⚕️ 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.
More questions answered
Which blood tests can be used when anaemia is being investigated?
There is usually no single blood test that answers every question about anaemia. Depending on the clinical situation, testing may include full blood count, reticulocyte count, iron studies and other targeted tests together with more general laboratory measurements. The purpose of each test is different, so a result that supports one part of the assessment may not establish the diagnosis by itself. A healthcare professional chooses and interprets tests according to symptoms, examination findings and other evidence.
Can blood tests confirm anaemia on their own?
Blood tests can provide important evidence, but they are not automatically diagnostic of anaemia by themselves. Results are interpreted with clinical findings, medical history and sometimes imaging, pathology or genetic information. Some abnormal laboratory patterns can also occur in iron deficiency, vitamin deficiency, chronic disease, bleeding and inherited conditions, which is why context matters. Personal results should be interpreted by an appropriately qualified healthcare professional.
Why can a full blood count be useful in anaemia?
A full blood count measures red cells, white cells and platelets, so it can reveal changes that may accompany anaemia or its complications. The exact pattern depends on the condition and on other factors affecting blood-cell production or survival. Similar changes can occur for unrelated reasons, so the count is not specific. Clinicians interpret it alongside full blood count, reticulocyte count, iron studies and other targeted tests and the wider clinical picture.
Why might laboratory results change over time in anaemia?
Laboratory values can change as anaemia changes, as treatment affects the body or as unrelated illnesses occur. Looking at a trend over time can therefore be more informative than comparing one result with a single cut-off. Laboratories also consider differences in sample timing, method and biological variation. A healthcare professional decides whether a change is clinically meaningful.
Can another condition cause blood-test changes that look similar to anaemia?
Yes. Laboratory findings associated with anaemia are often shared with iron deficiency, vitamin deficiency, chronic disease, bleeding and inherited conditions or with temporary physiological changes. This overlap is why clinicians avoid diagnosing from one marker or one abnormal result. They combine multiple sources of evidence to decide which explanation fits best. If an individual result is concerning, it should be discussed with a healthcare professional.
Why might several different tests be ordered for anaemia?
Different tests answer different questions about anaemia, such as whether inflammation is present, whether red blood cell production and survival is affected or whether a characteristic marker can be detected. A panel may therefore include full blood count, reticulocyte count, iron studies and other targeted tests rather than relying on one value. Results can complement each other even when none is specific enough to stand alone. The combination is interpreted in the context of the person’s clinical assessment.
What laboratory tests may be used to monitor anaemia?
Monitoring may involve full blood count and cause-specific laboratory tests, although the exact plan depends on the condition and treatment. Some tests track disease activity, while others look for treatment effects or organ changes. A result outside its usual range does not automatically mean the condition has worsened because other factors may influence it. The monitoring schedule should come from the treating healthcare team.
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