- Last Update On : 2013-01-27
Myoglobin is an oxygen carrying heme protein present in high concentrations in the cytoplasm
of cardiac and skeletal muscle. It constitutes 2% of total muscle protein. Small amounts of myoglobin are cleared from the plasma in one to six hours and metabolized to bilirubin. When plasma myoglobin levels exceed 1.5 mg/dL, it is excreted by the kidney.
Rhabdomyolysis is defined as an acute increase in plasma concentrations of creatinine kinase to more than 5 times the upper limit of normal in the absence of a myocardial infarction. High concentrations of myoglobin are released into the plasma during muscle injury. Visible myoglobinuria (tea or cola colored urine) occurs when urinary myoglobin exceeds 250 ug/mL, corresponding to the destruction of more than 100 grams of skeletal muscle.
The major causes of myoglobinuria include skeletal muscle trauma, excessive muscle use, primary skeletal muscle diseases, hyperpyrexia, seizures, and gangrene. Insect and snakebites can also cause myoglobinuria. The most common drugs causing myoglobinuria are alcohol, cocaine, amphetamines, opiates, phencyclidine, neuroleptics, and statins.
Patients with skeletal muscle injury also have elevated serum CK, AST, LD, uric acid, potassium and creatinine levels. Creatine kinase levels may exceed 100,000 IU/L. Serum calcium is often decreased due to calcium binding by damaged muscle proteins and phosphate. Acute renal failure results from renal vasoconstriction, intraluminal myoglobin cast formation, and heme protein nephrotoxicity.
Myoglobinuria can be inferred by a positive urine dipstick test for heme, in the absence of red blood cells on microscopic examination of urine sediment. Urine myoglobin can be confirmed by nephelometry. Reference range is 0 - 2 ug/mL.
Specimen requirement is 5 mL of a random urine collection.