Why is an AHG (serologic) crossmatch required when the antibody screen is positive?

A Clinical Laboratory Scientist (no region specified) asks why is an AHG (serologic) crossmatch required when the antibody screen is positive? If the antibody is identified and the units are screened for the antigen why isn’t an immediate spin (IS) crossmatch adequate? The CLS states that this seems like a basic question, but had trouble finding the reasoning behind this requirement in the literature.

Editors’ Notes: AABB standard 5.15.1 states that a crossmatch must demonstrate ABO incompatibility and clinically significant antibodies to red cell antigens and shall include an antigloblin test.

There is a large literature over the past several decades discussing omission of the serologic crossmatch if the patient has a negative antibody screen and no history of clinically significant antibodies. If the patient’s antibody detection test is negative, the risk of a hemolytic reaction is very low. The discussion link and references may be germane to the discussion:

2009 e-Network Forum discussion: Is a computer crossmatch in the absence of an immediate-spin antibody screen adequate for persons identified to be at increased risk of forming new blood group antibodies?
Oberman HA. The present and future crossmatch. Transfusion 1992;32:794-5.
Meyer EA, Shulman IA. The sensitivity and specificity of the immediate-spin crossmatch. Transfusion 1989;29:99-102.
Shulman IA, Odono V. The risk of overt acute hemolytic transfusion reaction following the use of an immediate-spin crossmatch. Transfusion 1994;34:87-88.

The reasons for performing the serologic crossmatch in a patient with a positive screen may not be explicit in the literature but is long-held practice due the following:

Patients who have a history of forming one RBC antibody have a much greater chance of forming additional RBC antibodies.
The serologic crossmatch (although not 100% sensitive) can identify antibodies that otherwise may have been missed. If the serologic crossmatch with an antigen negative unit is positive unexpectedly, either there is a problem with the antigen typing or the patient has an additional antibody not identified on the antibody screen.

Reference:

Schonewille H, van de Watering LM, Brand A. Additional red blood cell alloantibodies after blood transfusions in a nonhematologic alloimmunized patient cohort: is it time to take precautionary measures? Transfusion 2006:46;630-5.

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Ira A. Shulman, MD
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