sTfR-Ferritin Index Calculator

Separate iron deficiency from anaemia of inflammation using the ratio of soluble transferrin receptor to the log of ferritin.

Last updated 31 Aug 2026

sTfR-Ferritin Index

sTfR ÷ log ferritin

sTfR (mg/L)Assay-specific — see the note below.

Ferritin (µg/L)µg/L and ng/mL are identical.

2.06indexExample

sTfR 3.4 mg/L, ferritin 45 µg/L

Formula

sTfR-F index = sTfR (mg/L) ÷ log₁₀ ferritin (µg/L)

sTfR: soluble transferrin receptor, mg/L — not comparable across assay platforms
log₁₀ ferritin: base-10 logarithm compresses the wide ferritin range and dampens the acute-phase effect

Worked example

sTfR 3.4 mg/L, ferritin 45 µg/L
log₁₀(45) = 1.653
3.4 ÷ 1.653 = 2.06
Above 2 → iron deficiency likely

Interpretation

Cut-offs of 1 and 2 apply to the Roche assay. Other platforms use different thresholds — confirm with your laboratory before applying them.
Index	Pattern	Response to iron
< 1	Anaemia of inflammation, stores adequate	Unlikely to help
1 – 2	Indeterminate	Consider if saturation < 20%
> 2	Iron deficiency, with or without inflammation	Likely to help

Why the ratio works

The two components of this index fail in opposite directions, which is what makes their ratio informative. Ferritin reflects iron stores but rises with inflammation, so it can look reassuring in a patient who is genuinely deficient. Soluble transferrin receptor reflects iron-starved erythroid precursors and is unaffected by the acute-phase response, but it also rises with any expansion of the erythroid marrow.

Dividing receptor by the logarithm of ferritin combines both signals while damping the inflammatory distortion. The logarithm matters: ferritin spans three orders of magnitude in clinical practice, and taking logs keeps the index on a usable scale and reduces the leverage of a single very high acute-phase value.

The index earns its place in exactly the patients where the standard panel is ambiguous — chronic kidney disease, inflammatory bowel disease, rheumatoid arthritis, heart failure, malignancy — where a normal ferritin cannot be trusted. Its main practical limitation is the absence of assay standardisation: the widely quoted cut-offs of 1 and 2 come from Roche-based studies and cannot be transferred to another platform without local validation.

Frequently asked questions

What is a normal sTfR-ferritin index?

Below 1 on the Roche assay suggests adequate iron stores with inflammation as the cause of anaemia. Above 2 suggests iron deficiency. Between 1 and 2 the result is indeterminate.

Can I use these cut-offs with any sTfR assay?

No. Reference intervals for sTfR differ several-fold between manufacturers, so the index thresholds shift with them. Ask your laboratory for platform-specific cut-offs.

When is this better than ferritin alone?

Whenever inflammation is present. In chronic kidney disease, inflammatory bowel disease, rheumatoid arthritis and malignancy, ferritin can sit in the normal range despite absent iron stores, and the index remains interpretable.

Which ferritin units does the formula use?

µg/L, which is numerically identical to ng/mL. The logarithm is base 10.

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References

Punnonen K, Irjala K, Rajamäki A. Serum transferrin receptor and its ratio to serum ferritin in the diagnosis of iron deficiency. Blood. 1997;89(3):1052–7.
Weiss G, Goodnough LT. Anemia of chronic disease. N Engl J Med. 2005;352(10):1011–23.