⚕️ Educational content only. This article covers anaemia from a biomedical science and haematology perspective. It is not medical advice. Consult a healthcare professional for personal health concerns.
Anaemia is defined as a reduction in haemoglobin concentration below the reference range for age and sex. It is one of the most common laboratory findings worldwide and has numerous causes. The classification of anaemia by red cell size (MCV) provides a practical framework for investigation in the haematology laboratory.
Key Takeaways
- Anaemia is classified by MCV: microcytic (<80 fL), normocytic (80–100 fL), or macrocytic (>100 fL).
- Iron deficiency is the commonest cause of microcytic anaemia worldwide.
- Macrocytic anaemia requires B12/folate testing and consideration of alcohol, drugs, and haematological malignancy.
- The blood film provides additional morphological information beyond the FBC indices.
Definition and Haemoglobin Thresholds
The WHO defines anaemia as haemoglobin <130 g/L in adult males, <120 g/L in non-pregnant adult females, and <110 g/L in pregnant women. These thresholds vary slightly between laboratory reference ranges. Anaemia reduces the oxygen-carrying capacity of blood and presents clinically with fatigue, dyspnoea, pallor, and tachycardia. However, many patients are asymptomatic when anaemia is mild and develops gradually.
Classification by MCV
Microcytic Anaemia (MCV <80 fL)
Caused by reduced haemoglobin synthesis. Iron deficiency anaemia (IDA) is the most common cause globally — caused by insufficient dietary intake, chronic blood loss (menstruation, GI bleeding), or malabsorption (coeliac disease). Other causes include thalassaemia (reduced globin chain synthesis), anaemia of chronic disease (functional iron restriction), and sideroblastic anaemia (impaired haem synthesis). Iron studies, serum ferritin, and haemoglobin electrophoresis help differentiate these.
Normocytic Anaemia (MCV 80–100 fL)
Causes include: acute blood loss (haemoglobin falls before MCV changes), anaemia of chronic disease, haemolytic anaemia (increased red cell destruction), renal failure (reduced erythropoietin), and early combined deficiency (mixed micro- and macrocytic causes cancelling out). The reticulocyte count helps here — an elevated reticulocyte count suggests haemolysis or blood loss; a low count suggests hypoproliferative bone marrow.
Macrocytic Anaemia (MCV >100 fL)
Causes include vitamin B12 deficiency, folate deficiency (both megaloblastic), hypothyroidism, liver disease, alcohol excess, myelodysplastic syndrome (MDS), and drugs (methotrexate, hydroxycarbamide, azathioprine). The presence of hypersegmented neutrophils on blood film and raised MCV suggests megaloblastic change; B12 and folate levels confirm the cause.
Haemolytic Anaemia
Haemolytic anaemia results from increased red cell destruction. It can be intravascular (within blood vessels) or extravascular (in the spleen/liver). Laboratory features include: raised bilirubin (unconjugated), elevated LDH, reduced haptoglobin, raised reticulocyte count, and blood film showing spherocytes, fragmented cells (schistocytes), or sickle cells depending on cause. Causes include autoimmune haemolytic anaemia (AIHA), hereditary spherocytosis, G6PD deficiency, and sickle cell disease.
The Blood Film in Anaemia
The peripheral blood film adds morphological detail to the FBC: hypochromic microcytes and pencil cells in IDA; oval macrocytes and hypersegmented neutrophils in megaloblastic anaemia; target cells in thalassaemia and liver disease; spherocytes in AIHA or hereditary spherocytosis; sickle cells in sickle cell disease; and schistocytes in microangiopathic haemolytic anaemia (MAHA). Skilled film interpretation is a core competency in haematology laboratory practice.
References
- WHO. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. who.int
- NHS Blood and Transplant. Blood components: red cells. nhsbt.nhs.uk
- Hoffbrand AV, Moss PAH. Hoffbrand’s Essential Haematology. 7th ed. Wiley-Blackwell; 2016.