Direct antiglobulin test is used to demonstrate in vivo coating of red blood cells with IgG antibodies and complement (C3d). The assay uses Coombs reagent incubated with the patient’s washed red blood cells. [1]
Normal findings: Negative; no agglutination [2]
A direct antiglobulin test is positive in the following situations:
Warm autoimmune hemolytic anemia (SLE, Evan’s syndrome, idiopathic) Hemolytic disease of the newborn Alloimmune reactions to recently transfused cells, and Drug-induced hemolysis (alpha methyldopa, levodopa, quinidine or high dose of penicillin etc).Medications may cause the formation of antibodies, either against the medication itself or against intrinsic red blood cell antigens resulting in a positive DAT, immune red cell destruction, or both. Some of the antibodies produced appear to be dependent on the presence of the drug (eg, penicillin, quinidine, ceftriaxone), whereas others are independent of the continued presence of the inciting drug (eg, methyldopa, levodopa, procainamide, cephalosporins, fludarabine). [3]
A negative antiglobulin test can result in the following cases:
Hemolytic anemias caused by intrinsic red cell defect (eg, hemoglobinopathies or G6PD)See the list below:
Specimen: Blood anticoagulated with EDTA Container: Lavender or red-top vacuum tubeAll samples must be sent in a sealed, leak-proof container marked with a biohazard sticker to comply with Occupational Safety and Health Administration (OSHA) safety standards.
Clinical features of hemolysis help to differentiate the hemolytic anemia from other anemias. Clinical characteristics of hemolysis include pallor, icterus, and discoloration of urine, and, in some cases, hepatosplenomegaly. Bony changes due to overactivity of the bone marrow (frontal bossing) may occur, although they are never as severe as they are in thalassemia. [4]
The laboratory characteristics of hemolytic anemias are related to hemolysis itself and the erythropoietic response of the bone marrow. Features of hemolysis include increase in unconjugated bilirubin and aspartate transaminase in serum; and urobilinogen both urine and stool. [4] In case of intravascular hemolysis, increased hemoglobin in serum resulting in hemoglobinuria, increase in lactate dehydrogenase, and reduced haptoglobin are notable laboratory features. However, serum bilirubin level may be normal or only mildly increased. The main sign of the erythropoietic response from the bone marrow is an elevated reticulocytes count. [4]
Direct antiglobulin test is used to demonstrate in vivo coating of red blood cells with IgG antibodies and complement (C3d). The assay uses Coombs reagent incubated with the patient’s washed red blood cells. [1]