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DIRECT COOMBS’ TEST
What is a Coombs’ test?
A Coombs’ (or direct antiglobulin) test detects the presence of immunoglobulins
(antibodies) on the surface of red blood cells.
Immunoglobulins are proteins made by white
blood cells (specifically plasma cells). The
Coombs’ test detects these immunoglobulins
using specific antiserum that reacts against
different types of immunoglobulins. If this
antiserum detects immunoglobulins on the
surface of the red blood cells, it will cause the
red blood cells to agglutinate or clump in the test
tube. This test is performed at a veterinary
reference laboratory and requires a single blood
sample.
When is a Coombs’ test indicated?
A Coombs’ test is indicated if it is suspected that a pet has immune-mediated hemolytic
anemia.
What is immune mediated hemolytic anemia (IMHA)?
Immune mediated hemolytic anemia is one of the most prevalent immune mediated
disorders in dogs and cats. It is a condition whereby the red blood cells become coated
with immunoglobulins (antibodies), leading to
the premature destruction of these red blood
cells by the animal’s own body. Coated red
blood cells will either be destroyed (hemolysed)
within the blood stream or by specialized white
blood cells (macrophages) within the spleen or
liver.
The blood smear is part of the complete
blood count.
The causes of immune mediated hemolysis can
generally be divided into two categories, which
are primary (also called idiopathic) and
secondary.
With primary immune mediated hemolytic
anemia the underlying reason that antibodies
are being produced against an animal’s red
blood cells remains unclear or unknown.
With secondary immune-mediated hemolytic anemia, the production of antibodies
against red blood cells is secondary to another condition such as infection, allergy,
inflammation or neoplasia (cancer). Certain drugs are known to trigger immune mediated
hemolytic anemia in susceptible animals – however, the effects of these drugs on
individual animals cannot be predicted or foretold.
From: Laboratory Urinalysis and Hematology by
Carolyn Sink
Published by Teton NewMedia2004
with permission
Are there any other ways to diagnose immune mediated hemolytic anemia?
The condition will be suspected in any animal that has clinical symptoms such as pale
gums or unexplained weakness and characteristic laboratory evidence of immune
mediated red blood cell destruction.
Laboratory evidence of immune mediated red blood cell destruction includes a mild to
severe anemia accompanied by strong evidence of new red blood cell production. The
presence and degree of anemia is determined by a complete blood count (CBC), and
the accompanying blood smear will be used to identify the presence of increased red
blood cell production. The new red blood cells are called polychromatic erythrocytes
(erythrocyte is Latin for red blood cells), or if a special stain is used, reticulocytes.
Another cell that may be present in higher numbers on the blood smear is the
spherocyte. Spherocytes are red blood cells that are very dense and rounded in
appearance, and are formed when a portion of the red blood cell membrane is removed
in the spleen or liver because of the presence of immunoglobulins on the membrane
surface. Occasionally there will be evidence of red blood cell clumping or agglutination
on the slide. This clumping is also caused by the presence of immunoglobulins attached
to the surfaces of red blood cells.
A serum chemistry profile may indicate an increase in bilirubin. Bilirubin is a compound
formed in the liver due to the breakdown of red blood cells. Normally bilirubin is found in
small amounts in a serum sample but with immune mediated destruction of red blood
cells, the amount of bilirubin will increase.
Taken together, the physical findings and the
laboratory data may suggest that immunemediated destruction of red blood cells is the
most likely cause of your pet’s anemia. The
Coombs’ test may provide further confirmation
of the presence of immunoglobulin on the red
blood cell surface.
What does a positive test result mean?
A positive Coombs’ test means that the red
blood cells are coated with hundreds of
From: Laboratory Urinalysis and Hematology
immunoglobulins.A positive result, taken along
by Carolyn Sink
Published by Teton NewMedia2004
with other corroborative laboratory data and
with permission
clinical signs, is supportive of immune mediated
destruction of red blood cells. However, a positive Coombs’ test does not allow us to
determine if the red blood cell destruction is due to primary or secondary immune
mediated hemolysis.
Because positive Coombs’ test results can sometimes occur in animals without immune
mediated hemolytic anemia, it is important to determine whether the CBC and serum
biochemistry panels contain supportive evidence for immune mediated red blood cell
destruction.
What does a negative test result mean?
A negative result can indicate that your pet does not have immune mediated hemolytic
anemia. However it is important to know that a negative Coombs’ test result is seen in
approximately 10% to 30% of dogs with immune mediated hemolytic anemia.
This may be due to the fact that, as in people, red blood cell hemolysis can occur with as
few as 20 to 30 molecules of antibody attached to the red blood cell whereas the
Coombs’ test requires the presence of more than 200 to 300 molecules before a positive
result is noted.
Recent treatment with steroids may also cause a Coombs’ test to be negative.

 This client information sheet is based on material written by Kristiina Ruotsalo, DVM, DVSc, Dip ACVP &
Margo S. Tant BSc, DVM, DVSc.
© Copyright 2004 Lifelearn Inc. Used with permission under license. February 18, 2016