A complete blood count (CBC) is one of the most commonly ordered blood tests. It provides a broad overview of the cells in your blood and can flag anaemia, infections, inflammation and clotting disorders. Despite being so common, most people are unsure what each component actually means. This guide walks you through every element.
What is measured?
A CBC analyses three main cell types: red blood cells (oxygen transport), white blood cells (immune system) and platelets (blood clotting), along with characteristics like cell size (MCV), haemoglobin content and haematocrit. Together, these values provide a comprehensive snapshot of your overall health. An abnormality in any component can be the first sign of an underlying issue, often before symptoms develop.
Red blood cells
Erythrocytes
Erythrocytes are your red blood cells. Normal: men 4.5-5.5, women 3.9-5.0 (x10^12/L). The difference is due to testosterone stimulating red blood cell production. Low values may indicate anaemia, blood loss, nutritional deficiencies (iron, B12, folate) or bone marrow problems. High values can occur with dehydration, chronic lung disease, smoking or, rarely, polycythaemia vera.
Haemoglobin
Haemoglobin is the iron-containing protein that carries oxygen. It is the primary marker for anaemia and probably the most recognised value in blood results. Normal: men 8.5-11.0 mmol/L, women 7.5-10.0 mmol/L. Low haemoglobin causes insidious symptoms: fatigue, pallor, shortness of breath on exertion, dizziness and reduced concentration. The most common cause in the Netherlands is iron deficiency, followed by chronic disease and B12/folate deficiency. Haemoglobin at the lower end of normal can already noticeably affect athletic endurance.
Haematocrit
Haematocrit represents the percentage of blood volume occupied by red blood cells. Normal: men 40-50%, women 35-45%. It moves in parallel with haemoglobin. The most common cause of elevation is dehydration. Persistently elevated haematocrit increases blood viscosity and thrombosis risk.
MCV
MCV measures the average size of red blood cells (normal: 80-100 fL). It is a crucial directional indicator:
- Low MCV (microcytic) - small red blood cells, typically iron deficiency or thalassaemia. Directs follow-up towards iron status (ferritin, transferrin).
- High MCV (macrocytic) - large red blood cells, typically vitamin B12 or folate deficiency, excessive alcohol use, hypothyroidism or certain medications. Directs follow-up towards B12, folate and thyroid function.
- Normal MCV with anaemia - normocytic anaemia, suggesting chronic disease, kidney failure or acute blood loss.
White blood cells
Leukocytes form the core of your immune system. Normal total: 4.0-10.0 x10^9/L. A differential count breaks them down into subtypes: neutrophils (60-70%, first-line bacterial defence), lymphocytes (20-40%, viral defence and antibodies), monocytes (2-8%, cleanup), eosinophils (1-4%, allergy and parasites) and basophils (under 1%). This pattern tells your doctor which type of immune response is active: bacterial infections raise neutrophils, viral infections raise lymphocytes, allergic reactions raise eosinophils.
Elevated leukocytes (leukocytosis) most commonly indicate infection, inflammation, stress, smoking or medication effects. Low leukocytes (leukopenia) may signal viral infections, bone marrow problems, autoimmune conditions or medication effects, and mean your immune defence is reduced.
Platelets
Platelets (normal: 150-400 x10^9/L) are cell fragments essential for blood clotting. Low platelets (thrombocytopenia) increase bleeding risk, sometimes noticeable as unexplained bruising, tiny red dots on the skin, gum bleeding or prolonged bleeding from cuts. Causes include viral infections, autoimmune conditions (ITP), medications, liver disease and bone marrow problems. High platelets (thrombocytosis) are usually reactive (temporary response to infection, iron deficiency, surgery or inflammation) and rarely indicate a bone marrow disorder.
When to get tested
A CBC is valuable in multiple situations: as a preventive check (especially from age 35), when experiencing unexplained fatigue, pallor, frequent infections or easy bruising, when monitoring medication effects (chemotherapy, immunosuppressants, methotrexate), with chronic conditions (diabetes, kidney disease, autoimmune disorders), and during pregnancy (standard prenatal check due to increased anaemia risk).
Frequently asked questions
Do I need to fast?
No. Eating and drinking do not affect cell counts. Fasting may be required if glucose or cholesterol are tested at the same time. Water is always fine.
What if one value is slightly abnormal?
A mild single abnormality is not always concerning. Reference ranges cover 95% of healthy people, so 5% fall outside without being ill. Your doctor considers the full picture: symptoms, previous results, the combination of values and the degree of abnormality. A repeat test after a few weeks can distinguish a persistent issue from a temporary fluctuation.
How often should I have a CBC?
For healthy adults without symptoms, every 1-2 years is a good baseline. With risk factors, chronic conditions or medication use, your doctor may recommend more frequent testing.
Can exercise affect my blood count?
Yes. Intense exercise shortly before blood collection can temporarily raise leukocytes (stress response), affect haematocrit through sweating, and elevate CK and ASAT from muscle damage. Ideally, schedule your blood draw on a rest day.
Is a CBC enough as a health check?
A CBC is a valuable foundation but does not measure cholesterol, blood sugar, liver or kidney function, thyroid hormones or vitamins. For a comprehensive preventive picture, combine it with these additional values.
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