If you have been feeling fatigued, have
shortness of breath, cold hands and feet, and very pale skin, you may have an
insufficient amount of red blood cells. This condition is called anemia, and
there are many causes. If it’s confirmed that you have a low red blood cell
count, the Coombs test is one of the blood tests that your doctor may order to
help find out what kind of anemia you have.
Why Is the Coombs Test Done?
The Coombs test checks the blood to see if it
contains certain antibodies. Antibodies are proteins that your immune system
makes when it detects that something may be harmful to your health. These
antibodies will destroy the harmful invader. If the immune system’s detection
is wrong, it will make antibodies that don’t destroy the invader. This can
cause many kinds of health problems.
The Coombs test will help your doctor
determine if you have antibodies in your bloodstream that are causing your
immune system to attack and destroy your own red blood cells. If your red blood
cells are being destroyed, this can result in a condition called hemolytic
There are two types of Coombs tests: the
direct Coombs test and the indirect Coombs test. The direct test is more common
and checks for antibodies that are attached to the surface of your red blood
cells. The indirect test checks for unattached antibodies that are floating in
the bloodstream. It is also administered to determine if there was a potential
bad reaction to a blood transfusion.
How Is the Coombs Test Done?
A sample of your blood will be needed to
perform the test. The blood is tested with compounds that will react with
antibodies in your blood.
The blood sample is obtained through
“venipuncture,” in which a needle is inserted into a vein in your arm or hand.
The needle draws a small amount of blood into tubing, and the sample is stored
in a test tube.
This test is often done on infants who may
have antibodies in their blood because their mother has a different blood type.
To do this test in an infant, the skin is pricked with a small sharp needle
called a lancet, usually on the heel of the foot. Blood is collected into a
small glass tube, on a glass slide, or on a test strip.
How Do I Prepare for the Coombs Test?
No special preparation is necessary. Your
doctor will have you drink a normal amount of water before going to the
laboratory or collection site.
You may have to stop taking certain medications
before the test is performed, but only if your doctor tells you to do so.
What Are the Risks of the Coombs Test?
When the blood is collected, you may feel
moderate pain or a mild pinching sensation, though this is usually for a very
short time and very slight. After the needle is removed, you may feel a
throbbing sensation, and you will be instructed to apply pressure to the site at
which the needle entered your skin. A bandage will be applied, and it will need
to remain in place typically for 10 to 20 minutes. You should avoid using that
arm for heavy lifting for the rest of the day.
Very rare risks include:
a bruise in which blood accumulates under the skin
usually prevented by the skin being cleaned before the needle is inserted
bleeding (bleeding for a long period after the test may indicate a more
serious bleeding condition and should be reported to your doctor)
What Are the Results for the Coombs Test?
Results are considered normal if there is no
clumping of red blood cells.
Abnormal Results in a Direct Coombs Test
It’s an abnormal
result if there is clumping of the red blood cells during the test. Clumping
(agglutination) of your blood cells during a direct Coombs test means that you
have antibodies on the red blood cells and that you may have a condition that
causes the destruction of red blood cells by your immune system (hemolysis). The
conditions that may cause you to have antibodies on red blood cells are:
hemolytic anemia (your immune system reacts to your red blood cells)
toxicity where you develop antibodies to your red blood cells; drugs that
can cause this include cephalosporins (an antibiotic), levodopa (for
Parkinson’s disease), dapsone (antibacterial), nitrofurantoin
(antibiotic), NSAIDs such as ibuprofen, and quinidine (heart medication)
reaction where your immune system attacks donated blood
blood types between mother and infant (erythroblastosis fetalis)
lymphocytic leukemia and some other leukemias
(systemic lupus erythematosus), an autoimmune disease
with mycoplasma (a type of bacteria that many antibiotics can’t kill)
Sometimes, especially in older adults, a
Coombs test will have an abnormal result even without any other disease or risk
Abnormal Results in an Indirect Coombs Test
An abnormal result to an indirect Coombs test
means you have antibodies circulating in your bloodstream that could cause your
immune system to react to any red blood cells that are considered foreign to
the body, particularly regarding those that may be present during a blood
transfusion. Depending on your age and circumstances, this could mean a mother
and infant have different blood types (erythroblastosis fetalis), an
incompatible blood match for a blood transfusion, or hemolytic anemia due to an
autoimmune reaction or drug toxicity.
Infants with erythroblastosis fetalis may have
very high levels of bilirubin in their blood, which leads to jaundice. This
reaction occurs when the infant and mother have different blood types (Rh
factor positive or negative, or ABO type differences), and the mother’s immune
system attacks the baby’s blood during labor. This condition must be watched
carefully because it can result in death of the mother and child. A pregnant
woman is often given an indirect Coombs test to check for antibodies during
prenatal care, before labor.