How do you approach the work-up for a patient with iron deficiency anemia who is not responding to oral iron therapy?

Have the patient's iron loss sources been controlled? GI blood loss due to gastritis, IBD, and other issues is the number one cause. Menstrual bleeding is common in post-menarchal girls. I really like the menstrual chart in the Better You Know campaign from the CDC and NBDF. This can identify bleeding that is abnormal, but not so bad that it stands out to the family. Urine loss can also add up.

I am fond of using IV iron early.