Myoglobin Dx

Description: Myoglobin is an oxygen-binding intracellular muscle protein that stores and delivers oxygen to mitochondria. It is found in skeletal and cardiac muscle and may be released into the circulation after damage to muscle cells.

Method: Myoglobin is measured by radioimmunoassay in serum and by dipstick in urine. Myoglobin is chemically similar to hemoglobin and is detected by the usual dipsticks used for urinalysis. A strongly positive hemoglobin indicator on the dipstick in the absence of erythrocytes may signal myoglobinuria and should prompt measurement of urine myoglobin.

Normal Values: Concentration in serum is normally <90 ng/mL and 0 to 2 mg/mL in urine.

Increased in: Myoglobin is released into the circulation after muscle damage, most commonly caused by injuries such as blunt trauma, heat stroke, ischemia, or inflammation. Release may occur during the course of inflammatory muscle disorders such as polymyositis. In some but not all cases, muscle swelling or tenderness occurs.

Elevated serum levels are also seen in patients with overlap disease with myositis, alcoholic myopathy, hypothermia, shock, severe renal failure, myocardial infarction, or recent cardioversion.

Myoglobinuria may be detected in any instance of excessive serum myoglobin and may also be found in those with familial myoglobinuria, transient “march” myoglobinuria, diabetic ketoacidosis, marked hypokalemia, and severe infections.

Indications: Myoglobin is determined to assess the magnitude of muscle damage or the risks to renal function after muscle damage.

Comment: Myoglobin is cleared by the kidney and excreted in the urine. For reasons that are not well understood, myoglobin is toxic to renal tubular cells, and excess myoglobin (myoglobinuria) may cause acute tubular necrosis and renal failure.

Cost: Urine dipstick is inexpensive; serum radioimmunoassay, $90–130; urine radioimmunoassay, $95–130.