Informational indices. Validation of criteria and tests.

The informational indices comprise sensitivity, specificity and predictive value, and they give information on the probability of rheumatic disease being present or not. It is interesting that in clinical situations where the probability of the disease is appraised by the physician before the test, the probabilities may be strongly modified after the test. For example, if one estimates the pre-test probability of ankylosing spondylitis at 50% in a given patient, the HLA B27 antigen test will modify the probability of the disease as follows: B27 positive: probability 90% B27 negative: probability 7% The second part of the study discusses modern methods for the validation of diagnostic criteria and assessment tests. Validation of the latter includes inter- and intra-observer reproducibility, coefficient of variation, discriminating power of each assessment test in a double-blind "reverse" trial, in which the difference between placebo and active drug is the known base, the value of the tests being the unknown.