Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (901 page)

   Ratio between 1.2 and 1.6: LA may be present, but in a low titer.

   Limitations

   LA antibodies may vary in their properties, and the results may be positive in the dRVVT assay but not in other assays. Because of that, at least two type of assays have been recommended for each patient.

   Heparin levels >1 U/mL prolong the first-stage assay.

Suggested Reading

Lambert M, Ferrard-Sasson G, Dubucquoi S, et al. Dilute Russell viper-venom time improves identification of antiphospholipid syndrome in a lupus anticoagulant-positive patient population.
Thromb Haemost.
2009;101:577–581.

DIRECT AND INDIRECT ANTIGLOBULIN TESTS (DAT AND IAT)
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   Definition

   Previously known as the direct and IATs, these assays play a major role in transfusion medicine as well as in the diagnosis of immune hemolytic anemias (see p. 377), because they detect antibodies either bound to RBCs (the DAT), or in serum (the indirect antiglobulin test, IAT). In patients that have not been transfused within the preceding 3 months, a positive DAT almost always reveals autoimmune antibodies.

   The IAT is used to demonstrate in vitro reactions between RBCs and antibodies that sensitize red cells that express the corresponding antigen. The patient’s serum or plasma is incubated with red cells, which are then washed to remove unbound globulins. Agglutination that develops when the antiglobulin reagent is added indicates a reaction between serum antibodies (usually the result of immunization from previous transfusions) and red cells.

   The antiglobulin reagent consists in most cases of rabbit antibodies directed against human IgG. Other reagents used in the DAT assay are anticomplement (anti-C3dg) or a mixture of anti-IgG and anti-C3dg. If the DAT is positive following recent transfusions, the antibodies can be eluted from RBCs and the eluted antibodies must be identified.

   Use

   The DAT is used whenever hemolysis of red cells is suspected as being caused by autoantibodies. The assay determines if red cells have been coated in vivo with immunoglobulins, complement, or both.

   The utility of the IAT in blood banking stems from its great sensitivity in detecting various IgG antibodies in the recipient’s serum prior to transfusions. It is part of the antibody screening test. It is used to detect the presence of alloantibodies directed against non-ABO blood group antigens.

   In cases of severe autoimmune hemolytic anemia, both the DAT and the IAT may be positive because the excess antibodies elute from the red cell membranes and spill out into serum.