How to Read a Complete Blood Count

A complete blood count (CBC), or full blood count, counts and measures the three kinds of cells in your blood: red cells, which carry oxygen; white cells, which fight infection; and platelets, which help blood clot. It’s one of the most common blood tests, and most of its numbers are best read in groups: haemoglobin with red cell size, and the white cell count with its differential.

This article is general information, not medical advice. A result slightly outside the range is common and often means nothing; your clinician reads it with your history.

Red Cells

• Haemoglobin (Hb): the oxygen-carrying protein, the main test for anaemia. The World Health Organization’s 2024 guideline on haemoglobin cut-offs defines anaemia in non-pregnant adults as under 120 g/L (12.0 g/dL) for women and under 130 g/L (13.0 g/dL) for men, with lower cut-offs in pregnancy and adjustments for altitude and smoking.
• Haematocrit (Hct): the share of blood volume taken up by red cells.
• Red cell count (RBC): the number of red cells.
• Mean corpuscular volume (MCV): the average red cell size. It points to a cause of anaemia: small cells (low MCV) suggest iron deficiency; large cells (high MCV) suggest a lack of vitamin B12 or folate, as the British Society for Haematology’s guideline on their deficiency describes, or alcohol or liver disease.
• Mean corpuscular haemoglobin (MCH) and its concentration (MCHC): how much haemoglobin each cell carries.
• Red cell distribution width (RDW): how much red cells vary in size. It often rises early in iron or B12 deficiency.

White Cells and the Differential

The white cell count (WBC) is the total; the differential splits it into five types, as absolute counts and percentages:

• Neutrophils, the first responders to bacterial infection; they rise with infection, stress, and steroids.
• Lymphocytes, central to viral infection and immunity.
• Monocytes, which clean up and fight longer-lasting infections.
• Eosinophils, which rise with allergies, asthma, and parasites.
• Basophils, the rarest, involved in allergic reactions.

Absolute counts are usually more meaningful than percentages, since a percentage can change because another type rose.

Platelets

The platelet count shows clotting cells; mean platelet volume (MPV) their average size. Low counts can come with some infections and medicines; high counts with inflammation and iron deficiency.

Reading a Result

Your lab’s printed ranges apply to its analyser and population. Ranges are set so that about 5% of healthy people fall outside them, so one result just outside often isn’t significant; a trend or several related results outside together means more. See What the Reference Range on Your Lab Results Actually Means.

Following It in Kaizora

Every part of a CBC is its own metric on Labs, from Blood Hemoglobin and Mean Corpuscular Volume to each white cell type’s count and percentage, logged with the laboratory and the range it printed, so a result outside it is flagged. A draw’s results share their time and laboratory, so your iron panel and B12 line up beside the CBC from the same blood; see How Do You Read Ferritin, Iron, and Transferrin Saturation Results?

The Short Version

• A CBC counts red cells, white cells, and platelets.
• The WHO defines anaemia as haemoglobin under 120 g/L in women and 130 g/L in men.
• Red cell size (MCV) points to the cause of anaemia; the differential breaks down the white cells.

Sources

• World Health Organization, Guideline on haemoglobin cutoffs to define anaemia in individuals and populations (2024).
• Devalia V, Hamilton MS, Molloy AM, “Guidelines for the diagnosis and treatment of cobalamin and folate disorders,” British Journal of Haematology 166, no. 4 (2014): 496–513. doi:10.1111/bjh.12959