Clinical utility of the Coombs test

The article by J. Manuel Zarandona and Mark Yazer on the clinical utility of the Coombs test1 prompts consideration of the pitfalls of a negative test result. Specifically, further evaluation will be required if a patient with clinically significant hemolysis has a negative result on Coombs testing.

A negative result on the direct antiglobulin test (DAT) may occur if there are low-affinity antibodies coating the erythrocyte surface that are removed in the washing process or by prior use of corticosteroid therapy. IgG antibodies and C3d complement are detected by the DAT; however, IgM and particularly IgA antibodies may not be detected by the polyclonal antihuman-globulin serum. Therefore, DAT using anti-IgA or anti-IgM antibodies may be required if the results of DAT with anti-IgG antibodies do not correlate with the clinical manifestations. Patients whose erythrocytes are coated with multiple immunoglobulins will have severe hemolysis and may need intensive therapy with immunosuppressive agents2 or even splenectomy.3

At a major immunohematology referral centre in the United Kingdom, 124 of 5235 patients had warm-reactive elutable IgA antibodies and of these, 6 had only IgA autoantibodies coating erythrocytes.4 It is known that IgA complexes can activate the alternative pathway, causing complement activation. Therefore, measurement of complement components C3 and C4 could prove useful in some situations. Splenic sequestration of IgA-sensitized erythrocytes may occur, and in-vitro analysis has shown that monocytes are involved in hemolysis of the sensitized erythrocytes.5 Naturally occurring antibodies of the IgM isotype, which bind complement and autoantibodies of this class, occur in conjunction with anti-IgG and or anti-IgA immunoglobulins. In-vivo autoagglutination of IgM antibodies can lead to multiorgan failure and is usually associated with a high mortality rate.6 Sokol and colleagues7 described 2 patients with only IgM autoantibodies who had chronic hemolysis, one of whom required splenectomy.

It is important that clinicians be aware of the limitations of biological tests and investigate further when a patient with severe hemolysis and organ involvement has a negative Coombs test result.