Background

The direct anti-human globulin test (DAT) detects coating of red blood cells (RBC) with immunoglobulins (Ig) and/or complement factors. A positive DAT can be harmless as it may occur in healthy donors. If clinical significance is suspected and IgG coating is found, an elution is recommended. The value of weakly positive DAT for alloantibody elution in transfused patients was found using the classical tube testing. We reevaluated this in the era of the more sensitive gel card DAT method.

Methods

Elution procedures of 107 samples (of 90 patients) were evaluated for a period of 6 months in a university hospital. Acid elutions (BAG, Germany) were performed for either (A) first time diagnosis of warm autoantibodies; (B) detection of RBC-bound new alloantibodies within 14 days since the last transfusion, or (C) in DAT negative samples when haemolysis was evident or reported within 14 days since the last transfusion. Eluates were analysed by antibody screening (gel centrifugation) and differentiation as well as isoagglutinin screening where appropriate. DAT results were classified according to the agglutination strength in negative (n=7), weakly positive (up to 2+; n=71), or strongly positive (3+ or 4+; n= 27).

Results

36 eluates from 30 patients (33%) were reactive. 24 patients had autoantibodies. One patient had a passenger lymphocyte syndrome caused by Anti-A after an allogeneic haematopoietic stem cell transplantation (recipient AB, donor B). This and two novel Anti-K and Anti-Fy(a) antibodies were found exclusively in the eluates and not in the plasma (informative eluates). Three more patients had new alloantibodies (Anti-Fy(a), Anti-Cw, and Anti-Lu(a), respectively) in the plasma, with similar or incomplete reaction pattern in the eluate. Reactive eluates were found in 16 (59%) of the strongly positive, 19 (27%) of the weakly positive and in 1 (14%) negative DAT. All six RBC-bound new alloantibodies or isoagglutinins had weakly positive DAT.

Conclusion

Our study confirmed that weakly positive DAT rather than strongly positive DAT have the potential to reveal new alloantibodies. Therefore, elution of weakly positive DAT remains useful even in the gel card era. The study also confirmed that a reactive eluate is most likely when the DAT is strongly positive. The efforts of elution in weakly positive DAT samples should be focussed on promising cases. As a conclusion, we advise clinicians to specifically consider and request an elution if needed.

Offenlegung Interessenkonflikt:

AR acknowledges research support (not for the data presented here) by Bio-Rad DiaMed and Sanofi-Aventis.