Confirm iron deficiency and investigate the cause

I review haemoglobin, ferritin and their trends alongside symptoms, bleeding and inflammation. When ferritin is difficult to interpret, I add transferrin saturation. Ret-He may provide an additional clue to whether iron supply to developing red cells is restricted.

Treat confirmed deficiency while investigating blood loss or malabsorption. A response to iron supports the diagnosis but does not identify its cause.

Guidance behind this advice

1. Start with the clinical picture

Review the FBC, ferritin trend, symptoms and bleeding history. Check CRP and renal function when inflammation or kidney disease is possible. Recent iron treatment can affect the results.

Awanui Auckland’s lower ferritin reference limit for adults aged 20 and over is 30 µg/L, effective from 5 October 2026.

2. Read ferritin, TSAT and Ret-He together

With ferritin between 30 and 100 µg/L, low TSAT and Ret-He strengthen the evidence of iron restriction. Assess blood loss and inflammation to decide whether stores are depleted, iron is poorly available, or both.

Adult iron studies at a glance
Marker	What it tells you	Practical interpretation
Ferritin	Iron stores, influenced by inflammation.	A value at or below 30 µg/L supports depleted stores, even with normal haemoglobin. Inflammation can raise ferritin and obscure deficiency.
TSAT	The proportion of transferrin carrying iron.	Below 20% supports restricted iron availability. This can occur with depleted stores or inflammatory iron restriction.
Ret-He	Recent iron supply to developing red cells.	A low result can help identify iron-restricted red-cell production when ferritin is equivocal. Interpret alongside ferritin, TSAT and the FBC.

3. Look for the cause

Ask about menstrual and gastrointestinal blood loss, blood donation, diet, NSAIDs and previous gastrointestinal surgery. Check coeliac serology while the patient is eating gluten, and assess urinary blood loss when indicated.

Unexplained iron deficiency anaemia in an older individual usually warrants gastrointestinal assessment, often gastroscopy and colonoscopy. In menstruating patients, consider gastrointestinal investigation for concerning symptoms, recurrent anaemia, family risk or loss disproportionate to the menstrual history.

4. Replace iron and review the response

Start oral iron while investigating the cause. Once-daily dosing is usually sufficient; alternate-day dosing may improve tolerance. Consider intravenous iron for intolerance, inadequate response or impaired absorption.

Repeat the FBC after two to four weeks. If haemoglobin is not rising, check the dose actually taken, adherence, ongoing bleeding, absorption and other causes of anaemia. Document the repeat tests and investigation plan.

Clinical pearls

Iron deficiency can be present before haemoglobin falls; a normal MCV does not exclude it.
Ret-He can also be low in thalassaemia trait. Review unexplained microcytosis alongside the iron studies.
In an anaemic patient taking daily oral iron, a haemoglobin rise of at least 10 g/L within two weeks strongly supports iron deficiency. Investigate the cause even when treatment works.

Borderline ferritin: assessing iron deficiency