Are the UK ferritin cut‑off values (<15 µg/L, 15–30 µg/L, 30–100 µg/L, >100 µg/L) accurate for diagnosing iron deficiency, and how should inflammation be accounted for (e.g., CRP, transferrin saturation)?

UK Ferritin Cut-offs: Accuracy Assessment

The stated UK ferritin cut-offs are partially accurate but require important modifications: ferritin <15 µg/L is correct for high specificity, but the 15–30 µg/L range underestimates iron deficiency risk, and critically, the upper threshold should be 150 µg/L (not 100 µg/L) to exclude iron deficiency in inflammation, with a practical cut-off of 45 µg/L offering optimal sensitivity-specificity balance for clinical decision-making. 1

Ferritin <15 µg/L: Diagnostic Threshold

• Accurate: Ferritin <15 µg/L is highly specific (specificity 0.99) for iron deficiency and indicates absent iron stores 1
• This threshold is the most reliable for confirming iron deficiency without inflammation 1
• However, sensitivity at this level is only 59%, meaning many iron-deficient patients will be missed 2

Ferritin 15–30 µg/L: Low Iron Stores

• Accurate but conservative: This range does indicate low body iron stores and represents the lower limit of normal for most UK laboratories 1
• The British Society of Gastroenterology confirms ferritin <30 µg/L generally indicates low body iron stores 1
• Critical update: A ferritin cut-off of 45 µg/L provides superior diagnostic performance with sensitivity of 85% and specificity of 92%, making it the optimal practical threshold 1, 2

Ferritin 30–100 µg/L: The Inflammation Zone

This range requires significant nuance and the upper limit is incorrect:

• The 45 µg/L threshold is key: Below 45 µg/L warrants investigation, especially with chronic inflammation and anaemia 1
• Ferritin is an acute phase protein that rises with inflammation, potentially masking iron deficiency 1
• Transferrin saturation (TSAT) becomes essential: In inflammatory conditions, TSAT <20% confirms iron deficiency even when ferritin is 100–300 µg/L 3
• CRP should be checked concurrently to interpret ferritin in context of inflammation 1, 4

Practical Approach in Inflammation:

• Ferritin/CRP ratio ≤6 is a simple, effective biomarker for iron deficiency in systemic inflammation (sensitivity and specificity both high) 4
• In chronic heart failure, chronic kidney disease, and inflammatory bowel disease: ferritin <100 µg/L OR TSAT <20% diagnoses iron deficiency 3
• If ferritin is 100–300 µg/L in these conditions, TSAT <20% is required to confirm deficiency 3

Ferritin >100 µg/L: Critical Correction Needed

The stated threshold of >100 µg/L is INCORRECT according to UK guidelines:

• Ferritin >150 µg/L is unlikely to occur with absolute iron deficiency, even in severe inflammation 1
• This is the evidence-based upper threshold from the British Society of Gastroenterology 1
• The 100 µg/L threshold applies specifically to chronic inflammatory conditions (heart failure, kidney disease, IBD) where it serves as the lower diagnostic cut-off, not the upper exclusion threshold 3

Inflammation Assessment: CRP and TSAT

When ferritin is equivocal (30–150 µg/L), additional testing is mandatory:

• Check CRP: Elevated CRP indicates ferritin may be falsely reassuring 1, 4
• Measure TSAT: TSAT <20% confirms functional or absolute iron deficiency regardless of ferritin level in inflammatory states 3, 5
• TSAT is more strongly associated with adverse outcomes than ferritin in conditions like heart failure 5
• In inflammatory bowel disease, optimal thresholds vary: ferritin 65–80 µg/L and TSAT 11–16% depending on CRP level 6

Alternative Markers (Less Commonly Available):

• Soluble transferrin receptor (sTfR) is not influenced by inflammation and provides superior discrimination, but most UK hospitals do not offer this test 1
• The sTfR/log₁₀ ferritin ratio may be superior to either test alone in chronic disease 1

Common Pitfalls to Avoid

• Do not use ferritin <15 µg/L as the sole diagnostic threshold—this misses 41% of iron-deficient patients 2
• Do not accept ferritin 30–100 µg/L as "normal" without checking inflammatory markers 1
• Do not use 100 µg/L as the upper exclusion limit—the correct threshold is 150 µg/L 1
• In chronic inflammatory conditions, always measure TSAT alongside ferritin 3, 6
• Mean cell hemoglobin (MCH) may be more sensitive than MCV for detecting iron deficiency 1