Myoglobinuria is defined as the presence of myoglobin in the urine and it is usually noted as a positive (+) result in the urinalysis section.
Myoglobin is present in muscle tissues, where it serves as an oxygen-storing molecule. Therefore, the presence of myoglobin in urine is a sign of muscle damage.
Myoglobin contains an heme group, very similar to the one present in the hemoglobin (a protein found in the red blood cells).
When present in urine, myoglobin may cause the urine to appear clear red-brown. The concentration of myoglobin in the urine must be at least 25 mg/dL before the red pigmentation can be visualized.
The myoglobin urine test is performed in patients with conditions associated with muscle destruction such as rhabdomyolysis.
Rhabdomyolysis is a syndrome resulting from the breakdown of skeletal muscle fibers due to a direct or indirect muscle injury.
Myoglobin in urine may be also present in case of suffering some genetic disorders.
This test is performed along with other urine tests. The patient must collect a sample of the urine in a specific container using a special kit. This sample will be sent to the lab for the analysis.
Myoglobin in urine can be tested either with a dipstick or with a tablet that change of color in the presence of blood in the urine.
In laboratories, there are automated machines that perform the analysis, but it is also possible to buy strips that let you analyze a urine sample at home. They comprise up to 10 or more different reagents or chemical pads to perform different urine tests at the same time.
The finding of a positive reagent strip test result for blood may indicate:
Dipsticks cannot discern the presence of red blood cells (RBCs), hemoglobin, or myoglobin and give a positive result if there is any of these three substances in the urine.
If there is a positive result for blood in the urine and the examination of the urine sediment under the microscope does not show red blood cells (RBCs), the cause is hemoglobinuria or myoglobinuria.
The diagnosis of myoglobinuria is usually based on the patient's history and elevated serum levels of the enzymes creatinine kinase (CK) and lactic dehydrogenase (LDH).
The laboratory can differentiate between the presence of hemoglobin and myoglobin in a urine specimen using the precipitation test; 2.8 g of ammonium sulfate are added to 5 mL of centrifuged urine. After mixing and allowing the specimen to sit for 5 minutes, the urine is filtered or centrifuged, and the supernatant is tested for a reaction for blood with dipstick. The principle of this screening test is based on the fact that the larger hemoglobin molecules are precipitated by the ammonium sulfate, and myoglobin remains in the supernatant. Therefore, when myoglobin is present, the supernatant gives a positive result for blood in the dipstick. By contrast, hemoglobinuria will give a negative result for blood in the dipstick. Myoglobin is not stable in acidic urine and in this case may precipitate with the ammonium sulfate, so this is a fact to be also considered.
The urine myoglobin test must be negative to be considered within normality.
A positive (+) result for myoglobin in the urine may be a sign of conditions associated with muscle destruction (rhabdomyolysis):
Other rare diseases may cause myoglobinuria:
The heme portion of myoglobin is toxic to the renal tubules, and high concentrations can cause acute kidney failure.