In this mechanism, the drug is adsorbed directly onto the surface of the red blood cell (RBC). The antibody is formed against the drug itself. This results in drug-coated RBCs becoming coated with IgG. Complement is rarely involved. Sensitization of the RBCs can lead to increased RBC destruction. The drug most commonly associated with this mechanism is Penicillin, but only when dosing is higher than normal.
The DAT is strongly positive with anti-IgG. Antibody eluted from the red cells will only react with drug-coated red cells and not uncoated red cells. The antibody screen is usually negative unless an alloantibody is present. Crossmatches are compatible in all phases. The eluate is non-reactive.
Only a small percentage of patients will demonstrate hematologic complications. The anemia develops slowly because destruction occurs extravascularly. The patient usually improves once drug therapy has ceased.
Medications commonly implicated in the drug-adsorption mechanism include:
- Penicillin G
- Erythromycin
- Methicillin
- Carbromal
- Nafcillin
- Cefazolin
- Tetracycline
- Cefamandole
