Intravenous Iron: Indications, Dosing, and Monitoring
When IV Iron is Indicated
Intravenous iron should be used when oral iron fails to produce a hemoglobin rise of ≥1 g/dL within 3-4 weeks, when oral iron is not tolerated, or when rapid iron repletion is required. 1, 2
Primary Indications for IV Iron:
- Intolerance to oral iron despite trial of alternative formulations (ferrous fumarate, ferrous gluconate) 3
- Failure to respond to oral iron after 3-4 weeks of adequate dosing 1
- Malabsorption conditions: inflammatory bowel disease, celiac disease, post-bariatric surgery, or gastric/small bowel resection 3
- Chronic kidney disease patients, particularly those on hemodialysis 3
- Heart failure with reduced ejection fraction (HFrEF <40%) when ferritin <100 μg/L or ferritin 100-299 μg/L with transferrin saturation <20% 3
- Inflammatory bowel disease with moderate-to-severe anemia (Hb <100 g/L) 3
- Perioperative settings requiring rapid correction before surgery 4
- Pregnancy when oral iron is inadequate for maternal and fetal needs 4
Contraindications to IV Iron:
- Active bacteremia (treatment must be stopped) 3, 2
- Known hypersensitivity to IV iron preparations 3
- Evidence of iron overload 3, 2
- Anemia not due to iron deficiency 3
Dose Calculation
Calculate the total iron deficit using the Ganzoni formula or weight-based dosing tables, with typical requirements ranging from 1000-1500 mg for most adults with iron deficiency anemia. 5, 6
Ganzoni Formula (from FDA label):
For adults and children >15 kg:
- Dose (mL) = 0.0442 × (Desired Hb - Observed Hb) × LBW + (0.26 × LBW)
- Where LBW = lean body weight in kg
- Desired Hb = target hemoglobin (typically 14.8 g/dL for adults) 5
For children ≤15 kg:
- Dose (mL) = 0.0442 × (Desired Hb - Observed Hb) × W + (0.26 × W)
- Where W = weight in kg
- Desired Hb = 12.0 g/dL for this age group 5
Practical Dosing Considerations:
- Average iron deficit in IDA patients is approximately 1400-1500 mg, suggesting that standard 1000 mg protocols may be insufficient 6
- A cumulative dose of 1500 mg results in significantly lower retreatment rates (5.6%) compared to 1000 mg (11.1%) 6
- The formula accounts for both hemoglobin restoration and iron store repletion (the 0.26 × weight component) 5
Administration Protocols by Formulation
Ferric Carboxymaltose (Ferinject):
Maximum 1000 mg iron per week, administered as undiluted slow bolus over 15 minutes or diluted infusion. 3, 2
- Can give 1000 mg in a single 15-minute infusion 3, 2
- Most convenient option for rapid repletion 3
- Cost: £217.50 per gram of iron 3
Iron Sucrose (Venofer):
Maximum 200 mg per dose, given over 10 minutes as bolus dosing. 3, 2
- Maximum weekly dose: 500 mg 7
- Requires multiple visits to achieve total dose 3
- Cost: £70.80 per gram of iron 3
Iron Dextran (Cosmofer):
Can administer up to 20 mg/kg (typically 1000 mg) in a single 6-hour infusion. 3, 5
- Higher risk of serious reactions (0.6-0.7%) and anaphylaxis compared to non-dextran preparations 3
- Requires test dose of 0.5 mL administered over 30 seconds IV, with 1-hour observation before full dose 5
- Cost: £79.70 per gram of iron 3
- Non-dextran formulations (ferric carboxymaltose, iron sucrose) are preferred due to superior safety profile 3, 4
Safety Requirements for All Formulations:
- Resuscitation facilities and trained personnel must be available 3, 2
- Observe patients for at least 30-60 minutes after infusion 3
- Risk of anaphylaxis is <1:250,000 with modern non-dextran formulations 2
Monitoring and Follow-Up
Critical Timing Rule:
Do not measure ferritin or transferrin saturation within 4 weeks of IV iron infusion—values are falsely elevated and do not reflect true iron stores. 1, 2, 7
Optimal Monitoring Schedule:
Re-evaluate CBC and iron studies (ferritin, TSAT) at 4-8 weeks after the last infusion, then again at 3 months to assess need for additional repletion. 1, 2, 7
Expected Response Timeline:
- Hemoglobin begins rising within 1-2 weeks 1, 7
- Expect 1-2 g/dL hemoglobin increase by 4-8 weeks 1, 7
- Initial rise in Hb is more rapid with IV iron, but at 12 weeks outcomes are similar to oral iron 3
Long-Term Monitoring:
- Check iron status every 3 months for the first year after correction 3, 1
- Then monitor at 6-month intervals for the next 2-3 years 1
- For chronic conditions (CKD, IBD, CHF): assess iron status every 3 months indefinitely 3, 1
- For stable patients: 1-2 times per year is sufficient 3, 1
Thresholds for Withholding Further IV Iron:
Stop or hold IV iron if:
Additional Monitoring Considerations:
- Check baseline phosphate levels before IV iron due to risk of treatment-emergent hypophosphatemia, especially with ferric carboxymaltose 1, 7, 8
- Monitor phosphate after infusion, as hypophosphatemia is a recognized complication 7, 8
- In CKD patients on erythropoiesis-stimulating agents: check Hb twice weekly for 2-6 weeks after dose changes, then monthly during stable therapy 1
Common Pitfalls and How to Avoid Them
Pitfall #1: Premature Laboratory Assessment
Measuring ferritin within 4 weeks of IV iron leads to falsely elevated results that do not reflect true iron stores. 1, 2, 7 Wait the full 4-8 weeks before reassessing iron parameters.
Pitfall #2: Inadequate Total Dose
Standard 1000 mg protocols leave many patients under-repleted. 6 Calculate the actual iron deficit using the Ganzoni formula—most patients require 1400-1500 mg total.
Pitfall #3: Failure to Investigate Non-Response
If hemoglobin does not rise by ≥1 g/dL within 3-4 weeks of oral iron, or within 2 weeks of IV iron, investigate for:
- Ongoing blood loss 1, 2
- Malabsorption 1
- Alternative diagnoses (anemia of chronic disease, B12/folate deficiency) 1
Pitfall #4: Premature Discontinuation of Monitoring
Iron deficiency recurs in many patients, and cessation of monitoring can mask underlying pathology. 3, 1 Continue surveillance every 3 months for at least 1 year, then periodically thereafter.
Pitfall #5: Using Oral Iron in Inflammatory Conditions
In heart failure, inflammatory bowel disease, and CKD, oral iron is poorly absorbed due to hepcidin elevation and gut inflammation. 3 IV iron is the preferred route in these populations.
Pitfall #6: Ignoring Hypersensitivity Risk Factors
Patients with drug allergies, severe asthma, eczema, or autoimmune conditions (SLE, rheumatoid arthritis) have increased risk of hypersensitivity reactions. 3 Ensure resuscitation equipment is immediately available and extend observation time.
Special Population Considerations
Heart Failure:
- IV ferric carboxymaltose is recommended (Class IIa, Level A evidence) for symptomatic HFrEF patients with iron deficiency 3
- Improves exercise capacity, symptoms, quality of life, and may reduce hospitalizations 3
- Oral iron has no proven benefit in CHF and should be avoided 3
Inflammatory Bowel Disease:
- Oral iron should contain ≤100 mg elemental iron daily if used 3
- IV iron indicated for moderate-to-severe anemia (Hb <100 g/L) or oral iron intolerance 3
- Monitor every 3 months for at least 1 year after correction 3
Chronic Kidney Disease:
- IV iron is standard for hemodialysis patients 3
- For non-dialysis CKD: consider oral iron first, but switch to IV if no response or intolerance 3
- Monitor iron status at least every 3 months during ESA therapy 3