Iron Supplementation for Ferritin 35.7 µg/L
For a patient with serum ferritin of 35.7 µg/L, start oral iron supplementation with ferrous sulfate 325 mg (65 mg elemental iron) once or twice daily, continue for at least 3 months, and recheck complete blood count (CBC) and iron parameters (ferritin and transferrin saturation) at 4-8 weeks after starting therapy.
Rationale for Oral Iron First-Line
Your patient's ferritin of 35.7 µg/L indicates iron deficiency, though the severity depends on clinical context. The most recent expert consensus 1 establishes that in the absence of inflammation, the goal ferritin is 50 ng/mL, and your patient falls below this threshold. However, without knowing the patient's hemoglobin level, transferrin saturation (TSAT), or underlying condition, oral iron remains the appropriate initial approach for most patients.
Specific Medication Order
Generic Name & Dose
- Ferrous sulfate 325 mg (65 mg elemental iron) orally
- Frequency: Once daily OR twice daily
Duration
- Minimum 3 months of therapy to replete iron stores
- Continue until ferritin reaches ≥50 µg/L (ideally 50-100 µg/L range) 1
Alternative Oral Formulations
If ferrous sulfate is not tolerated:
- Ferrous gluconate or ferrous fumarate at equivalent elemental iron doses
- Consider heme iron preparations (better absorbed, fewer GI side effects) 3
Laboratory Monitoring Schedule
Initial Assessment (Before Starting)
You need baseline values if not already obtained:
- Complete blood count (CBC) with hemoglobin, MCV, MCH
- Serum ferritin (you have: 35.7 µg/L)
- Transferrin saturation (TSAT) - critical for determining if IV iron needed 1
- Consider: serum iron, total iron binding capacity (TIBC)
Follow-up Labs
First recheck at 4-8 weeks 1:
- CBC (hemoglobin should increase 1-2 g/dL within 4-8 weeks)
- Ferritin and TSAT
- Do NOT check iron parameters within 4 weeks of starting oral iron as circulating iron interferes with assays 1
Subsequent monitoring:
- If responding: Recheck every 3 months until ferritin ≥50 µg/L 4
- If not responding after 8 weeks: Consider IV iron (see below)
When to Switch to Intravenous Iron
Consider IV iron if:
- TSAT <20% despite oral iron 5
- Intolerance to oral iron (GI side effects)
- Malabsorption (inflammatory bowel disease, celiac disease, post-bariatric surgery) 1
- Chronic kidney disease on hemodialysis 4
- Cancer-related anemia with chemotherapy 5
- Need for rapid correction
- Failure to respond to adequate oral iron trial (3 months)
IV Iron Options & Dosing
If IV iron becomes necessary 1:
- Ferric derisomaltose (iron isomaltoside): 1000 mg single infusion (can give up to 20 mg/kg, max 1500 mg)
- Ferric carboxymaltose: 750-1000 mg per infusion
- Iron sucrose: 100-200 mg per dose, multiple doses needed
- Low molecular weight iron dextran: 1000 mg total dose infusion
- Iron gluconate: 125 mg weekly × 8 doses
Recheck labs 4-8 weeks after last IV iron dose (not sooner, as circulating iron interferes with assays) 1
Critical Caveats
Context Matters
The ferritin threshold for treatment varies by condition 6:
- Healthy adults: Ferritin <30 µg/L indicates depleted stores
- With inflammation/chronic disease: Ferritin <100 µg/L may still represent iron deficiency 7, 6
- CKD patients: Target ferritin ≥100 ng/mL and TSAT ≥20% 4
TSAT is Essential
You must obtain TSAT to properly guide therapy 1:
- TSAT <20% has high sensitivity for absolute or functional iron deficiency
- Ferritin alone can be misleading in inflammatory states (acute phase reactant)
- Discordant results (low TSAT, normal/high ferritin) suggest functional iron deficiency
Upper Safety Limits
Stop iron supplementation if 4:
- Ferritin >800 ng/mL (CKD patients)
- TSAT >50%
- Ferritin >300 µg/L in general population 1
Avoid Iron Overload
Do not continue indefinitely without monitoring—iron overload causes organ damage. The goal is repletion to 50-100 µg/L, not maximization 1.