Iron Studies

Iron studies assess iron supply, transport and storage using serum iron, transferrin, total iron binding capacity, transferrin saturation and ferritin, with soluble transferrin receptor as an adjunct. The panel separates iron deficiency from anaemia of chronic disease, iron overload and other causes of microcytosis.

Iron deficiency is the most common nutritional deficiency worldwide and the leading cause of anaemia. Iron overload is far less common. Hereditary haemochromatosis (HFE C282Y homozygosity) is most frequent in people of Northern European ancestry.

Physiological basis. Absorbed iron is carried by transferrin to the marrow and to stores. Hepcidin, made by the liver, blocks iron absorption and release from stores. In inflammation, hepcidin rises and produces a low serum iron with preserved or raised stores.

Indications. Microcytic or hypochromic anaemia, suspected iron deficiency without anaemia (fatigue, restless legs, hair loss, pica), anaemia of chronic disease, chronic kidney disease, heart failure, suspected haemochromatosis or transfusional overload, and monitoring of iron therapy.

Specimen. Fasting morning serum, with iron supplements withheld for at least 24 hours. Serum iron is very sensitive to timing and recent intake.

Adult reference ranges are given. Local laboratory ranges take precedence.

Serum iron. Iron bound to transferrin. Range: men 11 to 30 micromol/L (60 to 170 microg/dL), women 9 to 30 micromol/L (50 to 170 microg/dL). Raised: iron overload, haemolysis, ineffective erythropoiesis, acute hepatitis, recent iron ingestion, sideroblastic anaemia. Lowered: iron deficiency, inflammation, infection, malignancy.

It has marked diurnal variation, peaking in the morning, and varies by up to 30% from day to day. It must not be interpreted alone.

Transferrin and total iron binding capacity (TIBC). Transferrin is the iron transport protein, and TIBC reflects its binding capacity. Range: transferrin 2.0 to 3.6 g/L, TIBC 45 to 72 micromol/L (250 to 400 microg/dL). Raised: iron deficiency, pregnancy, oral contraceptives. Lowered: inflammation (transferrin is a negative acute phase protein), liver disease, malnutrition, nephrotic syndrome, iron overload.

TIBC rises in iron deficiency and falls in chronic disease, which makes it a useful discriminator when ferritin is uncertain.

Transferrin saturation (TSAT). Serum iron divided by TIBC, multiplied by 100. Range: 20 to 45%. Raised: haemochromatosis, transfusional overload, sideroblastic anaemia, acute liver injury. Lowered: iron deficiency, inflammation.

A value under 16 to 20% indicates iron restricted erythropoiesis. A fasting value over 45% suggests possible iron overload and prompts HFE genotyping. In haemochromatosis it rises before ferritin does.

Ferritin. Intracellular iron storage protein released in proportion to stores, and also an acute phase protein. Range: men 30 to 400 microg/L, women 15 to 150 microg/L. Raised: iron overload, inflammation, infection, malignancy, liver disease, alcohol, metabolic syndrome, adult onset Still disease, haemophagocytic lymphohistiocytosis. Lowered: iron deficiency.

A value under 30 microg/L indicates iron deficiency in the absence of inflammation. When inflammation is present, values up to 100 microg/L can still represent deficiency. In heart failure, deficiency is defined by ferritin under 100 microg/L, or 100 to 299 microg/L with TSAT under 20%.

Soluble transferrin receptor (sTfR). Truncated membrane receptor released in proportion to erythroid iron demand, and not affected by inflammation. Range: assay specific, approximately 0.76 to 1.76 mg/L. Raised: iron deficiency, haemolysis, thalassaemia, polycythaemia vera, ineffective erythropoiesis. Lowered: aplastic anaemia, pure red cell aplasia, erythropoietin deficiency.

It is most useful for separating combined iron deficiency and chronic disease from chronic disease alone. The sTfR to log ferritin ratio is high (typically over 2) in iron deficiency with inflammation and low (under 1) in chronic disease alone, with assay specific cut offs. It is not widely available.

Causes of iron deficiency. Blood loss (gastrointestinal, menstrual, urinary), inadequate intake, malabsorption (coeliac disease, Helicobacter pylori infection, atrophic gastritis, bariatric surgery, proton pump inhibitors), increased demand (pregnancy, growth, blood donation), hookworm and schistosomiasis in endemic regions. In men and post-menopausal women, iron deficiency indicates gastrointestinal blood loss until proven otherwise.

Causes of iron overload. Hereditary haemochromatosis, repeated transfusion, ineffective erythropoiesis (thalassaemia major and intermedia, sideroblastic anaemia, myelodysplasia), chronic liver disease, prolonged iron therapy, and dietary overload in parts of sub-Saharan Africa.

Critical values and findings needing immediate action.

• Serum iron over 90 micromol/L (500 microg/dL) measured 4 to 6 hours after an overdose indicates severe toxicity.
• TSAT over 45% with ferritin over 1000 microg/L, particularly with raised transaminases or thrombocytopenia, suggests iron overload with a risk of cirrhosis and needs liver assessment and specialist referral.

Interferences and pitfalls.

• Oral iron ingestion raises serum iron for up to 24 hours and falsely normalises the result.
• Haemolysed samples falsely raise serum iron.
• Oral contraceptives and pregnancy raise transferrin and TIBC.
• Ferritin rises with inflammation, liver disease and alcohol, so a normal ferritin does not exclude iron deficiency.
• Recent intravenous iron raises ferritin and TSAT for weeks.
• Recent transfusion alters all indices.
• Assay differences between laboratories can reach 30%.

Special groups.

• Pregnancy: TIBC rises, serum iron and ferritin fall. A ferritin under 30 microg/L in early pregnancy indicates deficiency.
• Children: iron deficiency is suggested by ferritin under 12 microg/L in children under 5 years, using age specific ranges.
• Chronic kidney disease: ferritin and TSAT thresholds for deficiency are higher than in the general population, and a ferritin up to 500 microg/L can coexist with functional iron deficiency.
• Older adults: inflammation commonly raises ferritin and masks deficiency.

Follow up tests. Full blood count, blood film, reticulocyte count, C-reactive protein, HFE genotyping, liver function tests, coeliac serology, Helicobacter pylori testing, upper gastrointestinal endoscopy and colonoscopy, magnetic resonance imaging of liver iron, bone marrow iron stain, haemoglobin electrophoresis, blood lead level.