A patient who was Rh-negative develops a positive direct antiglobulin test due to anti-D. What is the most likely cause?

A patient who was Rh-negative develops a positive direct antiglobulin test due to anti-D. What is the most likely cause?

• A. Autoimmune anti-D
• B. Alloanti-D from prior transfusion
• C. Maternal anti-D crossing placenta
• D. No anti-D; incorrect sample

Answer: (B)

Direct antiglobulin testing detects antibodies or complement bound to a patient’s red blood cells in vivo. If anti-D is responsible for the DAT, the antibody must be directed against the D antigen on the patient’s own RBCs. In an Rh-negative individual, anti-D only arises after exposure to the D antigen from someone else, which most commonly happens with a prior transfusion of Rh-positive blood. That exposure sensitizes the immune system and leads to formation of alloanti-D, which then coats circulating Rh-positive cells and yields a positive DAT. Autoimmune anti-D is unlikely because an Rh-negative person does not express the D antigen to target on their own cells. Maternal anti-D crossing the placenta would affect a fetus or newborn, not the Rh-negative patient themselves, and would explain a positive DAT in the baby rather than the patient. The option of no anti-D would not fit a DAT known to be due to anti-D. Thus, the most likely cause is alloanti-D from prior transfusion.

Direct antiglobulin testing detects antibodies or complement bound to a patient’s red blood cells in vivo. If anti-D is responsible for the DAT, the antibody must be directed against the D antigen on the patient’s own RBCs. In an Rh-negative individual, anti-D only arises after exposure to the D antigen from someone else, which most commonly happens with a prior transfusion of Rh-positive blood. That exposure sensitizes the immune system and leads to formation of alloanti-D, which then coats circulating Rh-positive cells and yields a positive DAT.

Autoimmune anti-D is unlikely because an Rh-negative person does not express the D antigen to target on their own cells. Maternal anti-D crossing the placenta would affect a fetus or newborn, not the Rh-negative patient themselves, and would explain a positive DAT in the baby rather than the patient. The option of no anti-D would not fit a DAT known to be due to anti-D.

Thus, the most likely cause is alloanti-D from prior transfusion.