A newborn with jaundice and hepatosplenomegaly is born to an Rh-negative mother sensitized during a previous pregnancy. The baby's red cells are tested with anti-human globulin reagent directly, without prior incubation with maternal serum, and agglutination occurs. What does this result represent?

• A Direct Coombs test positive, showing antibody already bound to fetal red cells ✓
• B Indirect Coombs test positive, showing free maternal antibody in fetal serum
• C Direct Coombs test negative, excluding hemolytic disease
• D Indirect Coombs test positive, confirming ABO incompatibility

Explanation

The direct antiglobulin (Coombs) test detects antibody or complement already bound to red cells in vivo: the patient's washed red cells are mixed directly with anti-human globulin, and agglutination indicates in vivo sensitization, as in hemolytic disease of the fetus and newborn. The indirect Coombs test instead incubates normal red cells with the subject's serum to detect free circulating antibody, and is used for antenatal screening of the mother and pre-transfusion testing. The direct test being positive here confirms antibody-coated fetal erythrocytes.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.