What is the normal serum iron concentration in a healthy adult?

Normal Serum Iron Levels in Healthy Adults

Normal serum iron concentration in healthy adults ranges from approximately 60–170 μg/dL (10.7–30.4 μmol/L), though this value shows marked diurnal variation and is less reliable than ferritin for assessing iron status.

Understanding Serum Iron as a Diagnostic Marker

Serum iron measures the amount of iron bound to transferrin in the bloodstream, representing iron currently in transit and available for immediate use in red blood cell production 1. However, serum iron alone provides an incomplete picture of iron status and must be interpreted alongside other parameters 1.

Key Limitations of Serum Iron Measurement

• Serum iron exhibits significant day-to-day variation with an average intrasubject coefficient of variation of 28.5%, making a single measurement less reliable than ferritin (which varies only 14.5%) 2.
• Diurnal fluctuation affects interpretation if samples are not collected at consistent times, with values typically higher in the morning 1.
• Serum iron reflects readily available iron but not total body iron stores, unlike ferritin which directly correlates with storage iron 1, 3.

Complementary Iron Status Parameters

Transferrin Saturation (TSAT)

• TSAT is calculated as (serum iron × 100) ÷ total iron-binding capacity (TIBC) and indicates the percentage of transferrin binding sites occupied by iron 1.
• Normal TSAT ranges from 20–50% in healthy adults; values <16–20% indicate iron deficiency regardless of ferritin level 4, 1.
• TSAT provides a more clinically useful assessment of iron availability for erythropoiesis than serum iron alone 1.

Serum Ferritin: The Gold Standard

• Normal ferritin values are 135 μg/L for men and 43 μg/L for women according to the CDC 1.
• Ferritin <15 μg/L has 99% specificity for absolute iron deficiency, making it the most reliable single marker 4.
• Ferritin 15–30 μg/L generally indicates low body iron stores that warrant intervention 4.
• One microgram per liter of serum ferritin equals approximately 10 mg of stored iron, providing a direct quantitative relationship 1.

Critical Clinical Caveats

Ferritin as an Acute-Phase Reactant

• Ferritin rises during inflammation, infection, or tissue damage, potentially masking depleted iron stores even when true deficiency exists 4, 1, 5.
• In chronic inflammatory conditions (inflammatory bowel disease, chronic kidney disease, heart failure), the ferritin threshold for iron deficiency increases to <100 μg/L 1.
• When inflammation is present, TSAT <20% confirms iron deficiency even if ferritin appears normal 4, 1.

Optimal Diagnostic Approach

• Never rely on serum iron or ferritin alone—interpret both together with TSAT and clinical context 1.
• Check inflammatory markers (CRP, ESR) when ferritin is 30–100 μg/L to distinguish true iron deficiency from anemia of chronic disease 1.
• A sudden increase in ferritin with a drop in TSAT suggests inflammatory iron block rather than true iron sufficiency 1.

Practical Clinical Algorithm

For a patient with suspected iron deficiency:

1. Order serum iron, TIBC, ferritin, and calculate TSAT 1.
2. If ferritin <15 μg/L: absolute iron deficiency confirmed—initiate treatment and investigate source of loss 4, 1.
3. If ferritin 15–30 μg/L: low iron stores likely—begin oral iron supplementation 4.
4. If ferritin 30–100 μg/L with TSAT <20%: check CRP/ESR to distinguish iron deficiency from inflammatory anemia 1.
5. If ferritin >100 μg/L with elevated CRP/ESR: anemia of chronic disease—treat underlying inflammatory condition 1.