Biomarker 101: Full blood count (FBC)

The full blood count is the most frequently ordered blood test there is. It appears on almost every panel, and it produces more individual numbers than anything else on your report.

What it measures

An FBC counts and characterises the three cell types in your blood: red cells, white cells and platelets. It is a broad opening question rather than a specific answer, which is exactly why it is ordered so often.

Red cell measures

Haemoglobin is the oxygen-carrying protein, and usually the headline figure. Low haemoglobin defines anaemia.

Haematocrit is the proportion of blood volume made up of red cells.

Red cell count is the number of cells themselves.

Mean corpuscular volume describes their average size, and it is more useful than it sounds. Smaller-than-usual cells point in a different direction to larger-than-usual ones — which is why an FBC often indicates what to test next rather than answering the question itself.

White cell measures

The total white cell count reflects immune activity. The differential breaks that total into types, and the proportions carry the information:

• Neutrophils, the most numerous, respond first to bacterial and fungal infection
• Lymphocytes, including B cells producing antibodies and T cells targeting infected cells
• Monocytes, which move into tissue and clear microbes and debris
• Eosinophils, involved in allergic responses and parasitic infection
• Basophils, uncommon but active in allergy and inflammation

Platelet measures

The platelet count reflects clotting capacity. Too few can mean easy bruising or bleeding; too many can raise clotting risk. Mean platelet volume describes average platelet size, which offers a clue about turnover.

Why it's ordered

The FBC appears in general health screening, in investigating fatigue, infection or bleeding, and in monitoring known conditions over time. Its breadth is its strength — a single sample touches on oxygen transport, immune activity and clotting at once.

Reading the report

Every measure is compared against a reference interval for people of similar age and sex, and anything outside is flagged. With this many individual values on one test, a flagged result is common and frequently unremarkable.

Where a pattern needs closer inspection, a scientist can examine a blood film under a microscope to assess cell size, shape and appearance directly.

What an FBC won't tell you

• It doesn't diagnose. It describes your blood cells; interpreting that requires clinical context.
• It doesn't cover much else. Cholesterol, glucose, thyroid, iron stores, liver and kidney function are all separate tests.
• Recent illness shows up. An infection in the preceding weeks moves several values.
• You can't change it directly. Counts respond to underlying health, not to targeted effort.

The bottom line

The FBC is broad by design. It rarely provides a final answer and frequently provides a useful direction — which is why it sits on almost every panel and is usually the first test ordered.