In the antibody screen shown below, cell 3 meets the criteria to be used as a rule-out cell.
Using the logic that if the rule-out cell is positive for a given antigen, it should have reacted with the corresponding antibody, the technologist can rule-out antibodies that correspond to antigen-positive cells. For example, cell 3 in the screen above is positive for big K. If anti-K were present, it should have reacted. It did not react. Therefore, anti-K is most likely NOT present and can be presumptively ruled-out.
To increase the probability that rule-out will not mistakenly eliminate a weakly-reacting antibody that exhibits dosage, most laboratories will rule-out using only cells that are homozygous for the corresponding antigen for those systems that generally show dosage. Generally these include: C, c, E, e, Fya, Fyb, Jka, Jkb, M, N, S, and s. For example, in the screen above, cell 3 is homozygous positive for c (c positive, C negative). Since the cell did not react, anti-c can be presumptively ruled-out. Exemptions are usually made for low-prevalence antigens, which are rarely expressed as homozygous (K, Kpa, Jsa, Lua).
Using these principles, in the antibody screen example above, the antibodies highlighted in red are presumptively ruled-out: