The gel method, utilized in both manual and automated environments, involves the use of a card filled with a viscous substance to facilitate the reaction. The cards have microtubules filled with a dextran acrylamide gel containing anti-IgG. The gel acts as a filter for agglutinates. If antibody and antigen interaction leads to agglutination, the gel serves as a barrier, preventing the agglutinated red blood cells from moving to the bottom of the card.
Patient serum and reagent red cells are added to the microtubules. The card is incubated at 37°C for a predetermined length of time and subsequently centrifuged to facilitate the antibody/antigen interaction if present. Washing and check cells in the gel methodology are not necessary. If no agglutinates are present, the red cells fall freely through the viscous gel and form a pellet at the bottom of the gel, indicating that no clinically significant antibodies were detected. Agglutination indicates an antibody is detected. If large agglutinates are present, the red cells form a layer at the top of the gel. Small agglutinates are dispersed throughout the bottom of the gel. A gel card depicting the gradient of graded reactions from 0 (no agglutinates) to 4+ (high avidity) is displayed below. (MF stands for mixed-field reaction and will not be discussed here.)
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.
