This patient should undergo evaluation by colonoscopy and upper endoscopy. Iron deficiency can result from blood loss or malabsorption. In men and nonmenstruating women, gastrointestinal blood loss is always the presumed cause of iron deficiency unless proven otherwise and may develop secondary to an undiagnosed colonic neoplasm. Colon cancer is the most commonly detected cancer causing iron deficiency anemia.

Remeasuring iron stores in 1 month will not be helpful and is not an appropriate choice without assessing the cause of the iron deficiency. In patients who receive adequate treatment for iron deficiency anemia, reticulocytosis can be expected within 7 to 10 days, and the hemoglobin level can be expected to increase in 1 to 2 days. Iron stores are not expected to normalize for 6 months.

Iron malabsorption can result from celiac disease, inflammatory bowel disease, or surgical resection (affecting the duodenum, as in gastric bypass). Some malabsorption syndromes, such as celiac disease, are not accompanied by diarrhea, steatorrhea, or weight loss. In developed nations, gastrointestinal bleeding is the most common cause of iron deficiency; iron malabsorption is much less common. Considering this patient's complete blood count was normal 1 year ago, celiac disease is an unlikely cause of iron deficiency, and screening for celiac disease with tissue transglutaminase IgA antibody is not appropriate at this time.

Wireless capsule endoscopy is an effective technology that provides visualization of the small bowel. Unlike angiography or technetium scans, wireless capsule endoscopy is effective even in the absence of active bleeding. It detects the source of occult bleeding in 50% to 75% of patients. In those with iron deficiency anemia, in whom bleeding can be episodic, capsule endoscopy is another way to investigate potential sources of blood loss after other investigations have been unrevealing. Wireless capsule endoscopy should not precede colonoscopy and upper endoscopy as the first diagnostic tests for adult patients with iron deficiency anemia and presumed gastrointestinal blood loss.