Abstract
Clinical biochemistry, as an independent discipline within medical science, has developed its own body of theory and practice, and as such it cannot only be concerned with collecting observations. A simple report (plasma potassium=5·3 mmol/L*) is used as a model to discuss the problems of understanding measured chemical changes in the body in disease, and how these lead towards a general theory. These include the nature of the analysand and the reference base; accuracy and identification of the analyte; how disturbances of the steady state contribute to changes in a static result; the implications of precision; differences between activity, concentration and content; the convention of arithmetical concentration; and the meaning of ‘abnormal’, and of derived terms such as ‘predictive value’ and ‘decision level’. Clinical biochemists/chemical pathologists, with their understanding of all these and related problems, must act as the necessary bridge between analysts and clinicians.