Iron testing and ferritin explained

Your ferritin number tells a story — but only if you know how to read it. For plant-based eaters especially, a low ferritin rarely means what most people assume. And for anyone with inflammation or chronic disease, a “normal” ferritin can mask a genuine deficiency. A single test is almost never enough.

The quick read

Marker	Deficiency threshold	Notes
Serum ferritin	below 15 µg/L	WHO 2020 (apparently healthy adults); rises to below 70 µg/L if inflammation present
Transferrin saturation (TSAT)	below 20%	Reliable even when ferritin is inflated
Hemoglobin (women)	below 120 g/L	Late-stage marker — misses early depletion
Hemoglobin (men)	below 130 g/L	Late-stage marker — misses early depletion
sTfR/log ferritin index	—	AUC 0.87; combined parameters lift detection from 41% to ~92%

The minimum useful panel is ferritin plus TSAT. Hemoglobin alone is an inadequate iron screen.

Why hemoglobin misses the point

Hemoglobin is the last marker to fall. Iron depletion follows a staged sequence: stores drop first (ferritin), then transport capacity falls (TSAT), then red blood cell production is finally impaired and hemoglobin declines. By the time hemoglobin drops below the anemia threshold — below 120 g/L in women, 130 g/L in men (NIH ODS, 2023) — iron stores have often been depleted for weeks or months.

Iron deficiency without anemia (IDWA) is at least twice as common as iron deficiency anemia, yet remains systematically underdiagnosed because hemoglobin is the most common screening test (Al-Naseem et al., 2021). IDWA carries its own symptom burden: fatigue, reduced exercise tolerance, impaired cognitive function, and hair thinning — all without the hemoglobin number flagging anything. If you are symptomatic but have “normal” bloodwork, ask which markers were actually checked.

The ferritin problem

Ferritin is the best available single marker for iron stores in healthy individuals — but it is not a simple iron gauge. Ferritin is an acute-phase reactant: it rises in response to inflammation, infection, obesity, rheumatoid arthritis, IBD, and chronic heart failure (Dignass et al., 2018). When C-reactive protein (CRP) is elevated, serum ferritin behaves as an acute-phase protein and rises independently of iron stores, making it an unreliable standalone marker in that context (Dignass et al., 2018). A ferritin of 80 µg/L looks fine on paper; with high inflammation it may reflect genuine iron depletion.

The WHO updated its ferritin thresholds in 2020 specifically to account for this. The current guidance sets the iron deficiency cutoff at 15 µg/L for apparently healthy adults, rising to 70 µg/L when inflammation markers are elevated (WHO, 2020). Many clinical labs report reference ranges from an older era; the 70 µg/L inflammation-adjusted threshold in particular is rarely applied in routine practice.

A ferritin below 12–15 µg/L means essentially zero iron stores regardless of inflammation. That number needs follow-up no matter what the rest of the panel shows.

What TSAT and sTfR add

Transferrin saturation (TSAT) measures the percentage of iron-binding capacity actually occupied by iron. Below 20%, the body is not receiving enough iron for normal function — even when ferritin appears normal or high (NIH ODS, 2023; Dignass et al., 2018). TSAT is not an acute-phase reactant, so it stays reliable in inflammatory states where ferritin misleads. In chronic inflammatory conditions, TSAT plus ferritin is the recommended minimum two-marker approach.

Soluble transferrin receptor (sTfR) goes further. It reflects iron demand at the cellular level and does not rise with inflammation. The sTfR/log ferritin index improves detection from roughly 41% (ferritin alone) to around 92% with combined parameters, with an AUC of 0.87 for the sTfR index (Metzgeroth et al., 2012). sTfR is most useful when ferritin is ambiguous — elevated by inflammation but possibly masking depletion — and is not yet routine in primary care due to variable laboratory standardization (Pfeiffer & Looker, 2017).

What lower ferritin means for plant-based eaters

Studies consistently show lower serum ferritin in vegetarians and vegans compared to omnivores. A systematic review and meta-analysis found the mean difference to be approximately −29.7 µg/L (Haider et al., 2018). That gap looks alarming at first. It is less alarming in context.

After controlling for BMI and inflammatory markers, iron deficiency prevalence was not significantly higher in vegetarian men (3.1% vs. 0% in omnivores, not statistically significant) or in non-menstruating women (Slywitch et al., 2021). The lower ferritin in plant-based eaters appears to reflect an adaptive downregulation of iron storage driven by non-heme iron’s regulatory pathways — the body absorbs and stores less because it can modulate uptake more readily than with heme iron. The lower ferritin typically sits in the 20–60 µg/L range, not near zero.

The exception matters: premenopausal menstruating women on vegetarian diets showed iron deficiency prevalence of 51.5% vs. 31.9% in omnivore peers (Slywitch et al., 2021). Menstrual blood loss is a large enough demand that the lower-ferritin adaptive baseline becomes a genuine liability. This group warrants active monitoring regardless of symptoms. See iron deficiency in vegan women for a full breakdown.

Practical guidance

• Ask for ferritin AND TSAT, not just a CBC with hemoglobin. If your doctor orders a “standard” iron panel, confirm TSAT is included.
• Add CRP or AGP to contextualize ferritin. Without an inflammation marker, a ferritin of 40–70 µg/L cannot be reliably interpreted.
• Apply the WHO 2020 thresholds — 15 µg/L without inflammation (apparently healthy adults), 70 µg/L with inflammation — not just the older reference ranges.
• If you are symptomatic (fatigue, cold intolerance, hair loss, reduced exercise capacity) and hemoglobin is normal, push for a full iron panel. IDWA is underdiagnosed and fully treatable.
• Menstruating plant-based eaters should monitor iron status at least annually, even without symptoms.
• Don’t self-supplement without data. Iron supplementation in people with adequate stores carries risks; the decision should factor in symptoms, the full panel, and the underlying cause.

Common misconceptions

• “My hemoglobin is fine, so my iron is fine.” Hemoglobin falls last. Normal hemoglobin does not rule out depleted stores or IDWA.
• “My ferritin is 35 — I’m above the limit.” The WHO 2020 cutoff is 15 µg/L in apparently healthy adults, rising to 70 µg/L with inflammation. If your lab’s reference range starts at 10–12 µg/L, that is an older threshold and still within the plausible range, but the 70 µg/L inflammation-adjusted cutoff is rarely displayed at all.
• “High ferritin means good iron status.” Not when inflammation is present. As an acute-phase reactant, ferritin rises with inflammation independently of iron stores (Dignass et al., 2018).
• “Vegans always have iron deficiency — look at the ferritin.” Lower ferritin is common in plant-based eaters but is largely adaptive, not pathological — provided it stays out of the near-zero range and menstrual losses are modest.
• “One ferritin test is all I need.” Without TSAT and an inflammation marker, a single ferritin produces both false negatives and false positives at clinically relevant rates.

The punchline

Iron status is a three-layer picture: stores (ferritin), transport (TSAT), and production (hemoglobin). Testing only the last layer — as most routine panels do — misses deficiency until it is severe.

For plant-based eaters the stakes are lower than the ferritin gap suggests, but they are not zero. Lower ferritin is normal and mostly benign; near-zero ferritin, TSAT below 20%, and IDWA symptoms are not. Read the full picture in iron and plant-based diets.