The microtiter plate method involves serial dilutions of the Coombs reagent from 1:2 to 1:2048 added to washed RBCs from the patient. In this dog with immune-mediated hemolytic anemia, the Coombs test is positive grossly (clumped cells in wells 1-6) and microscopically in all wells (left image: 1:2 dilution or first well; middle image: 1:2048 dilution or last well, arrow shows small agglutinates versus those present in the 1:2 dilution; right image: The auto-control has no agglutinates). The auto-control consists of patient RBCs without Coombs reagent.
