Example Data Table
| Sex | Ferritin (ng/mL) | sTfR (mg/L) | Iron (µg/dL) | TIBC (µg/dL) | Hb (g/dL) | MCV (fL) | CRP (mg/L) | IDI | TSAT | Score | Category |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Female | 18.0 | 3.2 | 55.0 | 320.0 | 11.8 | 78.0 | 2.0 | 2.549 | 17.2% | 90.0 | High |
| Male | 75.0 | 2.1 | 95.0 | 300.0 | 14.2 | 90.0 | 9.0 | 1.120 | 31.7% | 11.0 | Low |
Formula Used
- Iron Deficiency Index (IDI): IDI = sTfR / log10(Ferritin)
- Transferrin Saturation (TSAT): TSAT% = (Serum Iron / TIBC) × 100
- Composite Score (0–100): Weighted points from ferritin, IDI, TSAT, hemoglobin, and MCV.
How to Use This Calculator
- Enter Ferritin and sTfR (required).
- Optionally add Serum Iron and TIBC to compute TSAT.
- Add Hemoglobin, MCV, and CRP for better context.
- Press Calculate to see results below the header.
- Use Download CSV or Download PDF to export your report.
Why Combine Ferritin and sTfR
Ferritin reflects stored iron, while soluble transferrin receptor (sTfR) rises when tissues need more iron. Using them together reduces ambiguity that can occur when one marker is borderline. In this calculator, ferritin is entered in ng/mL and sTfR in mg/L. Very low ferritin values (for example, below 15 ng/mL) strongly support depleted stores, while sTfR adds sensitivity when symptoms appear before ferritin falls dramatically. When ferritin sits between 30 and 100 ng/mL, symptoms and other markers help determine whether supplementation is appropriate.
Interpreting the Iron Deficiency Index
The Iron Deficiency Index is calculated as IDI = sTfR / log10(Ferritin). Because ferritin is inside a logarithm, the index increases quickly when ferritin is low and sTfR is elevated. This tool flags higher values as more suggestive (for example, IDI at or above 2.0 is often more consistent with deficiency), but the exact cutoff can vary by assay platform and reference population.
Adding Transferrin Saturation for Context
If you provide Serum Iron and TIBC, the calculator estimates transferrin saturation: TSAT% = (Serum Iron / TIBC) × 100. Lower saturation may indicate limited circulating iron, especially when TSAT drops below 20%. The scoring model assigns more points when TSAT is below 15%, fewer points when it is 15–19.9%, and minimal points when it is 20–24.9%. This helps align the index with functional availability.
Hemoglobin and Red-Cell Indices
Hemoglobin and mean corpuscular volume (MCV) support interpretation when anemia is present. The score adds weight if hemoglobin is below 12.0 g/dL (female default) or below 13.0 g/dL (male default). Microcytosis can appear later, so an MCV under 80 fL adds points, while 80–84.9 fL adds fewer points. Normal hemoglobin and MCV do not exclude early deficiency.
Inflammation, CRP, and Result Confidence
Ferritin is an acute-phase reactant and can rise during infection, inflammation, or chronic disease. When CRP exceeds 5 mg/L, this calculator displays a warning and reduces the ferritin contribution in the composite score. A higher index with an elevated CRP may still be meaningful, but interpretation should consider clinical context, repeat testing, and physician guidance.
FAQs
1) What does the Iron Deficiency Index measure?
It combines sTfR and ferritin into one ratio to summarize iron need versus iron stores. Higher values generally suggest deficiency, but assay methods and populations can shift practical cutoffs.
2) Why are ferritin and sTfR required inputs?
The index formula uses both values. Ferritin must be positive because the calculator applies log10(ferritin). sTfR must be positive because it represents receptor concentration.
3) What if I only have serum iron and TIBC?
You can estimate TSAT, but the main index cannot be calculated without ferritin and sTfR. Enter the required markers when available to get both the index and the composite score.
4) How should I interpret results when CRP is high?
Elevated CRP can make ferritin look higher than true iron stores. The tool flags this and down-weights ferritin in scoring. Discuss inflammation, infection, or chronic disease with a clinician before conclusions.
5) Do pregnancy and childhood use different cutoffs?
Yes. Reference ranges and thresholds can differ by age, pregnancy status, altitude, and lab method. This calculator uses general adult-style guidance for scoring, so medical interpretation should be individualized.
6) Why might my score feel different from my lab report?
The score is a weighted summary of several markers, not a diagnostic label. Labs often report individual reference ranges, while this tool integrates patterns and adds an inflammation warning for context.