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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 liver function test measures selected substances in blood to investigate aspects of liver activity or injury. Common panels include albumin, bilirubin and enzymes associated with liver cells.

The components reflect different processes: protein production, handling of breakdown products and release of enzymes need not change together. Consequently the panel's name does not make every component a direct measurement of one overall liver function. The pattern and its clinical context contribute to clinical diagnosis; an abnormal value alone does not identify a unique disease.^[ref]