Rule-Out Procedures, continued

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The page below is a sample from the LabCE course Antibody Detection and Identification. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Rule-Out Procedures, continued

In the antibody screen shown below, cell 3 meets the criteria to be used as a rule-out cell as it is negative (non-reactive) in all three phases of testing.
Rh-HrKellDuffyKiddLewisMNSsPLutheranXg
Donor Cell NumberRhDCcEeCwKkKpaKpbFyaFybJkaJkbLeaLebMNSsP1LuaLubXgaIS37AHGCC
1R1R1++00+00+000+0+00+0+++0+01+002+
2R2R2+0++000+00+0+0+0+++++0++02+3+NT
3rr00+0+0++00+0+00+0+0+0++00002+
IS = immediate spin; AHG = antihuman globulin; CC = check cells; NT = not tested
Using the logic that if the rule-out cell is positive for a given antigen, it should have reacted with the corresponding antibody, the blood bank scientist 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 minimize the risk that rule-out will not mistakenly eliminate a weakly-reacting antibody that exhibits dosage, most laboratories will rule out using cells that are only homozygous for the corresponding antigen for those systems that show dosage. Generally, these include Rh (C, c, E, e), k, Duffy (Fya, Fyb), Kidd (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 whose zygosity is not indicated 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 an FDA requirement 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, from the antibody screen example above, the antibodies highlighted in light orange are presumptively ruled-out using Cell 3:
Rh-HrKellDuffyKiddLewisMNSsPLutheranXg
Donor Cell NumberRhDCcEeCwKkKpaKpbFyaFybJkaJkbLeaLebMNSsP1LuaLubXgaIS37AHGCC
1R1R1++00+00+000+0+00+0+++0+01+002+
2R2R2+0++000+00+0+0+0+++++0++02+3+NT
3rr00+0+0++00+0+00+0+0+0++00002+
Note that K could be ruled out as it is one of the low-prevalence antigens that are rarely expressed homozygously. A heterozygous rule-out is often used for K. On the other hand, k can only be ruled out with a homozygous expression which is why it was not marked off.