Isoniazid Induced Pure Red Blood Cell Aplasia

Pure red blood cell aplasia (PRCA) is one of the uncommon causes of anemia. Drug-induced PRCA is even more infrequent. Only a few drugs are implicated in PRCA. Isoniazid is a widely used drug for the treatment of tuberculosis all over the world. It is known to cause hepatotoxicity, but in rare instances, it can lead to PRCA.  A 72-year-old Caucasian male, who was started on isoniazid after the diagnosis of latent tuberculosis, presented two months later with episodes of syncope to primary care physician's office. The initial blood work showed severe anemia. There were no signs of acute or chronic gastrointestinal blood loss, and the stool hemoccult test was negative. Iron, vitamin B12, folate, lactate dehydrogenase, bilirubin, transaminases, and erythropoietin were within normal limits. Peripheral blood smear showed normochromic and normocytic anemia. A reticulocyte count was less than 1,000 per microliter. Thymoma, human immunodeficiency virus, and parvovirus B19 were ruled out. Further work-up with bone marrow biopsy confirmed pure red blood cell aplasia. A detailed review of recently started medications revealed isoniazid as the offending drug. Isoniazid was stopped. Reticulocyte count and bone marrow recovered a few days after stopping with eventual improvement in hemoglobin level, thus confirming the diagnosis of isoniazid induced red blood cell aplasia. Isoniazid is the first-line therapy for tuberculosis. Rarely, it can cause pure red blood cell suppression and severe anemia, an untoward effect, worth remembering.