Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1262 page)

   Limitations

   Normal in pregnancy with hyperthyroidism, nontoxic goiter, and in use of certain drugs (e.g., mercurials, iodine)

   In some cases of severe nonthyroid illness, RUR does not fully compensate and does not adjust the T
₄
into the normal range.

TROPONINS, CARDIAC-SPECIFIC TROPONIN I AND TROPONIN T
*

   Definition

   Cardiac troponin T and troponin I, also known as TnI, TnT, cTnI, cTnT, and cTn, are cardiac regulatory proteins specific to the myocardium that control the calcium-mediated interaction between actin and myosin. Troponin I remains increased longer than Ck-MB and is more specific, and cTnI is more sensitive but less specific.

   
Normal range:

   Troponin T: 0.0–0.1 ng/mL

   Troponin I: 0.0–0.04 ng/mL

   Use

   Cardiac troponin is the preferred test for diagnosis of acute coronary syndrome (ACS). cTn establishes the diagnosis of irreversible myocardial necrosis (e.g., anoxia, contusion, inflammation), even when ECG changes or CK-MB are nondiagnostic (which occurs in ≤50% of patients with ACS). It is important to note that several distinct pathobiologic may cause elevated troponin, not all of which involve myocyte necrosis.

   The diagnosis of myocardial infarction is predicated upon the rise and fall of cardiac troponin along with other clinical factors (see Chapter
3
). However, serial normal cTn rules out myocardial necrosis.

   In patients with a clinical syndrome consistent with ACS, a peak concentration exceeding the 99th percentile of values for a reference control group should be considered indicative of increased mortality, myocardial infarcts, and recurrent ischemic events.

   Patients with ACS patients and cTnI and cTnT results above the decision limit should be labeled as having myocardial injury and a high-risk profile.