In the solid phase method, manufactured microtiter wells, as shown in the image below, are lined with reagent donor red blood cells. Patient serum or plasma is added to each well along with an enhancing reagent like low ionic strength solution (LISS). The wells are incubated at 37°C, and washing is then required.
Recall in the test tube method, antihuman globulin (AHG) is introduced into the test environment post-washing to promote crosslinking between IgG and the target antigen. Conversely, in the solid phase method, the antigen source is affixed to the microtiter well's wall, enabling a distinct form of crosslinking across the well's layer. Instead of AHG, indicator cells (cells coated with anti-IgG) are added.
If sensitization between the patient's antibody and the reagent red blood cells occurs, indicator cells react with the antibody coating the well and form a diffuse pattern. The presence of a diffuse pattern, which is evaluated on a graded scale, indicates a positive reaction suggesting a high likelihood that an antibody is present. If no sensitization occurs, indicator cells are unable to crosslink the reaction and as a result form a pellet at the bottom of the well. The presence of a red cell button surrounded by a clear background is a negative reaction indicating no clinically significant antibodies are present. The image below illustrates negative and positive reactions.
Again note that a negative reaction signifies no detectable antibody/interaction was recorded, it does not signify the complete absence of antibodies. Each method has limitations.
