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Clinical Chemistry

Cardiac-related tests

Troponins (Cardiac-specific Troponin I and Troponin T)


Author: Larry Bernstein, M.D. (see Reviewers page)
Revised: 15 March 2011, last major update March 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.

Definition
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● Marker of acute myocarcial infarction; troponin I is more specific and sensitive than CK-MB (J Am Board Fam Pract 1999;12:214)

Physiology
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● Troponin is a complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle
● Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue
● In relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction
● When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm
● Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction

Clinical use
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● When heart muscle is damaged, as in myocardial infarction (MI), troponin I and troponin T leak out of cells and into the bloodstream; increased troponin levels indicate cardiomyocyte damage
● Troponins are not normally present in serum, so any amount present in serum (measured at the 99th percentile of the upper limit of normal at a 10% imprecision) indicates structural damage to the heart, although not necessarily AMI
● Use of the 99th percentile of the upper limit of normal for diagnosis still requires either typical chest pain or ECG changes of ST depression or T-wave inversion

Test methodology
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● Numerous diagnostic companies make troponin I immunoassays available on different platforms

Test indications
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● Typical or atypical chest pain, or ECG changes suspicious for non Q-wave AMI
● These may be combined with risk factors for AMI

Test limitations
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● Both troponin I (TnI) and troponin T (TnT) are affected by renal insufficiency, but TnT is to a greater extent
● 100% of TnT is excreted in urine, but 70% of TnI is degraded by vascular endothelium; this means that minor elevations of troponins have to be considered in the context of comorbidities, especially renal impairment, and risk factors
● Among heart failure patients, the objective parameter of NT-proBNP seems more useful to delineate the "cardiorenal syndrome" than the previous criteria of a clinical diagnosis of heart failure

Reference ranges
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● TnI: < 0.3 ng/ml
● TnT: < 0.03 ng/ml

High values
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● Suspect AMI for TnI above 0.7 ng/ml or for TnT at 0.07 ng/ml or higher
● Minor elevations above the defined upper limit of normal may be due to impaired renal clearance, or indicate a cardiac structural change that requires further investigation

Additional references
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Wikipedia, BMJ 2004;328:1028, Clin Chim Acta 2004;343:223, Circulation 1998;98:1831
J Am Coll Cardiol 1997;29:43, Eur Heart J 1997;18:762, BMJ 1999;319:1451, N Engl J Med 1997;337:1648
N Engl J Med 1996;335:1333, N Engl J Med 1996;335:1342, J Am Coll Cardiol 2001;38:478, Clinical Chemistry 2004;50:333, Eur Heart J 2000;21:1502

End of Clinical Chemistry > Cardiac-related tests > Troponins (Cardiac-specific Troponin I and Troponin T)


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