Laboratory Testing After 8-10 Weeks of Iron Supplementation
After 8-10 weeks of iron supplementation, order a complete blood count (CBC) along with iron parameters including serum ferritin and transferrin saturation (TSAT). 1, 2
Timing of Laboratory Assessment
- For oral iron supplementation: Check CBC and iron parameters (ferritin and TSAT) at 8-10 weeks to assess response 1, 3
- For intravenous iron: Wait at least 4 weeks, preferably 4-8 weeks after the last infusion before checking iron parameters, as ferritin levels are falsely elevated immediately after IV iron administration 2, 4
- Hemoglobin should increase by 1-2 g/dL within 4-8 weeks of therapy if treatment is effective 2, 3
Essential Laboratory Tests to Order
The following tests should be obtained:
- Complete Blood Count (CBC) to assess hemoglobin, hematocrit, mean corpuscular volume (MCV), and red blood cell indices 2, 4, 5
- Serum ferritin to evaluate iron stores (target >30 ng/mL in most patients, >100 ng/mL in chronic kidney disease) 1, 2
- Transferrin saturation (TSAT) calculated from serum iron and total iron-binding capacity (TSAT = serum iron/TIBC × 100) 2, 6
Interpreting Results and Next Steps
If Hemoglobin Has Not Increased by 1-2 g/dL:
- Consider malabsorption of oral iron, continued bleeding, or an unidentified lesion 3
- Investigate for occult blood loss, particularly gastrointestinal sources 1, 4
- Evaluate for other causes of anemia beyond iron deficiency 4
- Consider switching from oral to intravenous iron if absorption is impaired 1, 6
If Iron Parameters Remain Low:
- Low ferritin (<30 ng/mL) with low TSAT (<20%): Continue iron supplementation 1, 6
- Normal ferritin with low TSAT: May indicate functional iron deficiency or anemia of chronic disease rather than absolute iron deficiency 7
- Ensure oral iron is taken on an empty stomach for optimal absorption 1, 7
If Iron Parameters Are Adequate:
- TSAT >50% or ferritin >800-1000 ng/mL: Hold iron supplementation to avoid iron overload 4
- Continue monitoring iron status every 3 months if on ongoing therapy 2, 4
Common Pitfalls to Avoid
- Do not check iron parameters too soon after IV iron: Ferritin levels remain artificially elevated for at least 4 weeks after IV administration, making results unreliable 2
- Do not rely on CBC alone: Hemoglobin and hematocrit may appear normal even with iron depletion, as they decrease only with severe deficiency 5
- Do not continue oral iron indefinitely without reassessment: Lack of response by 8-10 weeks indicates need for alternative approach 1, 3
- Do not ignore normal ferritin with low serum iron: This pattern suggests functional iron deficiency or chronic inflammation rather than true iron deficiency, and increasing oral iron dosage will not help 7