American Board of Internal Medicine (ABIM) Certification Practice Exam

A positive direct antiglobulin test indicates which condition?

1. Iron deficiency anemia

2. Warm autoimmune hemolytic anemia

3. Chronic lymphocytic leukemia

4. Thrombocytopenia

The correct answer is: Warm autoimmune hemolytic anemia

A positive direct antiglobulin test (DAT), also known as a direct Coombs test, indicates that there are antibodies or complement proteins attached to the surface of red blood cells. This situation is typically seen in conditions involving immune-mediated hemolysis, where the body's immune system mistakenly targets and destroys its own red blood cells. Warm autoimmune hemolytic anemia is a classic example of this type of condition, as it is characterized by the production of antibodies (usually of the IgG type) that act against the patient's own red blood cells at body temperature. As a result, a positive direct antiglobulin test is a key diagnostic feature in identifying the presence of warm autoimmune hemolytic anemia. While iron deficiency anemia may lead to hemolysis in specific cases, it is not primarily associated with a positive DAT, as the underlying mechanism is different from that seen in warm autoimmune hemolytic anemia. Chronic lymphocytic leukemia and thrombocytopenia do not typically involve immune-mediated hemolysis, and therefore they do not correlate with a positive direct antiglobulin test in the same way. Thus, the association between a positive DAT and warm autoimmune hemolytic anemia is clear and crucial for diagnosis in appropriate clinical contexts.