How do I calculate the total iron deficit and determine the appropriate oral or intravenous iron regimen for a non‑pregnant adult woman weighing 70 kg with a current hemoglobin of 9 g/dL and a target hemoglobin of 13 g/dL?

Calculating Iron Deficit and Determining Replacement Regimen

Total Iron Deficit Calculation

For a 70 kg woman with hemoglobin 9 g/dL targeting 13 g/dL, the estimated total iron deficit is 1,500 mg elemental iron. 1

The simplified calculation uses body weight and baseline hemoglobin:

• Body weight 70 kg + hemoglobin 10–13 g/dL (men) or 10–12 g/dL (women): 1,000 mg total iron need
• Body weight ≥70 kg + hemoglobin 10–13 g/dL (men) or 10–12 g/dL (women): 1,500 mg total iron need
• Hemoglobin 7–10 g/dL regardless of weight: 1,500–2,000 mg total iron need 1

For this patient (70 kg, hemoglobin 9 g/dL), the deficit falls into the 1,500 mg category. 1

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Oral Iron Replacement Strategy

First-Line Oral Regimen

Prescribe ferrous sulfate 200 mg (65 mg elemental iron) once daily in the morning on an empty stomach, co-administered with 250–500 mg vitamin C. 2

• Ferrous sulfate remains the gold-standard formulation, costing approximately £1.00 per 28-day supply versus £47.60 for alternatives like ferric maltol. 2
• Once-daily dosing is superior to multiple daily doses because doses ≥60 mg elemental iron trigger hepcidin elevation lasting 24–48 hours, reducing absorption of subsequent doses by 35–45% and increasing gastrointestinal side effects without improving hemoglobin response. 2, 3
• Vitamin C enhances non-heme iron absorption by forming a soluble chelate and reducing ferric to ferrous iron. 2

Oral Dosing Cycle Approach

Each oral iron replacement cycle consists of 5,000 mg total elemental iron ingested over at least 1 month. 4

For this patient requiring 1,500 mg total deficit:

• At 65 mg elemental iron per day (one ferrous sulfate 200 mg tablet), the patient needs approximately 23 days to deliver 1,500 mg
• However, fractional absorption is only 10–20%, so the actual replacement cycle should extend 3 months beyond hemoglobin normalization to fully replenish iron stores. 2

Alternative Dosing if Standard Regimen Not Tolerated

Switch to alternate-day dosing with 100–200 mg elemental iron (1–2 ferrous sulfate tablets every other day) if daily dosing causes intolerable gastrointestinal symptoms. 2, 3

• Alternate-day dosing markedly increases fractional iron absorption compared to daily dosing by avoiding hepcidin-mediated blockade, though the early rate of hemoglobin rise may be slower. 2, 3
• If ferrous sulfate remains intolerable, switch to ferrous fumarate (69–106 mg elemental iron per tablet) or ferrous gluconate (35–38 mg elemental iron per tablet), though evidence does not support improved tolerability. 2

Monitoring Oral Iron Response

Check hemoglobin at 2 weeks; an increase of ≥10 g/L predicts treatment success with 90% sensitivity and 79% specificity. 2

• Failure to achieve ≥10 g/L rise by 2 weeks strongly predicts overall treatment failure and warrants evaluation for non-adherence, ongoing blood loss, malabsorption, or concurrent vitamin B12/folate deficiency. 2
• Continue oral iron for 3 months after hemoglobin normalizes (total treatment duration approximately 6–7 months) to fully replenish iron stores. 2
• Monitor hemoglobin and red-cell indices every 3 months during the first year. 2

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Intravenous Iron Replacement Strategy

Indications for Intravenous Iron

Consider intravenous iron when oral iron is not tolerated despite dosing modifications, ferritin fails to rise after 4 weeks of compliant oral therapy, or transferrin saturation remains <20% after an adequate oral trial. 2, 5

Additional absolute indications include:

• Active inflammatory bowel disease with hemoglobin <10 g/dL (oral iron may exacerbate intestinal inflammation) 1, 2
• Intolerance to at least two different oral iron preparations 2
• Post-bariatric surgery (duodenal iron absorption is anatomically disrupted) 2, 5
• Chronic kidney disease with eGFR <45 mL/min 2

Intravenous Iron Dosing Calculation

For patients ≥50 kg, administer ferric carboxymaltose 750 mg intravenously in two doses separated by at least 7 days, for a total cumulative dose of 1,500 mg per course. 6

Alternative FDA-approved regimen:

• Single-dose option: Ferric carboxymaltose 15 mg/kg body weight up to a maximum of 1,000 mg intravenously as a single dose per course 6
• For this 70 kg patient: 70 kg × 15 mg/kg = 1,050 mg, capped at 1,000 mg single dose 6

Preferred Intravenous Formulations

Choose high-dose formulations that replenish the iron deficit in 1–2 infusions to minimize infusion-related risk and improve convenience. 2, 5

• Ferric carboxymaltose: 750–1,000 mg per 15-minute infusion; two doses given ≥7 days apart provide 1,500 mg total 2, 6
• Ferric derisomaltose: 1,000 mg can be delivered as a single infusion 2, 7
• Iron dextran is not recommended as first-line therapy because it carries higher risk of anaphylaxis (≈0.6–0.7%); a test dose is required 2

Administration Protocol

Administer ferric carboxymaltose as an undiluted slow intravenous push at approximately 100 mg (2 mL) per minute, or dilute up to 1,000 mg in no more than 250 mL sterile 0.9% sodium chloride and infuse over at least 15 minutes. 6

• Monitor patients for signs and symptoms of hypersensitivity during and for at least 30 minutes after administration until clinically stable. 6
• Avoid extravasation, as brown discoloration may be long-lasting; discontinue infusion at that site if extravasation occurs. 6

Safety Profile

All approved intravenous iron products have comparable safety; true anaphylaxis is exceedingly rare (<1%), and most adverse reactions are complement-activation-related pseudo-allergic infusion reactions. 2, 8

• Serious anaphylactic/anaphylactoid reactions occur in 0.1% of patients receiving ferric carboxymaltose. 6
• Complement-mediated infusion reactions may occur in up to 1 in 200 patients but respond to antihistamines and corticosteroids rather than epinephrine. 2, 8
• Check serum phosphate levels in patients requiring repeat courses within 3 months, as hypophosphatemia is a recognized adverse effect. 6

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Critical Pitfalls to Avoid

• Do not prescribe multiple daily doses of oral iron, as this increases gastrointestinal side effects without improving efficacy due to hepcidin-mediated absorption blockade. 2, 3
• Do not discontinue iron therapy when hemoglobin normalizes; continue for an additional 3 months to restore iron stores. 2
• Do not persist with oral iron beyond 4 weeks without a hemoglobin rise; reassess adherence, malabsorption, ongoing blood loss, or need for intravenous iron. 2
• Do not use modified-release iron preparations, as they release iron beyond the duodenum (the primary absorption site) and are classified as "less suitable for prescribing." 2
• Do not assume all intravenous iron reactions are true anaphylaxis; most are pseudo-allergic and can be managed without abandoning future intravenous therapy. 2, 8