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Introduction ⬇

Creatine kinase (CK) is an enzyme found in higher concentrations in the heart and skeletal muscles and in much smaller concentrations in brain tissue. Because CK exists in relatively few organs, this test is used as a specific index of injury to myocardium and muscle. CPK can be divided into three isoenzymes: MM or CK3, BB or CK1, and MB or CK2. CK-MM is the isoenzyme that constitutes almost all the circulatory enzymes in healthy persons. Skeletal muscle contains primarily MM; cardiac muscle contains primarily MM and MB; and brain tissue, GI system, and genitourinary tract contain primarily BB. Normal CK levels are virtually 100% MM isoenzyme. A slight increase in total CPK is reflected from elevated BB from CNS injury. CPK isoenzyme studies help distinguish whether the CPK originated from the heart (MB) or the skeletal muscle (MM).

The CK (CPK) test is used in the diagnosis of MI and as a reliable measure of skeletal and inflammatory muscle diseases. CK levels can prove helpful in recognizing muscular dystrophy before clinical signs appear. CK levels may rise significantly with CNS disorders such as Reye syndrome. The determination of CK isoenzymes may be helpful in making a differential diagnosis. Elevation of MB, the cardiac isoenzyme, provides a more definitive indication of myocardial cell damage than total CK alone. MM isoenzyme is an indicator of skeletal muscle damage. Newer tests, such as CK isoforms, allow for earlier detection of MI than is possible with CK-MB.

Normal Findings ⬆ ⬇

Men: 38–174 U/L (0.63–2.90 μkat/L)

Women: 26–140 U/L (0.46–2.38 μkat/L)

Infants: two to three times adult values

Isoenzymes:

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample (red-topped tube). Serum must be used.

  2. 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.

  3. If a patient has been receiving multiple IM injections, note this fact on the laboratory requisition.

  4. Avoid hemolysis.

Clinical Implications ⬆ ⬇

  1. Total CK levels:

    1. Increased CK/CPK levels occur in the following conditions:

      1. Acute MI

        1. With MI, the rise starts soon after an attack (about 4–6 hours) and reaches a peak of at least several times normal within 24 hours. CK returns to normal in 48–72 hours.

        2. CK and CK-MB (CK2) peak about 1 day after onset, as does AST.

        3. CK-MB and total CK classically increase with acute MI. CK-MB increase, both in percentage and absolutely (each isoenzyme percentage times the respective total enzyme), peak, and then decrease.

      2. Severe myocarditis

      3. Post open heart surgery

      4. Cardioversion (cardiac defibrillation)

      5. Myocarditis

    2. Other diseases and procedures that cause increased CK/CPK levels include the following:

      1. Acute cerebrovascular disease

      2. Progressive muscular dystrophy (levels may reach 20–200 times normal), Duchenne muscular dystrophy, female carriers of muscular dystrophy

      3. Dermatomyositis and polymyositis

      4. Delirium tremens and chronic alcoholism

      5. Electrical shock, electromyography

      6. Malignant hyperthermia

      7. Reye syndrome

      8. Convulsions, ischemia, or subarachnoid hemorrhage

      9. Last weeks of pregnancy and during childbirth

      10. Hypothyroidism

      11. Acute psychosis

      12. CNS trauma, extensive brain infarction

      13. Neoplasms of prostate, bladder, or GI tract

      14. Rhabdomyolysis with cocaine intoxication

      15. Eosinophilia–myalgia syndrome

    3. Normal values are found in myasthenia gravis and multiple sclerosis.

    4. Decreased values have no diagnostic meaning and may be caused by low muscle mass and bed rest (overnight values can drop 20%).

  2. CK isoenzymes:

    1. Elevated MB (CK2) isoenzyme levels occur in the following conditions:

      1. Myocardial infarct (rises 4–6 hours after MI; not demonstrable after 24–36 hours; i.e., peak with rapid fall)

      2. Myocardial ischemia, angina pectoris

      3. Duchenne muscular dystrophy

      4. Subarachnoid hemorrhage

      5. Reye syndrome

      6. Muscle trauma, surgery (postoperative)

      7. Circulatory failure and shock

      8. Infections of heart—myocarditis

      9. CKD

      10. Malignant hyperthermia, hypothermia

      11. CO poisoning

      12. Polymyositis

      13. Myoglobulinemia

      14. Rocky Mountain spotted fever

    2. BB (CK1) elevations occur in the following conditions:

      1. Reye syndrome

      2. Some breast, bladder, lung, uterus, testis, and prostate cancers

      3. Severe shock syndrome

      4. Brain injury, neurosurgery

      5. Hypothermia

      6. Following coronary bypass surgery

      7. Newborns

    3. MM (CK3) is elevated in most conditions in which total CK is elevated.

    4. MB (CK2) is not elevated from:

      1. Exercise (total elevated)

      2. IM injections (total elevated)

      3. Strokes and other brain disorders in which total CK is elevated

      4. Pericarditis

      5. Pneumonia, other lung diseases; pulmonary embolism

      6. Seizures (CK total may be very high)

Clinical Alert

  1. After an MI, MB appears in the serum in 6–12 hours and remains for about 18–32 hours. The finding of MB in a patient with chest pain is diagnostic of MI. In addition, if there is a negative CK-MB for ≥48 hours following a clearly defined episode, it is clear that the patient has not had an MI.

  2. CK-MB and total CK classically increase with acute MI. CK-MB and the lactate dehydrogenase (LDH) isoenzyme LD1 increase both in percentage and absolutely (each isoenzyme percentage times the respective total enzyme), peak, and then decrease.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and need for at least three timed consecutive blood draws following episode.

  2. Note on requisition when suspected cardiac episode occurred and dates and times of blood draws.

  3. Do not allow exercise before test.

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

Posttest Patient Care

  1. Have patient resume normal activities.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Monitor as appropriate for MI, muscular dystrophy, and other causes of abnormal test outcomes.

  3. High levels of CK/CK-MB may suggest other tests should be done to support diagnosis of acute MI:

    1. Total leukocyte count and differential

    2. Cardiac troponin

    3. Myoglobin

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Strenuous exercise, weightlifting, and surgical procedures that damage skeletal muscle may cause increased levels of CK.

  2. Alcohol and other drugs of abuse increase CK levels.

  3. Athletes have a higher CK value because of greater muscle mass.

  4. Multiple IM injections may cause increased or decreased CK levels (see Appendix E).

  5. Many drugs may cause increased CK levels.

  6. Childbirth may cause increased CK levels.

  7. Hemolysis of blood sample causes increased CK levels.