To determine whether muscle has been injured
If you have muscle pain or weakness after muscle damage, particularly if your urine becomes dark reddish-brown in colour
Blood is being tested for the presence of myoglobin, a red 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 muscle is injured, myoglobin is released into the blood and appears in the urine.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm or a random urine sample.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Myoglobin is a small protein that leaks out of muscle cells soon after injury. Myoglobin blood concentrations 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 blood tests may be requested in people who had sustained muscle trauma, particularly crush injuries, burns or electrocution. They were also requested in those who had developed muscle pain and weakness
- after being immobile for a long time on a hard surface (for example during an operation or after a stroke, drugs or alcohol)
- following severe exercise
- after a fit
- during a severe infection
particularly if their urine had develops a reddish-brown colour and gave a positive dip-stick test for blood. (Myoglobin reacts with the dip-sticks in the same way as haemoglobin from blood cells.) However, the blood test has been superseded by the muscle enzyme creatine kinase (CK) which is more sensitive and the concentration remains elevated in the bloodstream for longer following injury.
What does the test result mean?
A high myoglobin concentration, or a result that goes up from the first to a second or later samples, generally indicates that there has been some recent muscle damage. Serial test results that peak and then begin to drop indicate that new muscle damage has diminished, while increasing and persistent elevations suggest continuing damage.
Is there anything else I should know?
The excretion of myoglobin by the kidney can lead to renal failure.
Myoglobin is no longer requested in people with chest pain who are suspected of having had a heart attack, having been superseded by troponin, a test which is specific for heart muscle and which also remains elevated for longer in the bloodstream.
How does myoglobin cause kidney failure?
Myoglobin released from damaged muscle into plasma 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.
How is a raised plasma myoglobin caused by a crush injury treated?
An infusion is given into a vein of a large amount of saline (salt solution) to increase the rate of urine formation and reduce the risk of precipitation.
Are any other lab tests needed?
Yes, it is important to monitor the patient for increasing plasma potassium and decreasing plasma calcium levels.