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Direct Antiglobulin Test
Direct antiglobulin test (DAT [e.g., Coombs’]) detects in vivo sensitization of RBCs with immunoglobulins and/or complement. DAT is useful in investigation of acute and delayed hemolytic transfusion reaction (AHTR and DHTR), hemolytic disease of the fetus and newborn (HDFN), autoimmune hemolytic anemia (AIHA), and drug-induced immune hemolysis (DIHA) ( Table 23.1 ). Positive DAT can occur with or without hemolysis. There are many causes of positive DAT; interpretation of a positive or negative result should include the patient’s history, clinical data, and results of other laboratory tests. Laboratory test results demonstrating evidence of increased RBC destruction include anemia, increased reticulocytes, increased LDH, decreased haptoglobin, hemoglobinemia, and hemoglobinuria. DAT can help differentiate immune from nonimmune causes of hemolysis. DAT may be positive in healthy individuals. Positive DATs are reported in 1:1000 up to 1:14,000 healthy blood donors, and 1:6 to 1:100 of hospitalized patients without signs of hemolysis or hemolytic anemia.
Table 23.1
Causes of a Positive Direct Antiglobulin Test
| Autoantibodies (warm autoimmune hemolytic anemia [WAIHA], cold agglutinin disease) |
| Alloantibodies to recently transfused antigen-positive RBCs (acute or delayed hemolytic or serologic transfusion reaction) |
| Passively transfused alloantibodies against the patient’s RBCs resulting from plasma-containing components (plasma, platelets) or a plasma derivative (intravenous immunoglobulin or Rh immune globulin) |
| Maternal alloantibodies coating the fetal RBCs (hemolytic disease of the fetus and newborn) |
| Drug-dependent antibodies reactive with drug-treated RBCs (penicillin, cefotetan) |
| Drug-dependent antibodies reacting with untreated RBCs in the presence of drug (ceftriaxone, piperacillin) |
| Drug-independent antibodies, indistinguishable from WAIHA (fludarabine, cladribine, methyldopa, levodopa, procainamide) |
| Nonimmunologic protein adsorption (cephalothin, diglycoaldehyde, cisplatin, oxaliplatin, clavulanate, tazobactam) |
| Antibodies derived from passenger lymphocytes as a result of either solid organ or hematopoietic stem cell (HSC) transplantation |
| Elevated levels of IgG or complement in patients with sickle cell disease, β-thalassemia, renal disease, multiple myeloma, autoimmune disorders, AIDS, etc. |
Indications
Antibody Identification
DAT is performed as part of the evaluation of unexpected RBC antibodies involved in AIHA (HDFN, AHTR, DHTR, and delayed serologic transfusion reaction [DSTR]). DAT and eluate may aid in antibody identification. Newly formed antibody can be bound to the circulating transfused cells and only detected in the eluate.
Autoimmune Hemolytic Anemia
A DAT is performed to determine whether hemolysis has an immune basis due to IgM or IgG. This differentiation is important because treatment may vary. Positive DAT with IgG (with or without complement) and panagglutinin in the eluate sample is consistent with diagnosis of warm autoimmune hemolytic anemia (WAIHA). DAT positive with complement only is seen in patients with cold agglutinin disease ( Chapter 51 ).
Drug-Induced Immune Hemolytic Anemia
DAT can be used to evaluate the presence of DIHA. Eluates in these cases are commonly nonreactive ( Chapter 51 ) unless the cells are incubated with the drug itself.
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