From the Guidelines
A positive IgG Coombs test indicates the presence of antibodies against red blood cells, suggesting potential autoimmune hemolytic anemia, hemolytic disease of the newborn, or risk of hemolytic transfusion reactions. This test is particularly relevant in patients with chronic lymphocytic leukemia, non-Hodgkin’s lymphoma, and those with a history of autoimmune disease, as stated in the 2007 American Society of Clinical Oncology/American Society of Hematology clinical practice guideline update 1. The Coombs test, also known as the indirect antiglobulin test, detects antibodies against red blood cells in a person's serum, which could be naturally occurring or developed due to previous blood transfusions, pregnancy, or certain autoimmune conditions.
Key points to consider with a positive IgG Coombs test include:
- The potential for hemolytic transfusion reactions if the patient receives incompatible blood
- The possibility of hemolytic disease of the newborn in pregnant women with antibodies against fetal blood cells
- The indication of autoimmune hemolytic anemia where the body attacks its own red blood cells
- The need for follow-up testing to identify the specific antibody and determine its clinical significance, as not all antibodies cause immediate or severe reactions
In clinical practice, a positive IgG Coombs test warrants careful consideration of the patient's history, including previous blood transfusions, pregnancies, and the presence of autoimmune diseases, as well as a thorough review of the peripheral blood smear and other diagnostic tests aimed at identifying causes of anemia 1. The test works by detecting these antibodies when they bind to test red blood cells in the laboratory, using anti-human globulin reagent to visualize the reaction. Given the potential implications of a positive result, it is essential to approach each case with caution and consider the clinical context in which the test is performed.
From the Research
Positive IgG Coombs Test
A positive IgG Coombs test indicates the presence of IgG antibodies on the surface of red blood cells. This can be a sign of autoimmune hemolytic anemia (AIHA), a condition in which the immune system produces antibodies that attack and destroy the body's own red blood cells.
Causes of Positive IgG Coombs Test
- Low-affinity IgG autoantibodies that are not detectable by standard direct antiglobulin tests 2, 3
- IgG sensitization below the threshold of detection by commercial antiglobulin reagents 3
- Red cell sensitization by IgA alone, or rarely, monomeric IgM alone, but not accompanied by complement fixation 3
Diagnosis and Detection
- The Coombs test, also known as the direct antiglobulin test (DAT), can detect IgG antibodies on red blood cells 4
- Flow cytometry can be used to detect low levels of IgG sensitization and is more sensitive than the conventional tube technique (CTT) 5
- The gel test (GT) can also detect red cell autoantibodies and is a good alternative to flow cytometry in resource-poor settings 5