The Full Blood Count — FBC, or CBC in North America — is the most frequently ordered blood test in medicine. Most hospital admissions include one. Most health checks include one. It gives your doctor a comprehensive snapshot of the three main components of your blood: red cells (which carry oxygen), white cells (which fight infection and regulate immunity), and platelets (which control clotting). Here's what every row on that report actually means.
The Three Blood Cell Lines
All three cell types are produced in the bone marrow. The FBC measures not just how many of each cell you have, but their size, shape, and characteristics — which helps diagnose anaemia, infection, inflammation, clotting disorders, and in some cases, more serious blood conditions.
Red Blood Cell Parameters
Haemoglobin (Hb)
The headline red cell marker. Haemoglobin is the protein inside red cells that binds to oxygen and transports it around the body. Normal levels: 130–170 g/L for men and 120–160 g/L for women. Below these levels = anaemia. The severity of anaemia guides urgency: mild anaemia (just below normal) may only need investigation; haemoglobin below 80 g/L often warrants urgent assessment.
Red Cell Count (RBC)
The total number of red cells per litre of blood. Normal: 4.5–5.5 × 10¹²/L for men, 3.8–4.8 × 10¹²/L for women. Usually moves in the same direction as haemoglobin. Elevated RBC (polycythaemia) can be caused by dehydration, lung disease, or a bone marrow disorder.
MCV — Mean Corpuscular Volume
MCV is the average size of red cells and is one of the most diagnostically useful FBC parameters. It immediately classifies the type of anaemia:
Low MCV (microcytic, below 80 fL): Red cells are smaller than normal. Most commonly caused by iron deficiency, or in people of appropriate ethnic background, thalassaemia trait.
Normal MCV (normocytic, 80–100 fL): Normal-sized cells. Anaemia here may indicate chronic disease, hypothyroidism, or acute blood loss.
High MCV (macrocytic, above 100 fL): Large red cells. The most common causes are vitamin B12 or folate deficiency, alcohol excess, hypothyroidism, or certain medications (methotrexate, hydroxyurea).
MCH and MCHC
MCH (mean corpuscular haemoglobin) is the average amount of haemoglobin per red cell. MCHC (mean corpuscular haemoglobin concentration) is the concentration of haemoglobin in a given volume of red cells. These refine the anaemia picture. Low MCH with low MCV strongly supports iron deficiency. High MCHC can indicate hereditary spherocytosis (a red cell membrane disorder).
RDW — Red Cell Distribution Width
RDW measures the variation in red cell size. A high RDW means cells are very uneven in size (anisocytosis) — early iron deficiency and mixed deficiencies (iron + B12, for example) often show a high RDW even before anaemia is established. It's a subtle but useful early flag.
White Blood Cell Parameters
Total White Cell Count (WBC)
White cells are your immune system's foot soldiers. Normal total WBC: 4.0–11.0 × 10⁹/L. Elevated (leukocytosis) suggests infection, inflammation, or — less commonly — a blood cancer. Low (leukopenia) can indicate viral infections, bone marrow suppression, autoimmune disease, or medication side effects.
Neutrophils
Neutrophils are the most abundant white cell and the primary responders to bacterial infection. Normal: 1.8–7.5 × 10⁹/L. A raised neutrophil count (neutrophilia) typically indicates bacterial infection, physiological stress (surgery, trauma), or steroid use. Severely low neutrophils (neutropenia, below 1.0 × 10⁹/L) seriously impair the ability to fight bacterial infection and require urgent investigation if found unexpectedly.
Lymphocytes
Lymphocytes include T cells and B cells — the components of adaptive immunity that remember previous infections and coordinate targeted immune responses. Normal: 1.0–4.0 × 10⁹/L. Elevated lymphocytes (lymphocytosis) suggest viral infection, or in persistent cases may indicate a lymphoproliferative disorder. Low lymphocytes are common in severe illness, steroid use, and some immunodeficiency conditions.
Monocytes, Eosinophils, Basophils
These three cell types are present in smaller numbers. Monocytes (normal 0.2–1.0 × 10⁹/L) help coordinate immune responses and can be elevated in chronic infections and inflammatory conditions. Eosinophils (normal 0.04–0.4 × 10⁹/L) are elevated in allergic conditions, asthma, parasitic infections, and some autoimmune diseases. Basophils (normal 0–0.1 × 10⁹/L) are rarely elevated but can be in allergic reactions and some haematological conditions.
Platelet Parameters
Platelet Count
Platelets are tiny cell fragments that clump together to form clots when a blood vessel is injured. Normal range: 150–400 × 10⁹/L. Low platelets (thrombocytopenia) can increase bleeding risk — significant below 50 × 10⁹/L; critical below 20 × 10⁹/L. High platelets (thrombocytosis) can occur reactively (in infection, inflammation, or after surgery) or, more rarely, in myeloproliferative disorders. Reactive thrombocytosis generally doesn't increase clotting risk.
MPV — Mean Platelet Volume
MPV is the average size of platelets. Larger platelets are more metabolically active. A high MPV with low platelet count can suggest the bone marrow is producing platelets rapidly in response to destruction — seen in immune thrombocytopenia (ITP).
Complete Reference Ranges
| Parameter | Adult Normal Range |
|---|---|
| Haemoglobin | 130–170 g/L (M) · 120–160 g/L (F) |
| Red Cell Count | 4.5–5.5 ×10¹²/L (M) · 3.8–4.8 ×10¹²/L (F) |
| MCV | 80–100 fL |
| MCH | 27–33 pg |
| MCHC | 315–360 g/L |
| White Cell Count | 4.0–11.0 ×10⁹/L |
| Neutrophils | 1.8–7.5 ×10⁹/L |
| Lymphocytes | 1.0–4.0 ×10⁹/L |
| Monocytes | 0.2–1.0 ×10⁹/L |
| Eosinophils | 0.04–0.4 ×10⁹/L |
| Platelets | 150–400 ×10⁹/L |
Contact your GP or NHS 111 promptly if your FBC shows: haemoglobin below 70 g/L, white cells below 2.0 or above 30 × 10⁹/L, neutrophils below 1.0 × 10⁹/L, or platelets below 50 × 10⁹/L — particularly if you are unwell or on chemotherapy/immunosuppressants.
The FBC in Context
It's important to recognise that one abnormal FBC value rarely tells the whole story. Clinicians look at the pattern across multiple parameters — for example, low haemoglobin + low MCV + low ferritin paints a clear picture of iron deficiency anaemia. Low haemoglobin + high MCV + low B12 points to B12 deficiency. Low haemoglobin + normal MCV + high CRP suggests anaemia of chronic disease.
The FBC is most powerful when combined with other results. If your GP has ordered an FBC, they'll usually be looking at it alongside CRP (inflammation marker), iron studies, B12, and folate — or alongside LFTs, U&Es, and thyroid function depending on the clinical question.
⚕️ Medical disclaimer: FBC results must be interpreted in clinical context. Many parameters vary naturally by age, sex, ethnicity, pregnancy, and fitness level. Don't interpret individual values in isolation — discuss the full picture with your GP.
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