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, k, 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). Cells that do not have their zygosity expressed on the antigram (D, P1) are eligible to be ruled out any time they are positive on a non-reacting cell. During initial screening procedures, it is recommended that at minimum the following antibodies be ruled out: Anti-D, -C, -c, -E, -e, -K, -k, -Fya, -Fyb, -Jka, -Jkb, -Lea, -Leb, -P1, -M, -N, -S and -s. Some facilities may rule out additional antibodies during initial screening (Anti-Lua, -Lub, -Kpa, etc.).
Using these principles, in the antibody screen example above, the antibodies highlighted in red are presumptively ruled-out: