A direct antiglobulin (Coombs) test is the most appropriate next diagnostic step for this patient with cold agglutinin disease. Autoimmune hemolytic anemias are characterized by the presence of antibodies directed toward antigens on the surface of erythrocytes; they are further classified by the type of immunoglobulin involved and the resulting tendency of hemolysis to occur in warm or cold environments. Direct IgG optimally binds erythrocytes at temperatures of 37.0 °C (98.6 °F), causing warm autoimmune hemolytic anemia. The immune response to the bound IgG causes erythrocytes to become spherocytic, resulting in hemolysis. Direct IgM binds more effectively at temperatures colder than 32.0 °C (89.6 °F), typically in the fingers, toes, and nose, causing cold agglutinin disease. IgM-coated erythrocytes agglutinate in the microvasculature, leading to cyanosis and ischemia in the cold extremities. The IgM antibodies fix complement and then detach from erythrocytes when they return to the warmer body core. This patient likely has cold agglutinin disease because he has an acquired hemolytic anemia associated with cold exposure and supported by his peripheral blood smear, showing agglutinated erythrocytes that disappear when warmed. Cold agglutinin disease may occur as a primary disorder or may be associated with a lymphoproliferative disorder or certain infections, such as Mycoplasma pneumoniae or Epstein-Barr virus. The primary treatment for cold agglutinin disease is avoidance of cold exposure.

Flow cytometry to detect the absence of specific cell surface antigens (CD55 and CD59) is useful in diagnosing paroxysmal nocturnal hemoglobinuria, which is another potential cause of hemolytic anemia. However, testing for this disorder is indicated only in patients without evidence of an autoimmune cause of hemolysis. Therefore, such testing would be inappropriate for this patient.

Multiple congenital hemoglobinopathies are associated with hemolysis, although this patient's clinical picture is consistent with an acquired hemolytic anemia. Therefore, hemoglobin electrophoresis would not likely be helpful in establishing the cause of his hemolysis.

The indirect antiglobulin test detects antierythrocyte antibodies in the serum and is used primarily before blood transfusion and in prenatal testing of pregnant women. It would not provide significant diagnostic information in this patient.