Laboratory examination may be divided into two general categories: screening and diagnostic. Screening studies are intended to identify individuals with disease in the early and asymptomatic stages. By definition, screening studies must be relatively simple and inexpensive, and are useful only when used to identify a disease which is relatively frequent (diabetes mellitus, anaemia, syphilis, blood disorders). Diagnostic examinations provide more specific information. The distinction between screening and diagnostic laboratory examination is not always rigid or absolute.

It must be remembered that laboratory examinations provide information that contributes to the diagnostic process. Seldom is this information of value by itself. The results must be interpreted in conjunction with other information that is available about the patient. It should also be noted that a laboratory value outside the normal range does not necessarily indicate disease. That value may represent normal for that specific patient. Usually, normal values are determined by testing supposedly healthy people, and these results are used to calculate the mean and normal range. Variables are not considered, and as a consequence, normal ranges are not always valid for all patients. Conversely, if a clinical diagnosis appears valid and is not substantiated by laboratory results, the tests should be repeated to rule out the possibility of laboratory error.