Rule-out (also referred to as exclusion or cross-out) is a process by which antibodies are identified as being unlikely in a given sample due to the absence of an expected antigen-antibody reaction. In other words, the absence of a reaction is noted with a cell that is positive for the corresponding antigen. Antibodies that could not be responsible for the reactions seen are excluded or ruled-out.
Rule-out, while very useful, can lead to error. Ruling out an antibody should be combined with other supporting data to increase confidence in the solution; the more data collected, the higher the probability that the final solution is correct.
Non-reactive cells are selected for rule-out. To be classified as non-reactive, a cell must NOT have reacted in any phase of testing in a given panel or screen.
In the case of cold antibodies: if reactions are only occurring at immediate spin and are negative in the AHG phase, then that panel cell can be used as a rule out cell for IgG reactive antibodies but not for antibodies that react at immediate spin (IgM).
If there is no reaction with a panel cell then it is possible that antibodies to the antigens on that panel cell are not present in the sample being tested.