A 27-year-old woman with iron deficiency anemia (Hb 7.8 g/dL) starts oral ferrous sulfate. She returns asking how soon improvement will be evident. The earliest laboratory marker of a therapeutic response is:

• A Normalization of mean corpuscular volume at 48 hours
• B Rise in serum ferritin within the first week
• C Reticulocytosis peaking between days 7 and 10 ✓
• D Correction of hemoglobin concentration within the first week

Explanation

With effective oral iron, the marrow releases a wave of new red cells, so the reticulocyte count begins rising around day 3 to 5 and peaks at 7 to 10 days, the earliest measurable sign of response. Hemoglobin then climbs about 1 g/dL every 2 to 3 weeks over roughly 6 to 8 weeks. Serum ferritin rises late and reflects stored iron, so therapy continues for about 3 months after hemoglobin normalizes to replenish reserves.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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