The full blood count (FBC) — called a complete blood count (CBC) in the United States and Canada — is one of the most commonly ordered blood tests in clinical medicine. It measures the three main types of blood cells and produces a detailed set of measurements about each one. This guide explains what those measurements are, what the common abbreviations mean, and why a single abnormal value rarely tells the whole story.
Educational note: This article explains blood count tests for educational purposes only. It is not medical advice. Any concerns about your blood test results should be discussed with a qualified healthcare professional.
Key takeaways
- The FBC (UK) and CBC (US/Canada) measure red blood cells, white blood cells, and platelets, plus a range of related indices.
- It is used to screen for anaemia, infection, clotting problems, and many other conditions, and is one of the most versatile tests in clinical medicine.
- Many abbreviations appear on FBC reports; this guide explains the most common ones in plain English.
- A single abnormal value on an FBC does not equal a diagnosis. The whole pattern, clinical context, and repeat testing all matter.
What a full blood count is
A full blood count is a panel of measurements made on a blood sample using an automated haematology analyser. The analyser counts and characterises the cells in the blood, measuring their numbers, sizes, and some of their properties. From a single sample — usually drawn into a lavender or purple-top EDTA tube — the instrument produces around 20 or more different measurements.
The FBC is used for a very wide range of clinical purposes: to investigate tiredness and breathlessness (often related to anaemia), to assess whether infection might be present, to screen for blood cancers, to monitor the effects of chemotherapy, to investigate bleeding or clotting problems, and as part of routine health checks. It is one of the most informative and cost-effective tests available. To understand how this sample is processed before reaching the analyser, see our guide on what happens to your blood sample in the lab.
What the test measures
The FBC divides into three main sections corresponding to the three main cell types in blood.
The red blood cell section measures the number of red cells and their characteristics, including the amount of haemoglobin they carry and their size.
The white blood cell section reports the total number of white blood cells and, in a differential count, breaks them down into their main subtypes.
The platelet section reports platelet count and related measurements relevant to clotting.
Red blood cell markers
RBC count, haemoglobin, haematocrit
The RBC count is the number of red blood cells in a given volume of blood (usually reported as cells per litre or per microlitre). Red blood cells carry oxygen from the lungs to tissues via the haemoglobin molecules inside them.
Haemoglobin (Hb or Hgb) is the iron-containing protein in red blood cells that binds and transports oxygen. The haemoglobin concentration — the total amount of haemoglobin per unit of blood — is the most clinically important value for diagnosing and monitoring anaemia. Low haemoglobin means less oxygen-carrying capacity, which is why anaemia causes fatigue and breathlessness.
The haematocrit (HCT), also called the packed cell volume (PCV), is the proportion of blood volume made up by red blood cells, expressed as a percentage or decimal fraction. It broadly mirrors haemoglobin and RBC count.
MCV, MCH, MCHC, RDW
These four indices describe the size and haemoglobin content of individual red blood cells. They help characterise the type of anaemia if one is present.
MCV (mean corpuscular volume) is the average size of a red blood cell, measured in femtolitres (fL). A low MCV indicates microcytic (small) cells, often seen in iron deficiency anaemia. A high MCV indicates macrocytic (large) cells, often associated with vitamin B12 or folate deficiency.
MCH (mean corpuscular haemoglobin) is the average amount of haemoglobin per red blood cell, in picograms (pg). It generally follows MCV: small cells tend to carry less haemoglobin.
MCHC (mean corpuscular haemoglobin concentration) is the average concentration of haemoglobin within red cells, expressed as grams per decilitre (g/dL) or grams per litre (g/L). Very high MCHC may suggest hereditary spherocytosis; very low MCHC supports iron deficiency.
RDW (red cell distribution width) measures how much variation there is in red cell size. A high RDW indicates that red cells vary more in size than normal (anisocytosis), which can help distinguish between types of anaemia.
White blood cells and differential
The WBC count (also called the total white cell count or leucocyte count) is the total number of white blood cells per unit of blood. White blood cells are the cells of the immune system. A high WBC may suggest infection, inflammation, or in some cases a haematological condition. A low WBC can indicate suppression of bone marrow function, for example due to chemotherapy or certain infections.
The differential count breaks the WBC into its main subtypes. The proportions of each subtype provide more specific clinical information.
| Cell type | Abbreviation | Main function |
|---|---|---|
| Neutrophils | NEUT or NEU | First responders to bacterial infection; most common white cell in adults |
| Lymphocytes | LYMPH or LYM | Adaptive immunity; includes T cells and B cells; elevated in viral infections |
| Monocytes | MONO | Phagocytosis and immune regulation |
| Eosinophils | EO or EOS | Response to allergic reactions and parasitic infections |
| Basophils | BASO or BAS | Involved in allergic responses; rare in blood normally |
Platelets and clotting basics
Platelets (PLT) are small cell fragments that play a key role in stopping bleeding. When a blood vessel is damaged, platelets are among the first responders: they aggregate at the injury site and form an initial plug, which is then reinforced by the clotting cascade. The platelet count is the number of platelets per unit of blood.
A low platelet count (thrombocytopenia) increases the risk of bleeding. A very high platelet count (thrombocytosis) can sometimes increase the risk of clotting, though mild elevations are often reactive (for example, following surgery or infection) and not clinically significant on their own.
The MPV (mean platelet volume) is the average size of platelets. Larger platelets are generally more active. MPV may be reported as additional context to the platelet count.
Why doctors order an FBC or CBC
The FBC is used in an enormous range of clinical situations because it surveys three major body systems (oxygen transport, immune defence, and clotting) simultaneously and inexpensively. Common reasons for ordering it include investigating symptoms such as fatigue, breathlessness, pallor, unusual bruising or bleeding, recurrent infections, or unexplained weight loss; monitoring the effects of treatments such as chemotherapy, immunosuppressants, or medications that affect bone marrow; and routine screening as part of a general health check or pre-operative assessment.
Common abbreviations on the report
Quick reference: common FBC/CBC abbreviations
- RBC — Red blood cell count
- Hb / Hgb — Haemoglobin / Hemoglobin
- HCT / PCV — Haematocrit / Packed cell volume
- MCV — Mean corpuscular volume (red cell size)
- MCH — Mean corpuscular haemoglobin
- MCHC — Mean corpuscular haemoglobin concentration
- RDW — Red cell distribution width
- WBC / WCC — White blood cell / white cell count
- NEUT — Neutrophils
- LYMPH — Lymphocytes
- MONO — Monocytes
- EOS — Eosinophils
- BASO — Basophils
- PLT — Platelets
- MPV — Mean platelet volume
What can affect the result
Many factors can shift FBC values without indicating a serious underlying condition. Dehydration can raise haematocrit and haemoglobin by concentrating the blood. Recent vigorous exercise can transiently increase white cell count. Altitude increases red cell production over time. Pregnancy causes a dilutional anaemia as blood volume expands faster than red cell mass. Smoking raises haemoglobin and can also affect white cells and platelets.
Medications are another common influence. Chemotherapy suppresses bone marrow and reduces all cell lines. Some antibiotics, antivirals, and immunosuppressants can affect white cells or platelets. If you are taking any regular medicines, your clinician should factor these into interpretation.
Pre-analytical factors also matter: a sample that has been processed late, stored incorrectly, or is partially clotted can produce inaccurate counts. Good sample quality starts at the point of collection. For more on this, see our guide on blood sample processing.
Why one abnormal value does not equal a diagnosis
The FBC is a screening and investigative tool. A single abnormal value on an FBC opens a line of inquiry; it does not close one. A mildly low haemoglobin in a menstruating woman, for example, has very different significance than the same haemoglobin in an older man who has had no recent blood loss. A mildly elevated white cell count after strenuous exercise is very different from the same count in a patient with fever.
Clinicians look at the pattern of abnormalities across the whole FBC, the magnitude of any deviation, the trend over previous results, and the full clinical picture. A flagged value on your report is a starting point for that conversation with your clinician, not a conclusion. To understand how reference ranges work and when a flagged result needs attention, see our guide on reference ranges explained.
References
- MedlinePlus. Complete Blood Count (CBC). US National Library of Medicine. medlineplus.gov
- Mayo Clinic. Complete Blood Count (CBC). mayoclinic.org
- Patient.info. What Is a Full Blood Count and What Can It Tell Us? patient.info
- NHS. Blood Tests. nhs.uk
- British Society for Haematology (BSH). Guidelines for the Use of the Automated FBC. b-s-h.org.uk