🔬 Ferritin and Iron Studies

Iron studies assess the body's iron stores and the transport of iron in the blood. Ferritin reflects total body iron stores and is the most useful single test for deficiency. Serum iron, Transferrin (or TIBC), and Transferrin Saturation provide a more complete picture, especially when inflammation is present or when iron overload is suspected. These tests are essential for diagnosing iron deficiency anaemia, anaemia of chronic disease, and hereditary haemochromatosis, and for guiding iron replacement therapy.

Indications include investigation of microcytic anaemia (low MCV), unexplained fatigue, or suspected iron overload (e.g., family history of haemochromatosis, bronzed skin, or hepatomegaly). It is also indicated for monitoring patients on iron replacement therapy, those with chronic conditions where functional iron deficiency is common (e.g., Heart Failure, IBD, CKD), and for pre-operative optimisation in patients with anticipated high blood loss. Screening for iron deficiency is also common in pregnancy and in patients with malabsorption syndromes like Coeliac disease.

A venous blood sample is taken, ideally in a fasting state for the most accurate serum iron and transferrin measurements (though not strictly necessary for ferritin). Ferritin is measured via immunoassay. Serum Iron measures the total iron in the blood, while Total Iron Binding Capacity (TIBC) or Transferrin measures the blood's capacity to bind iron. Transferrin Saturation is a calculated percentage: (Serum Iron / TIBC) x 100. Modern labs often report 'Soluble Transferrin Receptors' if iron deficiency is suspected despite inflammation, though this is less common in routine practice.

Normal ranges vary by lab, but generally: Ferritin 30–300 μg/L (males) and 15–200 μg/L (females). Serum Iron: 10–30 μmol/L. TIBC: 45–70 μmol/L. Transferrin Saturation: 20–45%. In a healthy person, iron stores are sufficient, and the transferrin is roughly one-third saturated. Normal results in the presence of microcytic anaemia should prompt investigation into other causes like Thalassemia (via Hb electrophoresis) or lead poisoning.

Ferritin is the first-line test; a very low level is diagnostic of deficiency. However, because ferritin is an acute-phase reactant, a 'normal' ferritin (e.g., 50-100 μg/L) does not rule out iron deficiency in the presence of inflammation (e.g., infection, malignancy, or RA). In these cases, Transferrin Saturation (Tsat) is more reliable; a Tsat <20% suggests deficiency regardless of the ferritin level. Hereditary Haemochromatosis is suspected with a high Tsat and progressively rising ferritin, requiring genetic testing (HFE gene) for confirmation.

Low ferritin (<30 μg/L) is the most specific marker for iron deficiency, even if the haemoglobin is currently normal (latent iron deficiency). High ferritin suggests iron overload (haemochromatosis) or, more commonly, an acute phase response. In iron deficiency anaemia, Serum Iron is low, TIBC is high, and Transferrin Saturation is low (usually <16-20%). In anaemia of chronic disease, ferritin is normal or high, but serum iron and TIBC are both low, reflecting 'trapped' iron stores that the body cannot utilise. High Transferrin Saturation (>45-50%) is a sensitive screening marker for hereditary haemochromatosis.

These studies are vital for differentiating types of microcytic anaemia. Iron deficiency in adults (especially men and post-menopausal women) must be investigated for an underlying cause, most crucially gastrointestinal malignancy. Conversely, diagnosing iron overload is essential to prevent organ damage (cirrhosis, diabetes, cardiomyopathy). For patients with chronic kidney disease (CKD) or heart failure, maintaining specific iron parameters is essential for symptom management and erythropoiesis-stimulating agent (ESA) efficacy.

A common pitfall is ignoring a 'borderline' ferritin in an elderly patient or one with high CRP; these patients often have significant iron deficiency that is masked by the acute phase response. Another is checking iron studies too soon after a blood transfusion or intravenous iron, which will yield falsely elevated results. Clinicians should also be aware that oral iron supplements should be stopped for 48 hours before testing to ensure serum iron levels are not artificially peaked.

The main limitation is that Ferritin rises in response to inflammation, liver disease, or alcohol consumption, which can mask an underlying iron deficiency ('functional iron deficiency'). Serum iron fluctuates significantly throughout the day and in response to recent dietary intake (including iron supplements), making it an unreliable marker when used in isolation. Interpretation requires a holistic view of the Full Blood Count (FBC) and inflammatory markers (CRP). Many labs will not perform a full iron panel if the ferritin is clearly low.

Students must remember that iron deficiency in a male or post-menopausal female is 'GI cancer until proven otherwise' and requires urgent 2-week-wait (2WW) referral for gastroscopy and colonoscopy. Understand the 'anaemia of chronic disease' pattern: low iron, low TIBC, high/normal ferritin. Note that ferritin >100 μg/L generally excludes absolute iron deficiency in most healthy patients, but in Heart Failure, NICE suggests treating iron deficiency if ferritin is <100 (or ferritin 100-299 if Tsat <20%).