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.