Direct antiglobulin test (Coombs test)

Used to detect autoimmune hemolytic anemia by screening a patient’s red blood cells for the presence of IgG or complement using broad spectrum reagents.

A positive test detects IgG or C3d on red blood cell surface.

The test involves washing the patient’s RBCs with saline to remove immunoglobulins and complement that are not bound to RBCs, leaving behind immunoglobulins and complement bound to RBCs.

The RBCs are mixed with an antihuman globulin reagent, containing polyspecific rabbit anti–IgG and monoclonal murine anti-complement antibodies.

In the presence of bound immunoglobulins and complement on RBC surface, the anti-human globulins produce cross-links between the patient’s RBC cells resulting in agglutination.

The results are reporting on a scale of 0 to 4+.

If the direct anti-globulin test is positive, it is repeated using monospecific antibodies to explicitly measure anti-IgG or anti-complement, usually C3d, reactivity.

Not an absolutely sensitive or specific tests for the presence of red blood cell antibody.

When the concentration of non-complement fixing immunoglobulin on the surface of the red blood cell is less than a few hundred molecules screening tests for hemolytic anemia may be negative.

When positive in a patients with hemolysis the diagnosis of an immune hemolytic anemia is supported in 83% of cases.

DAT is positive but not clinically significant in 7-8% of all hospitalized patients, even in those without evidence of hemolysis.

Its predictive value in random patients without evidence of hemolysis is very low at 1.4%.

Complement alone on red blood cells is seen in cold agglutinin disease and paroxysmal cold hemoglobinuria.

False positive results can occur due to spontaneous or RBC agglutination or technical issues such as insufficient washing, overcentrifugation, or clotted specimens.

Occurs in about 1 to 4 per 1000 adults annually.

DAT is positive but not clinically significant in 7-8% of all hospitalized patients, even those without hemolysis.

7-10% of patients with CLL develop autoimmune hemolytic anemia in their lifetime of which 90% is warm and 10% is cold.

Evidence of hemolysis with a low haptoglobin, high LDH, and high reticulocytosis and a positive IgG DAT indicates a warm autoimmune hemolytic anemia.

A warm AIHA can be primary or associated with a lymphoproliferative disorder such as CLL, autoimmune diseases, viral infections, and drugs.

Leave a Reply

Your email address will not be published. Required fields are marked *