Characterization and validation of diagnostic methods

Diagnosis is defined as the determination of disease, but not as the determ ination of the signs and symptoms thereof, The use of modern diagnostic met hodology in the clinic is hampered by cost considerations and by the still widespread belief that, e.g. caries lesions and periodontal breakdown are i rreversible processes that need to be detected and treated invasively, as e arly as possible, their measurement thus being irrelevant. Modem instrument al and quantitative methods allow early detection and introduction of nonin vasive preventive measures to control the development of the disease. Stich methods are also very beneficial in clinical research as they may describe the speed of progress or regress of disease. In epidemiology, such methods reduce the classical problem of calibration of observers. Repeatability, r eproducibility, accuracy and validity are defined as method-characterizing quantities, for which examples are given. To express the validity of quanti tative methods compared with a quantitative gold standard, the use of scatt er plots and correlation and regression methods is suggested. Validation of a dichotomous method with a dichotomous gold standard in terms of sensitiv ity and specificity is discussed. To validate a quantitative method with a dichotomous gold standard, the receiver operating characteristic curve is s uggested, with the requirement that the cutoff value should be determined i n relation to the use of the method. However, preferably a quantitative met hod should not be reduced to a dichotomous one by using a cutoff value, but instead all available information should be used by the diagnostician. It is argued that the use of a secondary standard instead or the accepted gold standard usually leads to inadequate results, even when the validity of th e secondary standard is known. Finally, it is argued that the choice of a g -Old standard is a matter of reasoning and weighing of arguments and not of following a prescribed procedure. Copyright (C) 2000 S. Karger AG, Basel.