The last of the three so-called “phases” or “stages” of testing serum and red cells for compatibility, most commonly in test tubes (the first two are the immediate spin and 37^oC phases). In classic tube testing, the AHG phase occurs after the serum and red cells have undergone an incubation at 37^oC followed by a washing step to remove free antibodies. During this step, we add anti-human globulin (yes, that’s where “AHG” comes from) to the serum-RBC mix, centrifuge the tube, and examine to see if the AHG agglutinates red cells that were coated with antibody (usually IgG) in earlier steps. This process is really just an indirect antiglobulin (Coombs) test. Most clinically significant antibodies will react at the AHG phase (and some may react at 37^oC as well).