Coomb’s test : Mnemonic

Components of Coomb’s test:

Mnemonic: ABCD

  1. A: Antibody (to be detected)
  2. B: Blood antigen (RBC)
  3. C: Coomb’s reagent (Anti-antibody)
  4. D: Detect agglutination
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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)

  1. Patient component: RBC +/- Antibody
  2. Coomb’s reagent: Anti-antibody
  3. 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)]

  1. Patient component: +/- Antibody (serum)
  2. Reagent: donor RBC + Coomb’s reagent (Anti-antibody)
  3. Result: Agglutinates if Antibody present in serum (Positive)

Clinical Examples

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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