When should ferritin be rechecked in a woman with normal hemoglobin but documented iron deficiency who is receiving oral or intravenous (IV) iron therapy?

When to Repeat Ferritin Levels in Women with Iron Deficiency on Iron Therapy

Recheck ferritin and hemoglobin 4-8 weeks after completing IV iron therapy, or after 3 months of oral iron therapy, then monitor every 3 months for the first year after correction, and every 6-12 months thereafter. 1, 2, 3

Timing Based on Route of Administration

After Intravenous (IV) Iron

• Wait 4-8 weeks after the last IV iron infusion before checking ferritin levels 1, 2
• Do not check ferritin within 4 weeks of IV iron administration, as circulating iron interferes with the assay and produces falsely elevated results that do not reflect true iron stores 1, 2
• For larger doses (≥1000 mg), wait at least 2 weeks; for doses of 200-500 mg, wait 7+ days 3
• Hemoglobin should increase by 1-2 g/dL within 4-8 weeks of IV iron therapy 1

After Oral Iron Therapy

• Recheck ferritin and hemoglobin after 3 months of oral iron supplementation 2, 3
• Continue oral iron for an additional 3 months after hemoglobin normalizes to fully replenish iron stores 3
• This ensures adequate repletion before transitioning to monitoring phase 3

Long-Term Monitoring Schedule

First Year After Correction

• Monitor ferritin every 3 months for at least one year after successful correction of iron deficiency 1, 2, 3
• This frequent monitoring is critical because iron deficiency recurs rapidly, particularly in women with ongoing menstrual blood loss 1

After First Year

• Monitor every 6-12 months thereafter 1, 2, 3
• More frequent monitoring (every 6 months) is appropriate for patients with risk factors for recurrence such as heavy menstrual bleeding 1

Target Ferritin Levels and Re-treatment Thresholds

Treatment Goals

• Target ferritin of 100-200 μg/L to ensure adequate iron stores and prevent rapid recurrence 2
• In the absence of inflammation, aim for ferritin ≥50 ng/mL regardless of sex 1
• Post-treatment ferritin levels >400 μg/L prevent recurrence of iron deficiency for 1-5 years better than lower levels 1

When to Re-initiate Treatment

• Re-start IV iron when ferritin drops below 100 μg/L or hemoglobin falls below 12 g/dL in women 1
• For oral iron, restart if hemoglobin or MCV falls below normal range 3
• Check ferritin in doubtful cases when hemoglobin or MCV decline 3

Key Parameters to Monitor

At each monitoring visit, measure: 1, 2

• Complete blood count (CBC) with hemoglobin and hematocrit
• Serum ferritin
• Transferrin saturation (TSAT) - calculated as serum iron divided by total iron binding capacity × 100

Clinical Context and Pitfalls

Rapid Recurrence as a Red Flag

• Rapid recurrence of iron deficiency in asymptomatic patients should raise suspicion for ongoing blood loss or subclinical disease activity 1
• Further investigation is necessary only if hemoglobin and MCV cannot be maintained with supplementation 3

Common Monitoring Errors to Avoid

• Never check ferritin too soon after IV iron (within 4 weeks) - this gives falsely elevated readings 1, 2
• Do not stop monitoring after initial correction - failure to continue monitoring for at least one year may miss early recurrence 2
• Do not discontinue iron supplementation prematurely - continue for 3 months after hemoglobin normalizes to replenish stores 3

Interpreting Ferritin in Context

• Ferritin is an acute-phase reactant and may be falsely elevated in inflammation or infection 1
• When ferritin and TSAT are discordant, TSAT <20% has high sensitivity for diagnosing iron deficiency even when ferritin appears normal 1
• Consider alternative tests like reticulocyte hemoglobin content (CHr) or soluble transferrin receptor if inflammation is present 1