What is being tested?
Myoglobin is a small protein found in heart and skeletal muscles that binds oxygen. It traps oxygen within muscle cells, allowing the cells to produce the energy required for muscles to contract. When heart or skeletal muscle is injured, myoglobin is released into the blood. Elevated levels 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. Large quantities of myoglobin are toxic to the kidneys. If significant amounts of myoglobin are released into the bloodstream, which can happen after severe trauma or muscle injuries, the excess myoglobin may cause damage to the kidneys and eventually result in kidney failure. Measurement of myoglobin in urine helps to detect this condition.
How is the test used?
A myoglobin blood test may be used detect muscle damage. 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 used 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.
When is it ordered?
A myoglobin blood test may be ordered when you have experienced damage to muscles caused by, for example, trauma or muscular dystrophy.
A urine myoglobin test may be ordered when there has been extensive injury to skeletal muscle, resulting in the rapid breakdown of muscle, and damage to the kidneys from excess myoglobin in the urine is suspected.
What does the test result mean?
An increase in blood myoglobin means that there has been very recent injury to muscle tissue. Some examples of causes of increased levels include:
- Accidents that result in muscle trauma
- 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 (see below).
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.
Some examples of conditions associated with myoglobin in the urine include:
- Excessive physical activity in untrained individuals
- Crush injuries and other trauma
- Viral infections (a common cause in children)
- Toxins or certain drugs
- Progressive muscle disease
- Heat injury
- Hereditary (genetic) or metabolic disorders that affect muscles
What is rhabdomyolysis?
Rhabdomyolysis is the rapid breakdown of muscle tissue. This condition can be caused by serious injury to muscles from a number of different sources. Examples include:
- Trauma, crushing injuries such as car accidents
- Electrical shock from a high-voltage sources
- Serious and/or extensive burns
- Blood clot (thrombosis) that blocks blood flow to tissues and organs
- Exposure to toxins such as heavy metals, snake venom, or carbon monoxide
- Infections such as HIV, influenza, Streptococcus (this can more commonly occur in children than adults)
- Genetic or metabolic disorders that affect muscles
- Diseases such as muscular dystrophy or underlying conditions such as diabetes that is not well controlled, hypothyroidism, or hyperthyroidism
- Several drugs
Complications can occur when, after the muscle is damaged, the cells release their contents rapidly into the blood. This has been known to cause damage to kidneys (acute kidney injury, AKI) and disseminated intravascular coagulation (DIC). Once this condition is diagnosed and depending on the severity, a person with rhabdomyolysis may be treated with intravenous fluids and other supportive care. Other procedures may be done to protect the person’s organs. For example, the person may undergo dialysis to prevent and/or limit damage to the kidneys.
What drugs can cause rhabdomyolysis?
Some examples include:
- Drugs of abuse such as alcohol (ethanol), heroin and cocaine
- Antibiotics (amphotericin B, ampicillin)
- Aspirin (salicylates)
- Statins (used to treat high cholesterol)
Should everyone with muscle pain and weakness have a myoglobin test?
General, routine testing for myoglobin is usually not necessary. Muscle pain and weakness are common symptoms of many temporary conditions that go away without specific treatment. However, if someone is taking a drug or has been exposed to a substance that has been linked with potential muscle damage, then testing may be warranted.
Are myoglobin tests used to help diagnose heart attacks?
Myoglobin blood tests have been used along with troponin tests as cardiac biomarkers to help detect a heart attack early. However, in the U.S., myoglobin testing is used less frequently for detecting heart attacks early and more recent studies have indicated that newer markers (e.g., troponin) are better 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.
If myoglobin does not increase within 12 hours following the onset of chest pain, a heart attack is very unlikely. Although a negative myoglobin result effectively rules out a heart attack, a positive result must be confirmed by testing for troponin.
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, 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.