COOMBS
TEST
Coombs test (also
known as Coombs' test, antiglobulin test or AGT) refers to two clinical blood tests used in immunohematology andimmunology.
The two Coombs tests are the direct Coombs test (DCT, also known as direct antiglobulin test or DAT), and the indirect Coombs test (also known as indirect antiglobulin test or IAT).
The more commonly used
test, the Direct Coombs test, is used to test for autoimmune hemolytic anemia.
In certain diseases or
conditions an individual's blood may contain IgG antibodies that can
specifically bind to antigens on the red blood cell (RBC) surface membrane, and
their circulating red blood cells (RBCs) can become coated with IgG
alloantibodies and/or IgG autoantibodies. Complement proteins may subsequently
bind to the bound antibodies. The direct Coombs test is used to
detect these antibodies or complement proteins that are bound to the surface of
red blood cells; a blood sample is taken and the RBCs are washed (removing the
patient's own plasma) and then incubated with antihuman globulin (also known as
"Coombs reagent"). If this produces agglutination of RBCs, the direct Coombs test is positive, a visual
indication that antibodies (and/or complement proteins) are bound to the surface
of red blood cells.
The indirect Coombs test is used in prenatal testing of pregnant women, and in
testing blood prior to a blood transfusion. It detects antibodies against RBCs that are present unbound in the
patient's serum. In
this case, serum is extracted from the blood, and the serum is incubated with
RBCs of known antigenicity.
If agglutination occurs, the indirect Coombs test is positive.
MECHANISM
The two Coombs tests are
based on the fact that anti-human antibodies,
which are produced by immunizing non-human species with human serum, will
bind to human antibodies, commonly IgG or IgM. Animal
anti-human antibodies will also bind to human antibodies that may be fixed onto
antigens on the surface of red blood cells (also referred to as RBCs), and in the appropriate
test tube conditions this can lead to agglutination of RBCs. The
phenomenon of agglutination ofRBCs is
important here, because the resulting clumping of RBCs can be visualised; when
clumping is seen the test is positive and when clumping is not seen the test is
negative.
Common clinical uses of
the Coombs test include the preparation of blood for transfusion in cross-matching,
screening for atypical antibodies in the blood plasma of pregnant women as
part of antenatal care,
and detection of antibodies for the diagnosis of immune-mediated haemolytic anemias.
Coombs tests are done on
serum from venous blood samples which are taken from patients by venepuncture.
The venous blood is taken to a laboratory (or blood bank), where trained
scientific technical staff do the Coombs tests. The clinical significance of
the result is assessed by the physician who
requested the Coombs test, perhaps with assistance from a laboratory-based hematologist.
DIRECT COOMBS TEST
The direct Coombs test
(also known as the direct
antiglobulin test or DAT) is
used to detect if antibodies orcomplement system factors have bound to RBC surface antigens in vivo.
The DAT is not currently required for pre-transfusion testing but may be
included by some laboratories.
Examples
of diseases that give a positive direct Coombs test
The direct Coombs test
is used clinically when immune-mediated hemolytic anemia (antibody-mediated destruction of RBCs) is suspected.
A positive Coombs test indicates that an immune mechanism is attacking the
patient's own RBC's. This mechanism could be autoimmunity, alloimmunity or a drug-induced immune-mediated mechanism.
Examples
of alloimmune hemolysis
§ Other
blood group incompatibility (RhC, Rhe, Kidd, Duffy, MN, P and others)
Examples
of autoimmune hemolysis
§ Idiopathic
Drug-induced
immune-mediated hemolysis
§ Quinidine (IgM mediated activation of
classical complement pathway and Membrane attack complex, MAC)
(A memory device to remember that the DAT tests the
RBCs and is used to test infants for haemolytic
disease of the newborn is: Rh Disease; R = RBCs, D = DAT.)
INDIRECT
COOMBS TEST
The indirect Coombs test
(also known as the indirect
antiglobulin test or IAT) is
used to detect in-vitro antibody-antigen
reactions. It is used to detect very low concentrations of antibodies present
in a patient's plasma/serum prior to a blood transfusion. In antenatal care,
the IAT is used to screen pregnant women for antibodies that may cause hemolytic
disease of the newborn. The IAT can
also be used for compatibility testing, antibody identification, RBC phenotyping, and
titration studies.
Examples
of clinical uses of the indirect Coombs test
Blood
transfusion preparation
The indirect Coombs test
is used to screen for antibodies in the preparation of blood for blood transfusion. The donor's and recipient's blood must be ABO and Rh D compatible. Donor blood for transfusion is
also screened for infections in separate processes.
§ Antibody
screening
A blood sample from the
recipient and a blood sample from every unit of donor blood are
screened for antibodies with the indirect Coombs test. Each sample is incubated
against a wide range of RBCs that together exhibit a full range of surface
antigens (i.e. blood types).
§ Cross
matching
The indirect Coombs test
is used to test a sample of the recipient's serum against a sample of the blood donor's RBCs. This is sometimes called cross-matching blood.
Antenatal
antibody screening
The indirect Coombs test
is used to screen pregnant women for IgG antibodies that are likely to pass through the placenta into the
fetal blood and cause haemolytic
disease of the newborn.
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