Meaning and Uses of the
Direct Antiglobulin Test
Marilyn Telen, MD
Duke University
Direct Antiglobulin Test
• DAT
• Direct Coombs test
• Coombs test
Direct Antiglobulin Test
• Test for in-vivo coating of patient’s red
blood cells with anti-IgG and/or
complement
• Washed patient’s red blood cells +
polyspecific anti-human globulin (AHG:
anti-IgG, anti-C3/C3d)
– If positive, test red cells with 2
monospecific antibodies (anti-IgG and antiC3)
– If IgG+ may remove (elute) antibody and
test for specificity
Autologous Control
• Positive autocontrol on panel can mean:
– Alloantibody coating donor (previously
transfused) red cells
– Autoantibody
Direct Antiglobulin Test
• Physician request – usually because
immune-mediated hemolysis is
suspected.
• Positive DAT could mean:
– Warm autoantibody
– Cold autoantibody
– Transfusion reaction
– Hemolytic disease of the newborn
– Drug induced antibody
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•
•
•
Drug adsorption - penicillin
Immune complex
Membrane modification
Induced autoimmunity
DAT in Autoimmune Hemolytic
Anemia (AIHA)
• Warm AIHA – 80% of all AIHA
–
–
–
–
99% have + DAT
20% have IgG only
67% have IgG + C3
13% have C3 only
• Cold AIHA – 18% of AIHA
– C3 only (antibody is IgM)
• Paroxysmal Cold Hemoglobinuria – 2%
of adult AIHA
– C3 only or negative DAT
Direct Antiglobulin Test
• Transfusion Reaction work-up
– DAT to detect antibody coating only donor
cells in post sample
• Immediate hemolysis
– Prior work up missed detectable antibody
• Delayed hemolysis
– Antibody was below detectable levels on
pretransfusion testing
Direct Antiglobulin Test
• Done with newborn type and screen
– Hemolytic disease of the newborn
• Maternal IgG antibody coating infant’s red
cell
– Anti-A, -B, -A,B
– Alloantibody
– Usually not due to Rh Immune Globulin
Direct Antiglobulin Test
• A positive DAT does not necessarily mean
hemolysis is really occurring.
• A large number of hospitalized patients (830%) have a positive DAT but no evidence
of hemolysis.
• Frequency of + DAT increases with
concentration of immunoglobulin, and thus
presence of inflammation.
• About 1% of autoimmune hemolytic
anemia will have a negative DAT.
Direct Antiglobulin Test
• Useful in confirming immune-mediated
hemolysis from a variety of causes.
• In combination with other tests, may help
define cause of hemolysis.
• NOT indicative of hemolysis by itself.