Samples of Coombs
Last reviewed: 23.04.2024
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Normally, there are no antibodies to red blood cells in the blood.
Direct Coombs test - antiglobulin test (agglutination in the gel, allows to identify complete bivalent antibodies), by which antibodies of the IgG class and the C3 complement component on the erythrocyte surface are determined. Typically, antibodies detected by direct Coombs breakdown have a wide specificity that is not associated with a well established antigen. Positive direct Coombs test clearly indicates the presence of hemolytic anemia in the patient, although not all patients with a positive direct antiglobulin test suffer from this disease. Approximately 10% of patients with antibodies or complement components on the erythrocyte membrane fail to detect a direct breakdown of Coombs (negative test), but nevertheless they suffer from autoimmune hemolytic anemia. To clarify the specificity of antibodies in such cases, tests with their elution are used. A direct Coombs test, positive only for complement, usually has to do with cold antibodies such as IgM. In this case, IgM antibodies are not present on erythrocytes at basal body temperature. However, due to the fact that IgM antibodies actively fix complement, and complement remains on erythrocytes, with this form of autoimmune hemolytic anemia (cold agglutinin disease), the Coombs test will be positive only to complement.
Direct Coombs test is positive for autoimmune hemolytic anemia caused by thermal antibodies, autoimmune drug anemia (with a methyldopa up to 20% of patients have a positive reaction), drug-adsorption type of hemolytic anemia, immunocomplex type of hemolytic anemia (the sample is positive only for C3), with autoimmune hemolytic anemia caused by cold antibodies - diseases of cold agglutinins (the sample is positive only in relation to C3). With paroxysmal cold hemoglobinuria, the direct Coombs test is negative.
Indirect Coombs test - indirect antiglobulin test (reveals incomplete antibodies) allows to identify atypical antibodies in blood, including alloantibodies, to foreign antigens of erythrocytes. Its name (indirect), it received because of what happens in 2 stages. Initially, the blood serum of the patient, containing incomplete antibodies, interacts with the added corpuscular Arg diagnostics without visible manifestations. In the second stage, the introduced antiglobulin serum interacts with incomplete antibodies adsorbed on antigens, with the appearance of a visible precipitate. Transfusion of homologous (allogeneic) erythrocytes or pregnancy is the most frequent reason for the formation of these anti-erythrocyte antibodies. The combination of a positive indirect Coombs test with negative direct breakdown does not provide anything for the diagnosis of autoimmune hemolytic anemia. Positive indirect Coombs test causes certain difficulties in the selection of blood for transfusion and the conduct of a cross sample for compatibility with canned blood, but has no other diagnostic value.
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