A method of testing for incompatibility between plasma antibodies and target RBC antigens. Solid phase testing occurs in small microwells, where antigens are bound to the bottom of the well and the patient’s plasma is incubated in the well. Any incompatibility is detected by the addition of anti-human globulin (AHG) that has an indicator red blood cell attached to the Fc portion of each AHG antibody. In positive reactions, the indicator RBCs are seen spread all over the bottom of the microwell in a diffuse “carpet,” while in a negative reaction, the indicator RBCs will simply slide to the bottom of the well and look like a dense button (you should familiarize yourself with the positive and negative appearances in solid-phase testing, because if you have not seen them before, you would be fooled and switch positive and negative interpretations; I discuss this in my pretransfusion testing video). Solid-phase testing, like gel testing, is considered more sensitive than tube testing, and it can be used with automated instruments from different manufacturers.