Myoglobin blood and urine test: muscle injury and why it is rarely used now

Myoglobin is a small protein inside heart and skeletal muscle cells that stores oxygen. When muscle is crushed, starved of blood or breaks down for other reasons, myoglobin leaks into the blood quickly, and because it is small it is filtered by the kidneys and appears in urine, sometimes turning it dark. Lab Tests Online UK describes the test as measuring myoglobin in blood or urine to help detect and monitor muscle damage, including rhabdomyolysis and injury to heart or skeletal muscle.

A high myoglobin concentration indicates that there has been some recent muscle damage. It rises early after injury but also clears from the blood relatively quickly.

Myoglobin was once the primary test for suspected muscle injury. Lab Tests Online UK explains that it has been replaced by creatine kinase, which is more sensitive and specific and remains elevated in the bloodstream for longer after injury. In people with chest pain suspected of a heart attack, it has been superseded by troponin, a test specific for heart muscle that also remains elevated for longer.

For those reasons myoglobin is no longer in routine use in the UK. If your notes mention it, it is more likely in a specialist or research setting, or in discussion of kidney risk after muscle damage.

No preparation is needed. The sample is either blood from a vein in the arm or a random urine sample. If a clinician suspects rhabdomyolysis, creatine kinase, kidney function, potassium and urine tests are the usual first steps instead.

Myoglobin released from damaged muscle is filtered by the kidneys into urine but can precipitate in the kidney tubules, obstructing urine formation. This is a particular risk if shock lowers blood pressure and reduces urine production. Infusing fluid into a vein can increase urine production and myoglobin loss, thereby lowering the risk of kidney failure.

Causes of severe muscle breakdown include major trauma and crush injuries, prolonged immobility after a fall, extreme exertion, some medicines, and seizures. Dark brown or tea-coloured urine, severe muscle pain and weakness after such an event are warning signs.

Chest pain, severe breathlessness, or collapse need 999. Dark urine after a crush injury, extreme exertion or a long period on the floor, with muscle pain or weakness, needs urgent assessment through NHS 111 or A&E. Treatment centres on fluids, monitoring kidney function and potassium, and treating the cause of the muscle damage.