Full Blood Count (FBC) Explained: What It Tests, What Results Mean & When to Worry

Red cell parameters

• Haemoglobin (Hb) — the oxygen-carrying protein. The single most important marker for anaemia.
• Red blood cell count (RBC) — total number of red cells per litre of blood.
• Haematocrit (Hct / PCV) — the proportion of blood volume occupied by red cells.
• Mean corpuscular volume (MCV) — the average size of each red cell. Crucial for classifying anaemia as microcytic (small cells, often iron deficiency), normocytic (normal size), or macrocytic (large cells, often B12 or folate deficiency).
• Mean corpuscular haemoglobin (MCH) — the average amount of haemoglobin per red cell.
• Red cell distribution width (RDW) — how much variation there is in red cell size. A high RDW can suggest mixed causes of anaemia or early nutritional deficiency.

White cell parameters

• White blood cell count (WBC) — total number of white cells. Elevated in infection, inflammation, stress, and some blood cancers. Low in viral infections, autoimmune conditions, and some medications.
• Differential count — breaks WBC into five subtypes: neutrophils (bacterial infection), lymphocytes (viral infection, immune function), monocytes (chronic inflammation), eosinophils (allergies, parasites), and basophils (rare, allergic reactions).

Low haemoglobin / low RBC (anaemia)

Anaemia affects approximately 1.8 billion people worldwide according to the WHO. In the UK, iron deficiency anaemia alone affects around 3% of men and 8% of premenopausal women.

The MCV tells your GP what kind of anaemia you have:

• Low MCV (<80 fL) — microcytic: Most commonly iron deficiency. Also thalassaemia trait (common in people of Mediterranean, South Asian, or African heritage), chronic disease, or lead exposure.
• Normal MCV (80–100 fL) — normocytic: Anaemia of chronic disease, chronic kidney disease, early iron deficiency (before MCV drops), acute blood loss, or hypothyroidism.
• High MCV (>100 fL) — macrocytic: Vitamin B12 or folate deficiency, excess alcohol (a common cause), hypothyroidism, liver disease, or certain medications (methotrexate, azathioprine). Rarely, myelodysplastic syndrome.

High haemoglobin / high haematocrit (polycythaemia)

Elevated red cell counts can result from dehydration (the most common cause of a mildly raised Hb on a single test), chronic lung disease, smoking, living at high altitude, or rarely polycythaemia vera — a bone marrow disorder. If your Hb is persistently above the reference range, your GP should investigate further.

Neutrophils (50–70% of WBC)

The first responders to bacterial infection. Raised neutrophils (neutrophilia) typically indicate bacterial infection, inflammation, physical stress, smoking, or corticosteroid use. Low neutrophils (neutropenia) can result from viral infections, certain medications (especially chemotherapy), autoimmune conditions, or bone marrow disorders. Severe neutropenia (<0.5 × 10⁹/L) significantly increases infection risk.

Lymphocytes (20–40% of WBC)

Key players in adaptive immunity, including T cells, B cells, and natural killer cells. Raised lymphocytes (lymphocytosis) commonly indicate viral infection (EBV, CMV, hepatitis) or chronic lymphocytic leukaemia in older adults. Low lymphocytes (lymphopenia) can occur with HIV infection, autoimmune conditions, corticosteroid use, or severe acute illness.

Monocytes (2–8% of WBC)

Part of the innate immune system. They migrate into tissues and become macrophages, cleaning up dead cells and pathogens. Persistently elevated monocytes can indicate chronic inflammation, autoimmune disease, or chronic infections like tuberculosis.

Eosinophils (1–4% of WBC)

Primarily involved in fighting parasitic infections and mediating allergic responses. Raised eosinophils (eosinophilia) are most commonly caused by allergies (hay fever, asthma, eczema), parasitic infections, or drug reactions. Less common causes include autoimmune diseases and certain cancers.

Low platelets (thrombocytopenia: <150 × 10⁹/L)

Causes include viral infections, autoimmune thrombocytopenic purpura (ITP), liver disease (reduced thrombopoietin production), medications (heparin, some antibiotics), bone marrow disorders, or simply a lab artefact from clumped platelets in the sample. Mild reductions (100–150) are often not clinically significant; below 50 warrants urgent investigation; below 10 is a medical emergency.

High platelets (thrombocytosis: >400 × 10⁹/L)

Most commonly reactive — caused by iron deficiency, infection, inflammation, surgery, or cancer. Less commonly, primary thrombocytosis from a bone marrow disorder (essential thrombocythaemia). Reactive thrombocytosis usually resolves once the underlying cause is treated.

Pattern 1: Low Hb + low MCV + low ferritin

Classic iron deficiency anaemia. The most common anaemia worldwide. Investigate cause: heavy periods, dietary insufficiency, coeliac disease, or GI blood loss. Treat with oral iron and retest at 8 weeks.

Pattern 2: Low Hb + high MCV

Macrocytic anaemia. Most commonly vitamin B12 or folate deficiency. Also seen with excess alcohol (which directly suppresses red cell maturation), hypothyroidism, or liver disease. Check B12, folate, and thyroid function.

Pattern 3: Raised WBC + raised neutrophils

Usually indicates bacterial infection or acute inflammation. Smoking also causes a chronic low-grade neutrophilia. Corticosteroid use raises neutrophils and suppresses lymphocytes. If there is no obvious cause, your GP may retest after 4–6 weeks.

Pattern 4: Raised WBC + raised lymphocytes

Often viral infection (EBV, flu, COVID-19). In adults over 50, persistent lymphocytosis (>5 × 10⁹/L for more than 3 months) should prompt investigation for chronic lymphocytic leukaemia (CLL) — the most common leukaemia in adults, but one that often requires only monitoring rather than immediate treatment.

What is a normal haemoglobin level in the UK?

The NHS reference range is 130–170 g/L for men and 120–150 g/L for women. During pregnancy, haemoglobin naturally drops due to haemodilution, with the lower threshold set at 110 g/L in the first trimester and 105 g/L in the second and third trimesters according to NICE NG201.

Do I need to fast before a full blood count?

No. The FBC is not affected by food. However, if your blood test also includes cholesterol, triglycerides, or fasting glucose, a fasting morning sample is recommended. Water is always fine and encouraged to make the blood draw easier.

Can stress or exercise affect my FBC results?

Yes. Acute physical stress and intense exercise can temporarily raise white blood cell count (particularly neutrophils) for 24–48 hours. Chronic psychological stress can also mildly elevate WBC. Dehydration raises haemoglobin and haematocrit (less plasma, same red cells = higher concentration). For the most accurate baseline, test on a rest day and stay well hydrated.

How often should I get a full blood count?

There is no universal NHS screening interval for healthy adults. If you have symptoms or risk factors (heavy periods, fatigue, plant-based diet, family history of blood disorders), annual testing is reasonable. For a proactive baseline, including an FBC in a comprehensive blood test every 1–2 years gives useful trend data. If you take medications that affect blood counts, your GP will set a monitoring schedule.

What does it mean if my MCV is high but haemoglobin is normal?

A raised MCV without anaemia can indicate early B12 or folate deficiency (before haemoglobin drops), regular alcohol consumption (alcohol directly increases MCV), hypothyroidism, or liver disease. It is worth checking B12, folate, and thyroid function to identify the cause before it progresses to overt anaemia.