If the antibody screen is positive, the specificity of the antibody is identified by testing the serum against a panel of 8 to 20 Group O RBCs of varying phenotypes. The pattern of positive and negative reactions with these cells identifies the antigen against which the antibody is directed. Antibody identification is accomplished by the “crossing out” method which consists of identifying each cell that is negative and crossing
out all of the antigens present on that cell. The panel should also be observed to: From this information, one can determine:
Thermal characteristic Pattern of Reactivity Autocontrol Interpretation Stronger at cold & weaker at warm temperature One or few cells positive Negative Consider cold alloantibody such as MN, P, Le, etc. Stronger at cold & weaker at warm temperature All cells positive Negative Consider Vel, Tja, etc. Stronger at cold & weaker at warm temperature All cells posiitve Positive Cold autoantibody such as anti-I Negative in cold & positive at warmer temperature One or few cells positive Negative Consider clinically significant alloantibody such as Rh, Kell, Duffy, Kidd, Ss, etc Negative in cold & positive at warmer temperature All cells positive Negative Consider alloantibody to high frequency antigen KPb, k, Lub, Jsb, Lan, Ge, Ata, U, etc Negative in cold & positive at warmer temperature All cells positive Positive Consider autoantibody with or without alloantibody Antibody Identification
Antibody Panel
Interpretation
If a clinically significant antibody is identified, only red cells negative for the relevant antigen will be selected for crossmatching and transfusion. For example, if the antibody is anti-K, RBC of the appropriate ABO and Rh type will be tested with anti-K anti-serum and only K-negative red cells will be selected for transfusion. For added safety, an AHG crossmatch is also performed. For clinically insignificant antibodies, it is permissable to crossmatch units that have not been antigen typed.
When the antibody screen is positive, additional time is required to identify the antibody(ies), to find antigen-negative red cells, and to perform AHG crossmatches. This time can range from an hour to days if multiple antibodies, antibodies against high frequency antigens, or a mixture of autoantiobody and alloantibodies are present. If transfusion is medically necessary before compatible blood can be obtained, the attending physician and the transfusion medicine physician need to discuss the risk:benefit ratio of transfusing potentially incompatible blood.
An autoantibody is produced against a person’s own red cells. When a patient has an autoantibody, the direct antiglobulin test and the autocontrol in an antibody panel will be positive. In addition, all cells in the panel will be reactive. If the antibody reactions are stronger at colder temperatures and weaker at warm temperatures, the patient probably has a cold autoantibody. If the antibody reactions are negative at colder temperatures and positive at warmer temperatures, the patient most likely has a warm autoantibody.
More properly called the “antibody detection test,” the antibody screen (as most blood bank types call it) is a test used to demonstrate the presence or absence of “unexpected (non-ABO) antibodies.” You can think of it as an initial test designed to predict whether the patient has antibodies that could be incompatible with donor red blood cells. It’s generally a “yes/no” proposition, meaning that it’s primarily designed to answer this question: Are there any antibodies in this person’s serum/plasma that could cause a problem when the person’s serum/plasma is mixed with someone else’s red blood cells? If the antibody screen is negative, that strongly suggests that the person can receive blood from someone else without further matching (other than ABO and RhD, of course!).
Antibody screens are performed as part of routine pretransfusion testing for blood recipients. In this test, serum or plasma is added to RBCs from between two and four group O persons specifically chosen by the manufacturer to carry antigens that could be the target of significant RBC antibodies. The test can be done using one of several platforms, primarily either tubes, gel testing, or solid-phase testing. If the antibody screen is positive, in most cases the next step would be to perform antibody identification. If the screen is negative, there is a very high likelihood that no significant antibodies are present (though some rare antibodies against low-incidence RBC antigens could still be present).
Important Caveat: Don’t let me catch you saying something silly like, “The antibody screen is negative, so there are no antibodies!” That is not correct; rather, you should say, “The antibody detection test is negative, so no antibodies are detected!” This test is not perfect (no test can be), so it is absolutely possible for significant antibodies (especially ones targeted against rare red cell antigens) to go undetected on routine tests.
Two important things to remember about antibody screening:
- Group O red cells are used to avoid interactions with ABO antibodies. Any incompatibility with the screen cells should be due to antibodies other than normally occurring ABO antibodies.
- Antibody screens are done on recipients, as mentioned, but many forget the requirement that they are also performed on every blood donor! Plasma-rich products from donors with significant antibodies cannot be used, but the RBCs may be used if labeled with the antibody specificity (but many hospitals will not accept such units).