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Troponins
Cardiac troponins T (TnT) and I (TnI) occur on the contractile apparatus of the
myocardium and are released when myocardial damage occurs.
Advantages:
- both
are highly sensitive and specific for myocardial damage; CK-MB is sensitive
but not specific
- remain
elevated for longer than CK-MB allowing retrospective diagnosis
- independent
prognosticator of adverse outcome in patients with unstable angina or non-STEMI
- the
higher the rise in troponin the higher the risk of adverse outcome and the
greater the treatment benefit
- useful
for risk stratification
- point-of-care
test readily available
Disadvantages:
- troponin
T gene, although suppressed in healthy adult skeletal muscle, is
re-expressed in diseased (polymyositic, dystrophic muscle) and chronically
stressed muscle. It is also found in the serum of uraemic patients. In these
cases specificity is reduced. Troponin I has greater specificity as it not
expressed in these situations
- may
be negative if measured too early (within 4 h)
- more
expensive than CK-MB
- may be raised in sepsis
Creatine kinase
- CK rises (to twice normal) after 8-24 h falling back to normal by about 48-72
h except in very large infarcts. CK-MB fraction is more specific although with
gel electrophoresis method of measurement (more common method) there are false
positives. Area under CK concentration-time curve good indicator of size of
infarct but peak CK is only a poor indicator.
- other causes of rise in total CK:
- myocarditis*
- skeletal muscle disease: muscular dystrophy, polymyositis, myopathies,
polymyositis, trauma (including IM injection, fits,
surgery)
- cardiac surgery*
- unstable angina (rises to less than twice normal)
- hypothyroidism
- cardiac catheterization
- stroke
- electrical cardioversion*
* = may cause rise in CK-MB
Lactate dehydrogenase
- rises after 24-48 h and remains high for a 7-14 days. Heart isoenzyme
also known as hydroxybutyrate. Not necessary to measure HBD unless there is a
possibility that rise in CK or CK-MB was missed
Other abnormalities
Include:
- non-specific rise in WCC to approx
12-15
- rise in ESR which persists for 1-2 weeks
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