An Rh-negative mother who is not immunised receives antenatal prophylactic anti-D immunoglobulin. Screening of maternal serum for free anti-D antibodies against indicator Rh-positive red cells, without using Coombs reagent, would be best performed by which technique?

• A Direct Coombs test on the mother's own red cells
• B Indirect Coombs test, since the antibody is incomplete and needs antiglobulin to cause visible agglutination ✓
• C Rose-Waaler test using sensitised sheep red cells
• D Widal tube agglutination with serial dilutions of maternal serum

Explanation

Anti-D is an IgG antibody that behaves as an incomplete antibody: it coats Rh-positive red cells but cannot bridge them into visible agglutination because its small size leaves the zeta potential intact. The indirect Coombs test adds antihuman globulin to bridge the coated cells, detecting free antibody in maternal serum. The direct Coombs tests cells already coated in vivo, such as fetal cells in haemolytic disease of the newborn, and is not used to screen maternal serum.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.