Chemistry, toxicology & urinalysis

• Sensitive and specific test for myocardial infarction, now widely replaced by troponin

• Catalyzes the conversion of creatine to phosphocreatine, consuming adenosine triphosphate (ATP) and generating adenosine diphosphate (ADP)

• CK isoenzyme MB rises some 4 - 6 hours after the onset of chest pain, peaks within 12 - 24 hours, and returns to baseline levels within 24 - 48 hours
• CK-MB is usually ordered, along with total CK in persons with chest pain to determine whether the pain is due to myocardial infarction
• May also be ordered in a person with a high CK to determine whether damage is in the heart

Test methodology

• Electrophoresis:
  • Serum creatine kinase (CK) is separated into 3 isoenzymes by electrophoretic separation on agarose gel
  • Colorimetric results allow for improved workflow management as the gels do not have to be scanned immediately
  • The permanent patterns combined with a clear gel background means scanning and quantitation are easy
• Immunoassays are also commonly used

Test indications

• Troponin has largely replaced CK-MB in many hospitals, although some centers still rely on CK-MB (Wikipedia - CPK-MB test)

Test limitations

• Some patients have a variant of CK-BB called "Macro CK", which complexes to IgG or IgA antibody
  • It migrates between MM and BB on the gel, and may falsely increase CK-MB values
• CK-MB can be elevated with massive rhabdomyolysis, even though the concentration is low in skeletal muscle
• Electrophoresis of CK with values of total CK under 100 U/L may cause false positive CK-MB values

Reference ranges

• If the value of CK-MB is elevated and the ratio of CK–MB to total CK (relative index) is more than 2.5 - 3, it is likely that the heart was damaged
• A high CK with a relative index below 2.5 - 3.0 suggests that skeletal muscle and not cardiac muscle was damaged

• Early studies:
  • Proc Natl Acad Sci U S A 1974;71:1384, Circulation 1975;51:855, Clin Chem 1980;26:861, Lancet 1977;2:319, Clin Chem 1983;29:590

This is an image of a creatinine kinase electrophoresis. Which of the following patients would likely have this electrophoretic pattern?

1. 32 year old healthy male who just finished a marathon
2. 45 year old female with widely metastatic breast cancer
3. 55 year old male with acute onset chest pain
4. 78 year old asymptomatic female

D. A 78 year old asymptomatic female - this is the CK electrophoretic pattern seen in macro CK, a benign finding in healthy, elderly females. Macro CK is a result of a CK antibody conjugate and typically appears between CK-MM and CK-MB.

Choices 'a' and 'b' would not have a macro CK band. A 45 year old female (c) may have an additional band that migrates to the right of CK-MM. This band is mitochondrial CK and is seen in patients with widely metastatic disease.

Comment Here

Reference: CK-MB (creatine kinase isoenzyme MB)