Editing Coombs test (section)

===Laboratory method===
The IAT is a two-stage test. (A cross match is shown visually in the lower half of the schematic as an example of an indirect Coombs test).{{cn|date=November 2021}}

====First stage====
Nonpatient, washed [[red blood cells]] (RBCs) with known antigens are incubated with patient serum containing unknown antibody content.  If the serum contains [[antibodies]] to antigens on the RBC surface, the antibodies will bind to the surface of the RBCs.{{cn|date=November 2021}}

====Second stage====
The RBCs are washed three or four times with isotonic saline solution and then incubated with antihuman globulin.  If antibodies have bound to RBC surface antigens in the first stage, RBCs will agglutinate when incubated with the [[Coombs test#Coombs reagent|antihuman globulin]] (also known Coombs reagent) in this stage, and the indirect Coombs test will be positive.

====Titrations====
By diluting a serum containing antibodies the quantity of the antibody in the serum can be gauged.  This is done by performing [[Serial dilution|serial dilutions]] of the serum and finding the maximum dilution of test serum that is able to produce agglutination of relevant RBCs.{{cn|date=April 2022}}