Managing a healthy blood donor who has a positive direct antiglobulin test (DAT)
A colleague in South Africa reports that they have discovered a blood donor who is direct antiglobulin test (DAT) positive, but who has a negative indirect antiglobulin test (IAT), and who has no evidence of obvious hemolysis. A review of the patient's clinical condition reveal no obvious cause for the positive DAT. This individual was deferred as a red cell donor but was inadvertently accepted as an apheresis platelet donor. The South African colleague wonders if this donor should be disqualified for additional platelet donations, on the grounds that the donor's plasma might contain antibodies that can cause problems for a recipient? The South African colleague is not aware of any clinical problems experienced by the recipients of this donor's prior donations.
The following responses have been received.
- The above scenario was previewed by a colleague who directs both a red cell immunohematology laboratory and a platelet serology laboratory in Southern California. Here is what that colleague had to say (verbatim): "Approximately 1 in 1500 - 3000 donors have a positive DAT, without any signs of a hemolytic anemia. As blood centers and many hospital do not perform DATs/autocontrols or AGT crossmatches, such donor RBCs (and plasma) are being transfused relatively commonly. Although the DAT+ unit would not be acceptable for most physicians, I personally feel it is too conservative to permanently defer the donor (even as a RBC donor); the DAT may well be negative on the next testing. I see no reason to defer the donor as an apheresis donor (especially as there are no detectable serum antibodies). The DAT should not be used as a screening test for disease, e.g., when used as a screen for autoimmune hemolytic anemia (AIHA), the predictive value of a positive DAT is only 1.4%. The DAT should be used to support an immune etiology in a patient with hemolytic anemia. We all have IgG on our RBCs, and one sometimes would expect to find weakly positive DATs in healthy individuals (i.e., Gaussian distribution). I realize that a coincidental finding of a positive DAT (e.g., in crossmatching) may lead to a diagnosis of an associated disease (e.g., SLE or AIHA), but the literature suggests that this is very rare. If the DAT is strongly (3-4+) positive, there may be more reason to investigate the donor further."
- Editor's note: I would defer the donor as long as his DAT is positive, not over fear of transfusing antibodies (which appear to be lacking; the IAT for red cell antibodies is negative, although no test to rule out anti-platelet antibodies was reported having been done), but because the donor may be brewing an illness that is manifesting itself by a positive DAT.
ADDENDA Aug. 5, 2003
- A colleague in Spain agrees with the colleague from Southern California (response #1 above) that a DAT should not be used as a screening test for disease. He reports that they recently performed a study (Cid J et al., (Immunohematology 2003;1916-8) about the current use of the DAT in a hospital setting, and they saw that the DAT was being used in her hospital as a screening tool. In fact, a complete laboratory evaluation of suspected hemolytic anemia was seen in only 179 (38.7%) of 434 cases in which they performed a DAT. They did observe that the incidence of a positive DAT was higher in patients with signs of hemolysis than in those without signs of hemolysis. He adds, however, that even though he agrees that the DAT should not be used as a screening test for disease, if a blood donor is discovered with a positive DAT, such a donor is not used in the blood bank where he is working.
ADDENDA Aug. 14, 2003
- A colleague affiliated with the National Blood Service in England reports that all donations in that country are screened for auto-antibodies by running a 1:50 dilution of donor plasma in Phosphate Buffered Saline against a 2% suspension of donor red cells through the Olympus system. According to the reporting colleague (verbatim) "The reactivity rate is low; any red cells which do react are tested by DAT; most are positive. Other studies have shown that the range of DAT positivity due to IgG in donations has ranged from about 1 in 1000 to 1 in 25,000, depending on the sensitivity of the DAT testing system. Although many of our hospital blood banks have abandoned IAT cross-matching, those that still perform it increasingly rely on column agglutination methods which may, in practice, be more sensitive than the classical tube IATs. These hospitals send back to the NBS several DAT positive donations each month which are detected by reactivity in the IAT cross-match when no alloantibodies are found in patient serum. The inference is that hospitals which have abandoned IAT crossmatching (when the antibody screen is negative) either for computerised matching, or for a rapid spin match, will miss DAT positive donations. This has been going on for years and so far no adverse effects from the inadverdent transfusion of DAT positive red cells have been noted. This may be because of lack of awareness of this potential risk, but also favors the continued trend away from IAT matching. There is a case for surveillance of recipients of immediate spin or computerised matched blood; but I strongly suspect that there is no significant rate of adverse effects on patients as a result of receiving blood which is undiagnosed IgG DAT positive. I suggest referring to Issitt and Anstee's text, 4th edition, p 1015."
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