Hemoglobin Threshold for Iron Infusion
Iron infusion is justified when hemoglobin falls below 10 g/dL (100 g/L) in the presence of confirmed iron deficiency, defined as ferritin <100 ng/mL or transferrin saturation <20%. 1, 2
Critical First Step: Confirm Iron Deficiency Before Any Iron Therapy
You cannot justify iron infusion based on hemoglobin alone—you must document iron deficiency with both ferritin AND transferrin saturation before proceeding. 1
- Absolute iron deficiency: ferritin <100 ng/mL OR transferrin saturation <20% 3, 1
- Functional iron deficiency: ferritin 500-1200 ng/mL with transferrin saturation <25% in inflammatory states 1
Common pitfall: Ferritin is an acute phase reactant and can be falsely elevated in inflammatory conditions (CKD, IBD, cancer, heart failure), masking true iron deficiency. 1 In these patients, rely more heavily on transferrin saturation <20-25% to identify functional iron deficiency. 1
Hemoglobin-Based Algorithm for Route of Iron Administration
Hemoglobin <10 g/dL (100 g/L): IV Iron Preferred
Proceed directly to IV iron when hemoglobin is below 10 g/dL with confirmed iron deficiency. 1, 2 This threshold represents severe anemia requiring rapid correction, and IV iron demonstrates superior efficacy, faster response, and better tolerability compared to oral iron. 2
Hemoglobin 10-11 g/dL (100-110 g/L): Consider IV Iron in Specific Contexts
IV iron is strongly indicated over oral iron when any of the following apply: 1, 2, 4
- Active inflammatory disease (IBD, CKD, heart failure, cancer)
- Malabsorption (celiac disease, post-bariatric surgery, atrophic gastritis)
- Ongoing blood loss
- Previous intolerance to oral iron
- Patient requires erythropoiesis-stimulating agents (ESAs)
- Second or third trimester of pregnancy
Hemoglobin >11 g/dL: Oral Iron First-Line
Oral ferrous sulfate 325 mg daily (or every other day to improve tolerability) is appropriate first-line therapy. 2, 4 Reassess at 14 days: if hemoglobin increase is <1.0 g/dL, transition to IV iron. 5 This 14-day checkpoint has 90% sensitivity and 79% specificity for predicting overall oral iron failure. 5
Special Population: Chronic Kidney Disease
For CKD patients (stages 3-5 not on dialysis), initiate iron therapy when hemoglobin <11 g/dL (110 g/L) with ferritin <100 ng/mL or transferrin saturation <20%. 3 IV iron produces significantly greater hemoglobin increases (7-10 g/L) compared to oral iron (4-7 g/L) in this population. 2, 6
For hemodialysis patients, IV iron is strongly preferred and produces hemoglobin increases of 0.83 g/dL greater than oral iron. 6
Dosing for Severe Anemia (Hgb 7-10 g/dL)
When using ferric carboxymaltose for severe anemia: 2
- Body weight <70 kg: Total dose 1500 mg
- Body weight ≥70 kg: Total dose 2000 mg
Critical Safety Considerations
Never administer IV iron without documented iron deficiency—this carries risks of infusion reactions (4.3% incidence), iron overload, and potential cardiovascular events without any benefit. 1 Administering iron when stores are adequate will not improve anemia and may cause harm. 1
Infusion reactions are rare but potentially life-threatening, requiring appropriate monitoring and emergency preparedness. 7
Why This Threshold Matters for Outcomes
The <10 g/dL threshold prioritizes morbidity and quality of life: patients with hemoglobin below this level experience significant symptoms including fatigue, exercise intolerance, dyspnea, difficulty concentrating, restless legs syndrome, and worsening heart failure. 4 IV iron provides faster symptom relief and improved quality of life compared to oral iron in this severely anemic population. 2, 5
In CKD patients specifically, correcting iron deficiency with IV iron can achieve target hemoglobin of 12 g/dL in 55% of patients without requiring erythropoietin, reducing ESA-related costs and cardiovascular risks. 8