When should a complete blood count be repeated after a ferric carboxymaltose (FCM) infusion?

When to Repeat CBC After Ferric Carboxymaltose (FCM) Infusion

Repeat the complete blood count (CBC) at 4–8 weeks after the last FCM infusion, and reassess full iron parameters (ferritin, transferrin saturation) at 3 months. 1

Immediate Hematologic Monitoring (1–2 Weeks)

• Hemoglobin begins rising within 1–2 weeks of FCM administration, with an expected increase of approximately 1 g/dL in the first 2 weeks in anemic patients. 1, 2
• Reticulocytosis occurs at 3–5 days post-infusion, signaling early bone marrow activation. 1, 2
• A hemoglobin check at 2 weeks is reasonable to confirm early response, particularly in patients with severe anemia or heart failure requiring close monitoring. 1, 2

Optimal CBC Timing (4–8 Weeks)

• The CBC should be repeated 4–8 weeks after the last FCM dose to assess the full hematologic response. 1, 3
• By this time, hemoglobin should increase by 1–2 g/dL from baseline. 1, 2
• This interval allows adequate time for erythropoiesis while avoiding the confounding effects of acute-phase ferritin elevation. 1, 3

Iron Parameter Reassessment (3 Months)

• Do not recheck ferritin or transferrin saturation before 4 weeks after FCM infusion, as circulating iron interferes with assays and ferritin acts as an acute-phase reactant, yielding falsely elevated results. 1, 3, 2
• Iron status (ferritin, TSAT) should be formally reassessed at 3 months after the initial FCM course. 4, 1, 3
• This 3-month interval is explicitly recommended by the European Society of Cardiology for heart failure patients and is generalizable to other populations. 4, 1

Dosing Context and Re-Treatment Decisions

• For patients receiving smaller weekly FCM doses (e.g., 100–200 mg), iron parameters can be measured without interrupting therapy. 3
• For larger single doses (≥1000 mg), the mandatory waiting period of at least 4 weeks—and ideally 3 months—is critical to avoid misinterpretation. 1, 3
• If hemoglobin fails to rise or decreases after FCM, investigate for ongoing blood loss, infection, or other causes of anemia before administering additional iron. 4, 1

Long-Term Surveillance

• After achieving target hemoglobin and iron repletion, recheck iron parameters 1–2 times per year in patients with chronic conditions (e.g., heart failure, inflammatory bowel disease, chronic kidney disease). 4, 1, 3
• In heart failure patients, the European Society of Cardiology recommends reassessing ferritin and TSAT at the next scheduled visit (preferably after 3 months), then 1–2 times annually or if clinical status changes. 4

Critical Pitfalls to Avoid

• Premature retesting (<4 weeks) will show falsely elevated ferritin, masking inadequate iron repletion and potentially leading to underdosing. 1, 3, 2
• Single 1-gram FCM infusions frequently fail to achieve complete iron repletion in patients with significant deficiency; most clinical trials used mean total doses of 1500–2000 mg. 1
• Monitor serum phosphate in patients receiving repeat FCM courses within 3 months, as treatment-emergent hypophosphatemia occurs in 47–75% of such cases. 1, 2

Summary Algorithm

1. Day 3–5: Reticulocytosis begins (optional early marker). 1, 2
2. Week 1–2: Hemoglobin starts rising; optional early CBC if clinically indicated. 1, 2
3. Week 4–8: Repeat CBC to assess full hematologic response (Hb should increase 1–2 g/dL). 1, 3, 2
4. Month 3: Reassess ferritin and TSAT; administer additional FCM if ferritin <100 µg/L or ferritin 100–299 µg/L with TSAT <20%. 4, 1, 3
5. Every 6–12 months: Ongoing surveillance in chronic conditions. 4, 1, 3