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A blood count quantifies one or more cell populations in a blood sample. Counts of red blood cells, white blood cells and platelets describe different populations, usually reported per unit volume rather than as a total number throughout the body.

A complete blood count combines several measurements, including cell counts and indices of cell size or composition. A differential count further separates types of white blood cells. These distinctions matter because two reported numbers may describe different components of the same specimen.^[ref]

A blood test measures a property of a blood sample. Different tests answer different questions: a blood count quantifies cellular components, whereas a liver function test measures selected substances associated with the liver.

A result requires a named measured quantity, units and an appropriate reference interval. Changes in cell counts and changes in a dissolved substance are different observations, even when both come from the same sample. Their contribution to clinical diagnosis depends on the question and accompanying findings.^[ref][ref]

A laboratory panel measuring blood-cell quantities and related characteristics, including red cells, white cells and platelets. The pattern can guide further investigation, but no single abnormal count identifies every cause.^[ref]