Components of Coomb’s test:
- A: Antibody (to be detected)
- B: Blood antigen (RBC)
- C: Coomb’s reagent (Anti-antibody)
- D: Detect agglutination
Direct Coomb’s/Antiglobulin test (DAT or DCT):
Mnemonic: In DAT/DCT we take A+B from patient and add C [(A+B) + C]
Another Mnemonic: DCBA (Direct Coomb’s detect Blood Bound Antibody)
- Patient component: RBC +/- Antibody
- Coomb’s reagent: Anti-antibody
- Result: Agglutinates if Antibody bound to RBC (Positive)
Indirect Coomb’s/Antiglobulin test (IAT):
Mnemonic: In IAT, we take A from patient and add B+C [A+(B+C)]
- Patient component: +/- Antibody (serum)
- Reagent: donor RBC + Coomb’s reagent (Anti-antibody)
- Result: Agglutinates if Antibody present in serum (Positive)
a. Direct Coomb’s test (DCT):
- Mix infant’s RBC with Coomb’s reagent: if cells agglutinate, maternal antibody is present
b. Indirect Coomb’s test (IAT):
- Prenatal testing: Mix mother’s serum with Rh (D)+ cells (antigen) and add Coomb’s reagent: cells agglutinate if there is anti-D antibody is present in mother (this may be due to Rh+ fetus or Rh+ blood transfusion)
- Rho-gam (anti-D antibody) is given to Rh- mother to neutralize circulating Rh antigen preventing the formation of antibody.
- Cross-matching: Mix recipient’s serum with donor’s blood with antigens and add Coomb’s reagent: cells agglutinate if there is antibody to donor blood antigen in recipeint’s serum.
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