What are the essential considerations for prescribing iron therapy?

Essential Considerations for Prescribing Iron Therapy

Iron therapy should be prescribed based on documented iron deficiency (ferritin <100 ng/mL and/or TSAT <20%), with the route of administration determined by the clinical context, patient tolerance, and urgency of correction. 1

Initial Assessment and Diagnosis

Before prescribing iron, obtain the following laboratory tests 1:

• Complete blood count with hemoglobin, red cell indices, white blood cell count, differential, and platelet count
• Absolute reticulocyte count
• Serum ferritin level
• Transferrin saturation (TSAT) - calculated by dividing serum iron by total iron binding capacity
• Vitamin B12 and folate levels to exclude other causes of anemia

Iron deficiency is diagnosed when ferritin is <100 ng/mL in most clinical contexts (though <15 ng/mL in the absence of inflammation), and TSAT <20% has high sensitivity for detecting absolute or functional iron deficiency. 1, 2 In inflammatory conditions, ferritin may be falsely elevated due to acute phase reactivity, making TSAT <20% the more reliable indicator. 1

Oral Iron Therapy

Oral iron is the first-line treatment for uncomplicated iron deficiency without inflammation or malabsorption. 1, 3

Dosing and Formulations

• Ferrous sulfate 200 mg twice daily remains the gold standard, providing up to 200 mg elemental iron daily 1, 3
• Lower doses may be equally effective and better tolerated in patients experiencing side effects 1
• Alternative formulations include ferrous fumarate, ferrous gluconate, or iron suspensions for those intolerant to ferrous sulfate 1
• Continue oral iron for 3 months after correction of anemia to replenish iron stores 1

Monitoring Response

• Hemoglobin should increase by 2 g/dL after 3-4 weeks of therapy 1
• Failure to respond indicates poor compliance, misdiagnosis, continued blood loss, or malabsorption 1
• Recheck hemoglobin and iron parameters at 4-8 weeks after starting therapy 1

Intravenous Iron Therapy

IV iron is indicated when oral iron fails, is not tolerated, cannot be absorbed, or when rapid correction is required. 1, 4

Specific Indications for IV Iron

IV iron should be used in 1:

• Hemodialysis patients receiving erythropoietin-stimulating agents (ESAs)
• Inflammatory bowel disease with active inflammation compromising absorption
• Post-bariatric surgery patients with disrupted duodenal iron absorption
• Intolerance to oral iron after trial of at least two different formulations
• Functional iron deficiency (TSAT <20% despite ferritin >100 ng/mL in inflammatory states)
• Patients requiring rapid correction (e.g., preoperative optimization)

Available IV Iron Formulations

The modern formulations allowing high-dose administration include 1:

Iron Dextran (low-molecular-weight preferred - INFeD)

• Test dose required: 25 mg IV push, wait 1 hour before main dose 1
• Dosing: 100 mg IV over 5 minutes, weekly for 10 doses (total 1000 mg) 1
• Can be given as total dose infusion over several hours 1
• Avoid high-molecular-weight iron dextran (Dexferrum) due to higher adverse event rates 1

Ferric Gluconate (Ferrlecit)

• Test dose at physician discretion: 25 mg slow IV push or infusion 1
• Dosing: 125 mg IV over 60 minutes, weekly for 8 doses (total 1000 mg) 1
• Individual doses above 125 mg not recommended 1

Iron Sucrose (Venofer)

• Test dose at physician discretion: 25 mg slow IV push 1
• Dosing: 200 mg IV over 60 minutes or 2-5 minutes, every 2-3 weeks 1
• Individual doses above 300 mg not recommended 1

Ferric Carboxymaltose (Ferinject/Injectafer)

• No test dose required 1
• Dosing: 750-1000 mg IV over 15-30 minutes 1
• Avoid in patients requiring repeat infusions due to risk of hypophosphatemia 1, 5
• Can cause treatment-emergent hypophosphatemia in 50-74% of patients 5

Ferric Derisomaltose (FDI)

• No test dose required 1
• Only formulation with FDA label for total dose infusion 1
• Dosing: 1000 mg infusion or up to 20 mg/kg (not exceeding 1500 mg) 1
• Dilute in 100 mL normal saline, infuse over 20-30 minutes 1

Prefer IV iron formulations that replace iron deficits with 1-2 infusions over those requiring multiple doses. 1

Safety Considerations

All IV iron formulations carry similar risks; true anaphylaxis is very rare (<1%). 1 Most reactions are complement activation-related pseudo-allergy (infusion reactions), not true anaphylaxis. 1

Critical safety measures 1:

• Administer in medical facilities with personnel trained in managing hypersensitivity reactions
• Have immediate access to epinephrine, diphenhydramine, and corticosteroids 1
• Do not give IV iron to patients with active infection 1
• Monitor for dose-related arthralgias/myalgias with iron dextran (minimize by limiting doses to ≤100 mg) 1
• Monitor phosphate levels with ferric carboxymaltose due to risk of hypophosphatemia 1, 3, 5

Monitoring After IV Iron

Do not check iron parameters within 4 weeks of a total dose infusion as circulating iron interferes with assays. 1 Optimal timing is 4-8 weeks after the last infusion. 1

Target parameters 1:

• Maintain TSAT ≥20% and ferritin ≥100 ng/mL in most patients
• Upper safety limits: TSAT <50% and ferritin <800 ng/mL to avoid iron overload
• In hemodialysis patients, monitor TSAT and ferritin every 3 months during maintenance therapy 1
• Goal is to improve erythropoiesis, not simply achieve specific laboratory values 1

Special Populations

Chronic Kidney Disease

• IV iron is preferable for hemodialysis patients 1
• Maintenance IV iron: 250-1000 mg within 12 weeks, given thrice weekly, weekly, or every other week 1
• For non-dialysis CKD patients with TSAT ≤30% and ferritin ≤500 ng/mL, trial IV iron (or 1-3 month trial of oral iron) if increase in hemoglobin without ESA is desired 1

Pediatric Dosing

For iron dextran in children 1:

• <10 kg: 25 mg (0.5 mL) per dose
• 10-20 kg: 50 mg (1.0 mL) per dose
• >20 kg: 100 mg (2.0 mL) per dose
• Test dose: 10 mg if <10 kg, 15 mg if 10-20 kg 1

Cancer Patients

• Use IV iron for functional iron deficiency in patients receiving ESAs 1
• Do not use ESAs when anticipated outcome is cure 1
• Discontinue ESA 6-8 weeks after completing chemotherapy 1

Pregnancy and Heavy Menstrual Bleeding

• Oral iron preferred initially 2
• IV iron appropriate when oral iron fails or rapid correction needed 4, 2

Common Pitfalls to Avoid

• Do not rely on ferritin alone in inflammatory conditions - use TSAT <20% as primary indicator 1
• Do not prescribe oral iron when IV iron is clearly indicated (e.g., hemodialysis, severe malabsorption) - this delays effective treatment 1
• Do not use ferric carboxymaltose for repeated dosing due to hypophosphatemia risk 1, 5
• Do not check iron studies too soon after IV iron - wait 4-8 weeks for accurate results 1
• Do not continue escalating doses without investigating underlying causes of poor response 1