Myoglobin

A myoglobin blood test may be used detect muscle damage. Myoglobin is a small, oxygen-binding protein found in heart and skeletal muscle. When heart or skeletal muscle is injured, myoglobin is released into the blood. Blood levels of myoglobin can rise very quickly with severe muscle damage and can be measured within a few hours following an injury.

Myoglobin is filtered from the blood by the kidneys and is released into the urine. Sometimes, a urine test is ordered to evaluate myoglobin levels in people who have had extensive damage to their skeletal muscles (rhabdomyolysis). Urine myoglobin levels reflect the degree of muscle injury and, since myoglobin is toxic to the kidneys, reflect the risk of kidney damage.

Sometimes, a myoglobin blood test may be used along with a troponin test as a cardiac biomarker to help detect a heart attack early, but in the U.S., it is used less frequently for this purpose.

The level of myoglobin in the blood starts to rise within 2-3 hours of a heart attack or other muscle injury, reaches its highest levels within 8-12 hours, and generally falls back to normal within one day. An increase in myoglobin is detectable sooner than troponin, but it is not as specific for heart damage and it will not stay elevated as long as troponin.

Although a negative myoglobin result effectively rules out a heart attack, a positive result must be confirmed by testing for troponin.

This test may be ordered when a person has experienced damage to muscles caused by, for example, trauma or muscular dystrophy.

Myoglobin is not widely used for diagnosing heart attacks because it has largely been replaced by troponin, which is much more specific.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 when a person with signs and symptoms, such as chest pain, first arrives at the emergency room, followed by a series of tests done over several hours.

Urine myoglobin may be ordered when there has been extensive injury to skeletal muscle, resulting in the rapid breakdown of muscle, and damage to the kidneys is suspected.

An increase in blood myoglobin means that there has been very recent injury to muscle tissue. Increased levels can occur, for example, in people who have had:

• Accidents that result in muscle trauma  
• Seizures
• Surgery
• Any muscle disease, such as muscular dystrophy
• Inflammation of skeletal muscles (myositis)
• Heart attack

A significantly elevated level of myoglobin may be caused by rhabdomyolysis.

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 people.

When used as a cardiac biomarker, an increase in blood myoglobin may mean that a heart attack has occurred recently but because myoglobin is also found in skeletal muscle, a troponin test is required for confirmation. If myoglobin does not increase within 12 hours following the onset of chest pain, a heart attack is very unlikely.

Increased myoglobin levels can occur after muscle injections or strenuous exercise. Because the kidneys remove myoglobin from the blood, the myoglobin level may be high in people whose kidneys are failing. Heavy alcohol consumption and certain drugs can also cause muscle injury and increase myoglobin in the 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 should be followed up with more specific testing for myoglobin.