Table 1.

The main differences between the diagnostic and the classification approaches

	Diagnostic approach	Classification approach
Aim	To establish the diagnosis of a disease in clinical practice	To define a homogeneous group of patients for research purposes
The starting point	Suspicion of a disease with a certain level of a pre-test probability	Established diagnosis of a disease
Differential diagnoses or other conditions that might explain symptoms	Always considered	Not considered
Values of the positive diagnostic tests	Different and depend on the test itself, earlier screening or diagnostic tests performed, geographic region and background population	Few levels with the same value of parameters on the same level
Values of the negative diagnostic tests	Negative test results are considered; their diagnostic values depend on the same factors as for positive test results	Not considered except the situation that there are not enough positive test results to fulfil the criteria
Outcome	Probability of the disease presence	Yes or no answer (classification criteria fulfilled or not fulfilled) with a certain level of sensitivity and specificity
External reference (‘gold standard’)	None	Expert opinion derived during classification criteria development