How do you calculate the dose of an iron (Fe) injection for a patient with iron deficiency anemia?

Calculating Iron Injection Dose

For most patients with iron deficiency anemia, use a simplified weight and hemoglobin-based dosing table rather than complex formulas, as this approach demonstrates superior efficacy and compliance compared to traditional calculations. 1

Recommended Dosing Approaches by Clinical Context

Simplified Dosing Method (Preferred for Most Patients)

The simple dosing scheme based on body weight and hemoglobin is more effective than Ganzoni's formula and should be used as first-line for ferric carboxymaltose. 1

For ferric carboxymaltose (FCM), determine dose based on:

• Body weight <50 kg with Hb <10 g/dL: 1500 mg total iron 1
• Body weight ≥50 kg with Hb <10 g/dL: 1500-2000 mg total iron 1
• Body weight <50 kg with Hb 10-14 g/dL: 1000 mg total iron 1
• Body weight ≥50 kg with Hb 10-14 g/dL: 1000-1500 mg total iron 1

Maximum single dose: 1000 mg iron (20 mL FCM) per week 1

Ganzoni Formula (Alternative Method)

While less convenient and prone to underestimating requirements, Ganzoni's formula can be used: 1

Total iron deficit (mg) = Body weight (kg) × [Target Hb - Actual Hb (g/dL)] × 0.24 + 500

The 500 mg accounts for iron store replenishment. 1

Important limitation: Patients with Hb <7.0 g/dL likely need an additional 500 mg beyond the calculated dose. 1

Iron Dextran Dosing (FDA-Approved Formula)

For iron dextran specifically: 2

Adults and children >15 kg:

• Dose (mL) = 0.0442 × (Desired Hb - Observed Hb) × LBW + (0.26 × LBW)
• Use lean body weight (LBW) or actual weight if less than LBW 2
• Males: LBW = 50 kg + 2.3 kg per inch over 5 feet 2
• Females: LBW = 45.5 kg + 2.3 kg per inch over 5 feet 2

Children ≤15 kg:

• Dose (mL) = 0.0442 × (Desired Hb - Observed Hb) × W + (0.26 × W)
• Target Hb for children ≤15 kg is 12 g/dL 2

Iron Replacement for Blood Loss

For ongoing blood loss without anemia: 2

Replacement iron (mg) = Blood loss (mL) × Hematocrit

Example: 500 mL blood loss with 20% hematocrit = 500 × 0.20 = 100 mg iron 2

Administration Guidelines

Dosing Schedules by Preparation

Ferric carboxymaltose:

• Single doses of 500-1000 mg (up to 20 mg/kg body weight) 1
• Can be delivered over 15 minutes 1
• Maximum 1000 mg per week 1

Iron sucrose:

• Single doses up to 7 mg/kg tested 1
• Repeated dosing limited to 200-300 mg per treatment episode 1

Iron dextran:

• Maximum 50 mg (1 mL) per minute IV 2
• IM: Maximum 100 mg (2 mL) daily for adults 2

Critical Safety Requirements

Mandatory test dose: 0.5 mL administered over at least 30 seconds, with observation for at least 1 hour before giving therapeutic dose 2

Observation period: Monitor patients for adverse effects for at least 30 minutes following each IV injection 1

Administration setting: Must have trained staff and equipment to manage hypersensitivity reactions 1

Special Population Considerations

Chronic Kidney Disease Patients

For hemodialysis patients with TSAT <20% and/or ferritin <100 ng/mL: 1

• Administer 100-125 mg IV iron at every hemodialysis session for 8-10 doses 1
• If deficiency persists, repeat the course 1
• Maintenance: 25-125 mg IV once weekly after achieving targets 1

Do not exceed TSAT >50% or ferritin >800 ng/mL - withhold iron for up to 3 months if these levels are reached 1

Iron Deficiency Without Anemia

Minimum of 500-1000 mg should be considered for patients with iron deficiency but no anemia 1

Critically Ill Patients

For anemic critically ill patients with iron deficiency confirmed by low hepcidin: deliver 1 g of iron as a single large dose using recent carbohydrate products 1

Monitoring and Re-dosing

Re-evaluate iron status at 3 months after initial replacement, not earlier, as ferritin levels increase markedly following IV iron and cannot accurately indicate iron status within 4 weeks 1

Target response: Hemoglobin increase of at least 2 g/dL within 4 weeks is acceptable 1

Avoid early re-evaluation (within 4 weeks) as ferritin levels are falsely elevated during this period 1

Common Pitfalls to Avoid

Do not use Ganzoni formula for blood loss replacement - it is only for iron deficiency anemia, not simple iron replacement 2

Do not mix iron preparations with other medications or add to parenteral nutrition solutions for IV infusion 2

Do not give iron during active bacteremia - treatment should be stopped in patients with ongoing infection 1

Do not administer in first 4 months of life unless specifically indicated 2

Avoid afternoon/evening dosing if oral iron is used - morning single doses on alternate days maximize absorption in iron-deficient women 3