How is it used?
As a
cardiac biomarker, myoglobin is used in conjunction with
troponin to help diagnose or rule out a
heart attack. Myoglobin levels start to rise within 2-3 hours of a heart attack or other muscle injury, reach their highest levels within 8-12 hours, and generally fall back to normal within one day. Its advantage over other markers is that it turns positive sooner than troponin. However, myoglobin may come from either heart or skeletal muscle, so an increase in serum myoglobin is not specific for damage to the heart. Consequently, a negative myoglobin result effectively rules out a heart attack, but a positive result must be confirmed by testing for troponin.
Sometimes, a test for myoglobin is used to detect elevated levels in the urine of patients that have had extensive damage to their skeletal muscles. Myoglobin is toxic to the kidneys. If severe muscle injury occurs, blood levels of myoglobin may rise very quickly and the kidneys, which clear myoglobin from the blood and excrete it in the urine, may be damaged by the increased amounts.
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When is it ordered?
Myoglobin is not widely used for diagnosing
heart attacks because
troponin is much more specific. If the myoglobin test is available, it may be ordered to assess persons with chest pain who are suspected of having a heart attack. Blood samples are drawn on admission and every 2-3 hours for up to 12 hours in patients who come to the emergency room with a possible heart attack.
Urine myoglobin may be ordered when there has been extensive traumatic injury to muscle, and damage to the kidneys is suspected.
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What does the test result mean?
An increase in blood myoglobin means that there has been very recent injury to the heart or skeletal muscle tissue. Additional tests, such as
troponin, are necessary to determine where the damage has occurred. If myoglobin does not increase within 12 hours following the onset of chest pain, a
heart attack is very unlikely, unless the symptoms began more than a day earlier. Because myoglobin is also found in skeletal muscles, increased levels can occur in patients who have accidents, seizures, surgery, or any muscle disease, such as muscular dystrophy.
Since this blood test is used to detect increased levels in people with chest pain, a low or normal level of myoglobin means that either a heart attack has not occurred or myoglobin has already cleared the bloodstream. A test for troponin or other cardiac markers may be used as follow-up to a low or normal result.
Myoglobin levels are normally very low or not detectable in the urine. High levels of urine myoglobin indicate an increased risk for kidney damage and failure. Additional tests, such as BUN, creatinine, and urinalysis, are done to monitor kidney function in these patients.
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Is there anything else I should know?
Increased myoglobin levels can occur after muscle injections or strenuous exercise. Because the kidneys remove myoglobin from the blood, myoglobin levels may be high in persons whose kidneys are failing. Heavy alcohol abuse and certain drugs can also cause muscle injury and increase myoglobin in blood.
A urine dipstick test for hemoglobin can also be positive in the presence of myoglobin. If the urine dipstick test is positive and myoglobin is suspected to be the cause, it can be followed-up with more specific testing for myoglobin.
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