What ferritin level indicates iron-deficiency anemia?

What Ferritin Level Indicates Iron-Deficiency Anemia

Ferritin <30 µg/L confirms iron deficiency in patients without inflammation, while ferritin <100 µg/L indicates iron deficiency when inflammation is present. 1

Diagnostic Thresholds Without Inflammation

• Ferritin <15 µg/L has 99% specificity for absolute iron deficiency and definitively confirms the diagnosis without requiring additional testing. 1, 2
• Ferritin <30 µg/L generally indicates depleted iron stores and warrants treatment in patients with normal inflammatory markers (CRP, ESR). 1, 2
• The American Gastroenterological Association recommends ferritin <45 ng/mL as the optimal diagnostic cutoff, providing the best balance between sensitivity (85%) and specificity (92%) for clinical practice. 1
• Ferritin >100 µg/L essentially rules out iron deficiency when inflammation is absent. 1

Diagnostic Thresholds With Inflammation

When inflammation is present (elevated CRP or ESR), ferritin thresholds must be adjusted upward because ferritin is an acute-phase reactant that rises independently of iron stores. 1, 3

• Ferritin 30–100 µg/L with elevated CRP/ESR indicates a mixed picture of true iron deficiency coexisting with anemia of chronic disease; transferrin saturation (TSAT) <20% confirms iron deficiency requiring treatment. 1
• Ferritin >100 µg/L with TSAT <20% and elevated CRP/ESR defines anemia of chronic disease with functional iron deficiency, not true iron deficiency. 1
• Ferritin >150 µg/L rarely represents absolute iron deficiency even in the presence of inflammation. 1, 2

Transferrin Saturation: The Critical Confirmatory Test

TSAT <20% is the primary confirmatory marker for iron deficiency, especially when ferritin lies between 30–100 µg/L or may be falsely elevated by inflammation. 1, 4

• Calculate TSAT as: (serum iron × 100) ÷ total iron-binding capacity. 1
• TSAT <16% combined with low MCV, elevated RDW, and microcytic-hypochromic red cells strongly supports iron deficiency anemia. 1
• In inflammatory states, diagnostic criteria for anemia of chronic disease are ferritin >100 µg/L AND TSAT <20% in the presence of elevated inflammatory markers. 1

Special Population Thresholds

Inflammatory Bowel Disease

• During remission: ferritin <30 µg/L reliably indicates iron deficiency. 1
• During active inflammation: ferritin <100 µg/L is used as a screening threshold; confirm with TSAT <20%. 1

Chronic Kidney Disease, Heart Failure, and Cancer

• Use ferritin <100 µg/L (instead of <30 µg/L) as the screening threshold for iron deficiency in these populations. 1, 3
• Absolute iron deficiency is defined as ferritin <100 ng/mL AND TSAT <20%. 1, 2

Diagnostic Algorithm

1. Measure ferritin, TSAT, and inflammatory markers (CRP or ESR) simultaneously. 1
2. If CRP/ESR are normal:
   • Ferritin <30 µg/L confirms iron deficiency
   • Ferritin >100 µg/L excludes iron deficiency 1
3. If CRP/ESR are elevated:
   • Ferritin 30–100 µg/L with TSAT <20% indicates true iron deficiency
   • Ferritin >100 µg/L with TSAT <20% indicates anemia of chronic disease 1
4. When ferritin and TSAT provide conflicting information, measure soluble transferrin receptor (sTfR); an elevated sTfR confirms true iron deficiency because it is not affected by inflammation. 1

Critical Pitfalls to Avoid

• Do not rely on ferritin alone when inflammation is present; always calculate TSAT because functional iron deficiency can exist with high ferritin but low TSAT. 1
• Using the traditional ferritin cutoff of <15 ng/mL misses the majority of iron deficiency cases because this threshold has high specificity but poor sensitivity. 1
• Normal-range ferritin (up to 100 µg/L) does not exclude iron deficiency in the setting of inflammation; iron deficiency may still be present. 1, 5, 3
• In rare cases of extreme inflammation (such as adult-onset Still's disease), ferritin can be elevated to >26,000 µg/L despite severe iron deficiency; TSAT and sTfR become essential diagnostic tools in these situations. 6