity; not recommended.

LD isoenzymes - reasonably sensitive and specific, remains abnormal for up to 7 days after onset of pain. It has been superseded by troponin.

The group of 'cardiac enzymes', CK, AST and LD, is unsatisfactory for the reliable diagnosis of myocardial infarction.

Myoglobin - very sensitive, but non-specific. Peaks within 4 hours of the onset of pain. Thus it is of particular value for early exclusion of myocardial infarction.

Serial CKMB or myoglobin can be used to document the response to thrombolytic or angioplastic therapy: rapid elevation indicates successful revascularisation.

Elevation of LD1 indicates a poor outcome is more likely.

Creatinine, urea, electrolytes, glucose; FBC. If shock is present: blood gases. If lipid 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.

See also Atherosclerosis: risk assessment.