Cardiac troponin is unique to the heart muscle and is highly concentrated in cardiomyocytes. These isoforms show a high degree of cardiac specificity. This protein is released with very small areas of myocardial damage as early as 1–3 hours after injury, and levels return to normal within 5–7 days. Troponin I (binds to myofilaments of the troponin complex and has an inhibitory character) remains increased longer than creatinine kinase-MB (CK-MB) and is more cardiac specific. Troponin T (binds to tropomyosin of the troponin complex) is more sensitive but less specific, being positive with angina at rest. These tests are becoming the most important addition to the clinical assessment of cardiac injury. Cardiac troponin is the preferred test to diagnose MI.

This test is used in the early diagnosis of small myocardial infarcts that are undetectable by conventional diagnostic methods. Cardiac troponin levels are also used later in the course of MI because they remain elevated for 5–7 days after injury. A single sample may be misleading; therefore, serial sampling 0, 4, 8, and 12 hours after chest pains may be ordered to rule out acute MI. See Table 6.11 for a list of cardiac markers.

Negative (Qualitative):

Reference decision limit is typically >99th percentile cutoff of assay.

Troponin I: <0.12 ng/mL or <0.12 μg/L

Troponin T: <0.01 ng/mL or <0.01 μg/L

Total CK: 0–120 ng/mL or 0–120 μg/L

CK-MB: 0–3 ng/mL or 0–3 μg/L

CK index: 0–3

Myoglobin: <55 ng/mL or <55 μg/L

Values will vary depending on the testing method used. Check with your laboratory for reference values.

1. Obtain a 5-mL venous blood sample in a red-topped tube within hours after onset of chest pain. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

2. If serial samples are ordered, record date and time of sampling.

1. Positive or elevated cardiac-specific troponin I (cTn-I) levels indicate:

   1. Small infarct; increases remain for 5–7 days

   2. Myocardial injury during surgery

2. Positive or elevated cardiac-specific troponin T (cTn-T) indicates:

   1. Acute MI

   2. Perisurgical MI

   3. Unstable angina

   4. Myocarditis

   5. Some noncardiac events

      1. CKD

      2. Acute trauma involving muscle

      3. Rhabdomyolysis, polymyositis, dermatomyositis

Pretest Patient Care

1. Explain that the test is a sensitive marker for minor myocardial injury in unstable angina.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. cTn-I levels may be increased in chronic muscle or kidney disease and trauma.

2. Levels are not affected by orthopedic or lung surgery.