A 65-year-old woman presents to the clinic with a 1-week history of painful, blue discoloration of her fingertips and toes that occurs when she is hanging laundry outside in the cold. She also notes that her urine is dark red in the morning. She recently recovered from an atypical pneumonia that was treated with azithromycin. Past medical history is otherwise unremarkable. Vital signs are within normal limits. Physical examination reveals mild scleral icterus and livedo reticularis on her extremities.
Laboratory studies reveal the following:
Hemoglobin: 9.5 g/dL
Reticulocyte count: 7%
Indirect bilirubin: 2.4 mg/dL
Lactate dehydrogenase (LDH): 510 U/L
Peripheral blood smear: Significant red blood cell clumping (agglutination)
Which of the following is the most likely result of a direct antiglobulin (Coombs) test in this patient?
The correct answer is:
B) Positive for C3 only
This patient is presenting with cold agglutinin disease (CAD), a form of autoimmune hemolytic anemia mediated by IgM antibodies. These antibodies typically react at temperatures below core body temperature (0–18°C). The patient's history of atypical pneumonia, such as that caused by Mycoplasma pneumoniae, is a classic trigger for the production of these cold-reacting IgM antibodies. The pathophysiology involves IgM binding to red blood cells in the cooler peripheral circulation (fingers, toes, nose), where it fixes complement (C3b). When the red cells return to the warmer core of the body, the IgM antibody dissociates from the cell. However, the C3b remains attached. The direct antiglobulin test (DAT) detects this remaining complement. Therefore, the test is typically positive for C3 but negative for IgG. The hemolysis is primarily extravascular, occurring as the C3b-coated cells are cleared by the liver.
Answer choice A: Negative for both IgG and C3, is incorrect. A negative DAT would point toward non-immune causes of hemolysis, such as microangiopathic hemolytic anemia (TTP/HUS) or paroxysmal nocturnal hemoglobinuria (PNH).
Answer choice C: Positive for IgA only, is incorrect. IgA-mediated hemolysis is an extremely rare clinical entity and is not associated with the classic cold-induced symptoms or Mycoplasma infection described here.
Answer choice D: Positive for IgG only, is incorrect. This is the classic finding in warm autoimmune hemolytic anemia (WAIHA). WAIHA is usually associated with systemic lupus erythematosus or chronic lymphocytic anemia and involves IgG-mediated destruction in the spleen, rather than IgM-mediated complement fixation in the periphery.
Answer choice E: Positive for IgG and C3, is incorrect. While some cases of WAIHA can show both, CAD is characterized by the dissociation of IgM in the warm testing environment, leaving only C3 behind for detection.
Key Learning Point
Cold agglutinin disease is characterized by IgM-mediated hemolysis. It often presents with acrocyanosis (blue extremities) upon cold exposure and is frequently triggered by infections like Mycoplasma pneumoniae or infectious mononucleosis (Epstein-Barr virus). The diagnostic hallmark is a direct Coombs test positive for C3 only, and the peripheral smear often shows red blood cell RBC agglutination (clumping).
