CPK (creatine phosphokinase) test

Introduction

CK is a molecule made up of two polypeptide subunits (M and B) coded by different genes. Three major isoenzymes are available.

• CK-MM is the major isoenzyme type available. This is present in skeletal and cardiac muscles

• CK-MB is the isoenzyme which is primarily found in the cardiac muscles. A small amount is present in the skeletal muscles as well.

• CK-BB is the isoenzyme which is mainly found in the brain matter. A small amount is also found in kidney, stomach, colon, and liver as well.

MM is the predominant CK type. Serum of a normal individual contains more than 97% of MM variety whereas rest consists of MB type. Normal CK level is 200-250 u/l There are several causes which can lead to the elevation of CK level in the blood .These include;

1. Artefactual causes ( Hemolysis)

2. Physiological causes(neonates, adult male> adult female, exercise)

3. Pathological causes (e.g. shock & circulatory failure, Myocardial infarction, muscular dystrophy, Rhabdomyolysis, Hypothyroidism, Alcoholism, Head injury and strokes)

Importance in cardiac injuries

Most important aspect of the CK is it’s relevant with cardiac injury. The level of CK rises within 4 to 8 hours after the cardiac event and it returns to normal level by 48 to 72 hours. The peak level can be detected within 16 to 24 hours after the event. The main drawback of measurement of CK level is its lack of specificity for cardiac injuries as it elevates in above mentioned causes as well.

Since the CK-MB is more specific for the cardiac muscle, measurement of CK-MB level can be useful. But the skeletal muscles also contain some amount of CK-MB, which is very small when compare with the amount containing in the cardiac muscles (Table A). Usually in serum the fraction of CK-MB is about 3%. If this fraction is higher than the normal level it will indicate the cardiac injury. But sever skeletal muscles damage can give rise to the higher level of CK-MB therefore sever skeletal muscles damage can mimic acute myocardial infarction. To avoid this false positive scenario, series of ECG should be done with the measurement of CK level. There are some other causes which lead to the rise in CK-MB level such as crush syndrome, Major surgery, Defibrillation and Malignant Hyperthermia.

Table A

Isoenzyme	% in heart muscles	% in cardiac muscles
CK-MM	78	98
CK-MB	22	2

Following condition give elevated CPK level

1. Hyperthermia

2. Necrotizing fasciitis caused by S. pyogenes

3. Legionnaires’ disease (Pneumonia)

4. Severe Leptospirosis (Weil’s Syndrome)

5. Complications of influenza infection

6. Crimean-congo hemorrhagic fever

7. Trichinellosis

8. Cardiogenic shock

9. Hypothyroidism

10. Hypophosphatemia

11. As a side effects of HMG-CoA reductase inhibitors

12. As a side effects of Antipsychotic medications