Myoglobin holds oxygen inside heart and skeletal muscle (muscles that attach to and move bones). It is continually released into the blood in small amounts due to normal turnover of muscle cells. Kidneys discard the myoglobin into urine.
When muscle is damaged, as in a heart attack, larger amounts of myoglobin are released and blood levels rise rapidly. Myoglobin is one of the first tests done to determine if a person with chest pain is having a heart attack, as it may be one of the first blood tests to become abnormal.
Damage or injury to skeletal muscle also causes myoglobin to be released into the blood.
Heart attack must be diagnosed quickly. Medications to prevent heart damage are effective only within a limited number of hours. Yet, because of their risk for excessive bleeding, these medications are given only after a diagnosis of heart attack is made.
Myoglobin is one of several cardiac markers used to make the diagnosis. Cardiac markers are substances in blood whose levels rise in the hours following a heart attack. Increased levels help diagnose a heart attack; persistent normal levels rule it out.
Each cardiac marker rises, peaks, and returns to a normal level according to its own timeline, or diagnostic window. Myoglobin is useful because it has the earliest diagnostic window. It is the first marker to rise after chest pain begins. Myoglobin levels rise within two to three hours, and sometimes as early as 30 minutes. They peak after six to nine hours. The levels return to normal within 24-36 hours.
Although a rise in myoglobin supports a diagnosis of heart attack, it is not conclusive. Simultaneous skeletal muscle damage could also cause the increase. Myoglobin rules out, rather than proves, a diagnosis in the following way. If myoglobin levels have not risen after more than five hours, a heart attack in unlikely. Normal levels in the first two to three hours do not rule out an infarction.
The myoglobin test is sometimes repeated every one to two hours to watch for the rise and peak. Results are available within 30 minutes.
Myoglobin in large amounts is toxic to the kidney. When a person has high amounts of myoglobin in the blood, kidney function must be monitored.
This test requires 5 ml of blood. Collection of the sample takes only a few minutes. A urine myoglobin test requires 1 ml of urine collected into a urine collection cup.
Discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs to the puncture site relieve discomfort.
Normal results vary based on the laboratory and method used.
Myoglobin levels and levels of other cardiac markers are usually considered before finally confirming a diagnosis of heart attack. A level that has doubled after one to two hours, even if the level is still in the normal range, indicates a significant rise that may be due to heart attack.
Increased levels are also found with skeletal muscle damage or disease, such as an injury, muscular dystrophy, or polymyositis. Myoglobin levels also rise during renal failure because kidneys lose their ability to clear myoglobin from blood.
Wu, Alan, editor. Cardiac Markers. Washington, DC: American Association of Clinical Chemistry (AACC) Press, 1998.
Cardiac marker— A substance in the blood that rises following a heart attack.
Diagnostic window— A cardiac marker's timeline for rising, peaking, and returning to normal after a heart attack.
Myoglobin— A protein that holds oxygen in heart and skeletal muscle. It rises after damage to either of these muscle types.
"Myoglobin Test." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. (August 15, 2018). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/myoglobin-test
"Myoglobin Test." Gale Encyclopedia of Medicine, 3rd ed.. . Retrieved August 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/myoglobin-test
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Antimyocardial Antibody Test
Antimyocardial Antibody Test
Testing for antimyocardial antibodies is done when evaluating a person for heart damage or heart disease.
Antimyocardial antibodies are autoantibodies. Normal antibodies are special proteins built by the body as a defense against foreign material entering the body. Autoantibodies are also proteins built by the body, but instead of attacking foreign material, they inappropriately attack the body's own cells. Antimyocardial antibodies attack a person's heart muscle, or myocardium.
This test may be done on a person who recently had trauma to the heart, such as heart surgery or a myocardial infarction (heart attack ). It also may be done on someone with heart disease, such as cardiomyopathy or rheumatic fever.
Although the presence of antimyocardial antibodies does not diagnose heart damage or disease, there is a connection between the presence of these antibodies and damage to the heart. The amount of damage, however, cannot be predicted by the amount of antibodies.
These antibodies usually appear after heart surgery or the beginning of disease, but they may be present before surgery or the onset of disease. In 30% of people with myocardial infarction and 70% of people having heart surgery, antimyocardial antibodies will appear within two to three weeks and stay for three to eight weeks.
A 5-10 mL sample of venous blood is drawn from the patient's arm in the region of the inner elbow. Antimyocardial antibodies are detected by combining a patient's serum (clear, thin, sticky fluid in blood) with cells from animal heart tissue, usually that of a monkey. Antimyocardial antibodies in the serum bind to the heart tissue cells. A fluorescent dye is then added to the mixture. This dye will attach to any antibodies and heart tissue cells bound together. The final mixture is studied under a microscope that is designed to show fluorescence. If fluorescent cells are seen under the microscope, the test is positive.
When the test is positive, the next step is to find out how much antibody is present. The patient's serum is diluted, or titered, and the test is done again. The serum is then further diluted and the test repeated until the serum is so dilute that fluorescence is no longer seen. The last dilution that showed fluorescence is the titer reported.
No fasting or special prepartion is needed. Before the test is done it should be explained to the patient.
Discomfort or bruising may occur at the puncture site after the blood is drawn or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs on the puncture site relieve discomfort.
Antibody— A special protein built by the body as a defense against foreign material entering the body.
Antimyocardial antibody— An autoantibody that attacks a person's own heart muscle, or myocardium.
Autoantibody— An antibody that attacks the body's own cells or tissues.
Myocardial infarction— A block in the blood supply to the heart, resulting in what is commonly called a heart attack.
Myocardium— The muscular middle layer of the heart.
Titer— A dilution of a substance with an exact known amount of fluid. For example, one part of serum diluted with four parts of saline is a titer of 1:4.
Antimyocardial antibodies are not normally seen in healthy individuals.
A positive result means that antimyocardial antibodies are present and that heart disease or damage is likely. Further testing may be needed as other autoantibodies could also be present, causing a false abnormal test.
Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.
"Antimyocardial Antibody Test." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. (August 15, 2018). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/antimyocardial-antibody-test
"Antimyocardial Antibody Test." Gale Encyclopedia of Medicine, 3rd ed.. . Retrieved August 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/antimyocardial-antibody-test