Myoglobin

Definition

Myoglobin is the primary oxygen-carrying protein of muscle tissues found only in skeletal and cardiac muscle. It is a small-sized molecule that is rapidly released from damaged tissue and is not protein bound and rapidly excreted in urine. Plasma half-life is 9 minutes. It is linked in a reversible manner with oxygen, playing an important part in cellular aerobic metabolism.
Normal range (may be wide): 6 " 90 ng/mL.
- Male: 28 " 72 ng/mL
- Female: 25 " 58 ng/mL

A cardiac biomarker, myoglobin is the one of the earliest markers for myocardial necrosis.
Myoglobin levels start to rise within 2 " 3 hours of myocardial infarction, reach their highest levels within 8 " 12 hours, and generally fall back to normal within 1 day.
A negative myoglobin result effectively rules out a heart attack, but a positive result must be confirmed by testing for troponin or another biomarker.
Sensitivity is >95% within 6 hours of onset of symptoms.
Myoglobin may precede release of CK-MB by 2 " 5 hours.

Within 1 " 3 hours in >85% of patients with AMI, myoglobin peaks in about 8 " 12 hours (may peak within 1 hour) to about 10 times the upper reference limit and becomes normal in about 24 " 36 hours or less; reperfusion causes a peak 4 " 6 hours earlier.
It is also increased in
- Renal failure (high levels of urine myoglobin indicate an increased risk of kidney damage.)
- Shock
- Open heart surgery
- Carriers of progressive muscular dystrophy
- Extensive trauma
- Myocarditis
- Acute infectious diseases
  - Seizure
  - Toxin exposure: cocaine, narcotics, sea snake venom
  - Malignant hypertension

With high-sensitivity troponins and 99% sensitivity cutoffs now currently used in MI diagnosis, myoglobin has been replaced by troponin as the preferred cardiac biomarker. Exceptions may be in rapid ACS protocols if local laboratory point-of-care troponin is unreliable. Nevertheless, myoglobin should not be the solitary biomarker used for diagnosis.
Increased values may occur with skeletal muscle damage, exhaustive exercise, or heavy alcohol abuse.
The myoglobin test displays a low specificity for AMI. Myoglobin may come from either heart or skeletal muscle, so an increase in serum myoglobin is not specific for damage to the heart.
Blood samples should be drawn every 2 " 3 hours for the first several hours after experiencing chest pain (myoglobin may be released in multiple short bursts) for accurate measurements.
Values are usually much higher in patients with uremia and muscle trauma compared to AMI.
Myoglobin should not be used for the diagnosis of MI but may be useful in conjunction with other biomarkers for prognosis postrevascularization.