Iron Deficiency Anaemia: Ferritin, Serum Iron & Blood Test Guide

Key Takeaways

Iron deficiency is the most common nutritional deficiency worldwide, affecting around 3% of men and 8% of women in the UK.
Ferritin is the most sensitive early marker — it falls before haemoglobin drops.
Iron deficiency and iron deficiency anaemia are not the same thing: you can be iron depleted without being anaemic.
A fasting blood sample is recommended for accurate serum iron and TIBC results.

What Is Iron Deficiency Anaemia?

Iron is essential for producing haemoglobin, the protein in red blood cells that carries oxygen around your body. When your iron stores become depleted, your body can no longer make enough healthy red blood cells, leading to iron deficiency anaemia (IDA). It is the most common cause of anaemia in the UK and globally, accounting for around half of all anaemia cases according to the World Health Organization.

It is important to understand the distinction between iron deficiency (low iron stores without anaemia) and iron deficiency anaemia (low iron stores plus a reduced haemoglobin level). Iron deficiency on its own can still cause significant symptoms including fatigue, poor concentration, and hair loss. Identifying it early through blood testing allows treatment before anaemia develops.

The Key Blood Tests for Iron Status

Your GP or private testing provider will typically request an iron studies panel alongside a full blood count (FBC). The iron studies panel includes several markers, each measuring a different aspect of your iron metabolism.

Ferritin

Ferritin is a protein that stores iron inside your cells. A serum ferritin test reflects your total body iron stores and is the single most useful test for detecting iron deficiency. In the UK, the normal reference range is typically 15–300 µg/L for men and 15–200 µg/L for women, although ranges vary slightly between laboratories.

NICE guidelines suggest that a ferritin level below 30 µg/L is consistent with iron deficiency, even if haemoglobin remains normal. However, ferritin is also an acute phase reactant, meaning it rises during inflammation, infection, or liver disease. This can mask underlying iron deficiency. If inflammation is suspected, your doctor may also request a CRP (C-reactive protein) test alongside ferritin.

Serum Iron

Serum iron measures the amount of iron circulating in your blood, bound to the transport protein transferrin. The normal range is approximately 10–30 µmol/L. Serum iron levels fluctuate significantly throughout the day and are affected by recent food intake, which is why a fasting morning sample is recommended for the most accurate result.

On its own, serum iron is not a reliable marker of iron deficiency because it varies so much. It is most useful when interpreted alongside TIBC and transferrin saturation.

TIBC and Transferrin Saturation

Total iron-binding capacity (TIBC) measures the maximum amount of iron that transferrin can carry. In iron deficiency, the body produces more transferrin to try to capture more iron, so TIBC rises. The normal range is approximately 45–80 µmol/L. A TIBC above 70 µmol/L is suggestive of iron deficiency.

Transferrin saturation is calculated as serum iron divided by TIBC, expressed as a percentage. A transferrin saturation below 20% suggests iron deficiency, while below 16% strongly supports the diagnosis. This is one of the most reliable indicators when interpreted alongside ferritin.

Full Blood Count Findings in Iron Deficiency Anaemia

When iron deficiency progresses to anaemia, characteristic changes appear on the FBC:

Low haemoglobin (Hb) — below 130 g/L in men or 120 g/L in non-pregnant women (WHO criteria).
Low mean cell volume (MCV) — red cells are smaller than normal (microcytic), typically below 80 fL.
Low mean cell haemoglobin (MCH) — each red cell contains less haemoglobin than normal (hypochromic).
High red cell distribution width (RDW) — increased variation in red cell size, often an early sign.

Common Symptoms of Iron Deficiency

Symptoms can develop gradually and are often dismissed as “just tiredness.” Common signs include:

Persistent fatigue and low energy
Shortness of breath on exertion
Pale skin, nail beds, and inner lower eyelids
Heart palpitations
Brittle nails or spoon-shaped nails (koilonychia)
Hair thinning or increased hair loss
Restless legs syndrome
Difficulty concentrating and poor memory
Unusual cravings for non-food items such as ice (pica)

Common Causes in the UK

Iron deficiency occurs when iron losses or demands exceed dietary intake. The main causes include:

Heavy menstrual bleeding — the most common cause in premenopausal women.
Inadequate dietary intake — vegetarian and vegan diets may provide less bioavailable iron.
Gastrointestinal blood loss — peptic ulcers, colon polyps, inflammatory bowel disease, or bowel cancer.
Malabsorption — coeliac disease, inflammatory bowel disease, or previous gastric surgery.
Pregnancy — increased iron demands for the growing foetus and placenta.
Frequent blood donation — regular donors may become iron depleted without supplementation.

NICE guidelines recommend that all men and postmenopausal women with newly diagnosed iron deficiency anaemia should be investigated for gastrointestinal causes, as it may be the first sign of bowel cancer or coeliac disease.

Treatment and Monitoring

First-line treatment is oral iron supplementation, typically ferrous sulphate 200 mg two to three times daily. The NHS recommends taking iron on an empty stomach with vitamin C (such as a glass of orange juice) to improve absorption. Common side effects include constipation, nausea, and black stools.

If oral iron is not tolerated or absorption is impaired, intravenous iron (such as ferric carboxymaltose) may be offered. This is particularly common during pregnancy or in patients with inflammatory bowel disease.

After starting treatment, your GP will typically repeat blood tests at 2–4 weeks to check for a rise in haemoglobin (expect an increase of around 10–20 g/L per month) and again at 2–3 months once haemoglobin has normalised. Iron supplementation should continue for a further 3 months after haemoglobin normalises to replenish ferritin stores. A follow-up ferritin check confirms stores have been adequately replaced.

Getting Tested at Home

If you are experiencing symptoms of iron deficiency, you do not need to wait for a GP appointment to get tested. A mobile phlebotomist can visit your home or workplace and take a blood sample for iron studies, ferritin, and a full blood count. Results are typically available within 24 to 48 hours. This is especially convenient if you need fasting blood tests, as you can have your sample taken first thing in the morning at home. You can also browse our home blood test services to find the right test for you.