Dyspnea after treatment of recurrent urinary tract infection.

A 71-year-old woman came to the hospital because of generalized weakness, fatigue, and exertional dyspnea. She had a history of anemia, recurrent urinary tract infections, and hyperactive bladder. She had been taking nitrofurantoin for a urinary tract infection and phenazopyridine for dysuria, and she noticed that her urine was dark-colored. She was of northern European descent. She was unaware of any family history of blood-related disorders. She had been admitted to the hospital 6 weeks earlier for symptomatic anemia after taking nitrofurantoin for a urinary tract infection. At that time, she received 2 units of packed red blood cells and then was discharged. Follow-up blood work done 2 weeks later—including a glucose-6 phosphate dehydrogenase (G6PD) assay— was normal. On physical examination, she was pale and weak. Her hemoglobin level was 5.5 g/dL (reference range 14.0–17.5), with normal white blood cell and platelet counts and an elevated reticulocyte count. A comprehensive metabolic panel showed elevated indirect bilirubin and lactate dehydrogenase levels. A direct Coombs test for autoimmune hemolytic anemia was negative, as was a haptoglobin assay to look for intravascular hemolytic anemia. G6PD levels were normal, yet a peripheral blood smear (figure 1) showed features of G6PD deficiency. What was the cause of her anemia?