A 52-year-old woman presents to the clinic with complaints of fatigue and difficulty breathing for the past 5 weeks. She has no past medical history and takes no medications. There is no family history of respiratory or hematologic disease. She consumes alcohol socially on weekends but does not use tobacco or any recreational drugs. Vital signs are temperature 36.8° C (98.2° F), blood pressure 110/81 mmHg, pulse 103 beats/min, and respirations 17/min. Oxygen saturation is 99% on room air. Physical examination reveals mild tachycardia, conjunctival pallor, and scleral icterus. Laboratory results are as follows:
A direct antiglobulin (Coombs) test is positive. Which of the following is most likely to be seen on a peripheral smear of the patient’s blood?
D) Spherocytes
This patient has autoimmune hemolytic anemia (AIHA), as suggested by the clinical features of anemia (conjunctival pallor) and jaundice (scleral icterus), along with low hemoglobin, elevated bilirubin (predominantly indirect), elevated lactate dehydrogenase (LDH), and positive direct antiglobulin (Coombs) test.
Patients with AIHA develop autoantibodies against their red blood cells (RBCs), which results in hemolysis. Hemolysis in AIHA is predominantly extravascular (in the spleen) but can also be intravascular (in blood vessels). Extravascular hemolysis results in normocytic anemia (low hemoglobin and a normal mean corpuscular volume), elevated bilirubin (predominantly indirect), and elevated lactate dehydrogenase (LDH), as seen in this patient.
In the appropriate clinical setting, a positive direct antiglobulin (Coombs) test confirms the diagnosis of AIHA. The Coombs test detects the presence of antibodies against circulating RBCs. It is of two types: indirect and direct. The indirect Coombs test is used for pre-transfusion testing. Here, the donor’s RBCs are added to a sample of the patient’s serum, following which antiglobulin (anti-Ig antibody) is added. If the patient’s serum has antibodies against donor RBC antigens, then these antibodies will attach to these antigens on the surface of the donor RBCs. The anti-Ig antibodies will then attach to this antigen-antibody complex and cause RBC aggregation (indirect Coombs positive). Thus, a positive indirect Coombs test suggests the presence of anti-donor RBC antibodies in the recipient’s serum.
The direct Coombs test is used for AIHA diagnosis but can also be positive in hemolytic transfusion reactions and hemolytic disease of the newborn. In this test, the antiglobulin (anti-Ig antibody) is added to the patient’s RBCs. If the patient’s RBCs are already coated with antibodies, as they are in AIHA, then the antiglobulin will bind to them and cause RBC aggregation (direct Coombs positive). Thus, a positive direct Coombs test suggests the presence of antibodies attached to the surface of the patient’s RBCs.
The peripheral blood smear of a patient with AIHA is likely to show spherocytes. These are small, round RBCs that lack central pallor. Because of their shape, spherocytes have difficulty moving through small blood vessels and stay in the spleen longer than normal, where they are subsequently destroyed by splenic macrophages (extravascular hemolysis). Spherocytes can also be seen in hereditary spherocytosis, another type of normocytic hemolytic anemia. It is an autosomal dominant condition characterized by a defect in RBC membrane-cytoskeleton tethering proteins (ankyrin, spectrin, band 3, protein 4.2) which results in RBCs being spherical rather than biconcave. The presence of spherocytes on peripheral smear along with a strong family history and negative Coombs test suggests hereditary spherocytosis, whereas the same with a negative family history and positive Coombs test suggests AIHA.
Answer choice A: Heinz bodies and bite cells, is incorrect. Heinz bodies are seen in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, a normocytic hemolytic anemia. This is an X-linked recessive condition in which deficiency of G6PD results in increased susceptibility of RBCs to oxidative stress, resulting in back pain and hemoglobinuria (due to intravascular hemolysis) following triggers (e.g., sulfa drugs, fava beans, antimalarials). A peripheral smear of these patients will show Heinz bodies (small inclusions of denatured hemoglobin within RBCs) and bite cells (irregularly shaped RBCs with ‘bitten-off’ membranes due to the removal of Heinz bodies by splenic macrophages).
Answer choice B: Howell-Jolly bodies, is incorrect. Howell-Jolly bodies are basophilic nuclear remnants that are seen as small purple inclusions within RBCs. Normally, they are removed by splenic macrophages when RBCs pass through the spleen. In patients with functional hyposplenia or asplenia (e.g., sickle cell disease, post-splenectomy), they persist and are hence seen on a peripheral smear.
Answer choice C: Sickle cells, is incorrect. Sickle cells are seen in sickle cell anemia, a normocytic hemolytic anemia that usually presents in childhood. It is an autosomal recessive condition characterized by the presence of a mutated hemoglobin called HbS. In conditions of hypoxia or acidosis, HbS polymerizes and results in sickling of RBCs. These sickle cells can then cause vaso-occlusive disease (pain crises, autosplenectomy, renal papillary necrosis) and anemia (due to extravascular and intravascular hemolysis).
Answer choice E: Target cells, is incorrect. Target cells are RBCs that have a central disc of hemoglobin along with a separate outer rim of hemoglobin adjacent to the cell membrane, giving them a target-like appearance. Target cells are associated with conditions like Hemoglobin C disease, asplenia, liver disease, and thalassemia.
Key Learning Point
Autoimmune hemolytic anemia occurs due to autoantibodies against red blood cells (RBCs) that result in predominantly extravascular hemolysis. A positive direct antiglobulin (Coombs) test confirms the diagnosis. A peripheral blood smear of these patients may show spherocytes, which are small, round RBCs that lack central pallor. Spherocytes can also be seen in hereditary spherocytosis.
