What is being tested?
Blood is being tested for the presence of myoglobin, a protein found in muscles. While haemoglobin brings oxygen to most of the body, myoglobin traps oxygen in muscle to allow muscle cells to work properly. When heart or other muscle is injured, myoglobin is released into the blood.
How is it used?
Myoglobin is a small protein, and it leaks out of cells soon after injury. Myoglobin levels start to rise within 1–3 hours of muscle injury, reach their highest values by about 8–12 hours, and generally fall back to normal by about one day after injury occurred.
When is it requested?
Myoglobin tests may be requested in people who have sustained muscle trauma, particularly crush injuries, burns or electrocution. They are also requested in those who have developed muscle pain and weakness particularly if their urine has developed a reddish-brown colour. However, the blood test has been superseded by the muscle enzyme creatine kinase (CK) which is more sensitive and the concentration remains elevated in the bloodstream for longer following injury. Myoglobin is not widely used for diagnosing heart attacks because troponin is much more specific.
What does the result mean?
A high myoglobin concentration, or a result that increases from the first to a second or later samples, generally indicates that there has been some recent muscle damage.
Myoglobin levels are generally very low, or undetectable in the urine. High levels of urine myoglobin indicate a higher risk for kidney damage and possibly failure. Additional tests, such as urea, creatinine and urinalysis, are performed to monitor kidney function.
Is there anything else I should know?
Increased myoglobin levels can occur after injections into muscles or strenuous exercise. Because the kidneys remove myoglobin from the blood, myoglobin levels may be high in persons whose kidneys are failing. Rarely, heavy alcohol abuse and certain drugs can cause muscle injury and increase myoglobin.
Myoglobin is no longer used in people with chest pain who are suspected of having had a heart attack, having being superseded by troponin, a test which is specific for heart muscle and which also remains elevated for longer in the bloodstream.
Common questions
Generally, routine testing for myoglobin is not necessary. Muscle pain and weakness are common symptoms of many temporary conditions that go away without specific treatment.
Myoglobin released from damaged muscles into the bloodstream is filtered by the kidneys into the urine but can precipitate in the kidney tubules, obstructing urine formation. This is a particular risk if shock lowers blood pressure and reduces the rate of urine production.
When other muscles are damaged due to overwork e.g. running a marathon or a prolonged epileptic fit or due to other causes such as crush injuries, toxins, extremely high body temperature and a variety of other rare causes, myoglobin is released into the blood and can cause kidney failure. In these circumstances the serum CK will be extremely high and the dipstick urine test usually used to detect haemoglobin in urine will also be positive since myoglobin and haemoglobin have some similarities.
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