[Interpretation of efficacy measures derived from 2 X 2 tables for the evaluation of diagnostic tests and treatment].

BACKGROUND

To describe the efficacy of diagnostic tests and the effect of treatment a number of measures are used, which can be derived from 2 x 2 tables of frequencies. For the comprehension of these measures the knowledge of their properties in the framework of probability theory is necessary.

MATERIAL AND METHOD

After an introduction of basic terms such as probability, odds, joint and conditional probability the usual measures sensitivity, specificity, likelihood ratio, positive and negative predictive value, relative risk, odds ratio, relative risk reduction, absolute risk reduction, and number needed to treat are presented and explained. In particular, the importance of disease prevalence and baseline risk for the interpretation of these measures is pointed out by means of examples.

CONCLUSION

If the disease prevalence or the baseline risk is not appropriately taken into account, the efficacy of a diagnostic test and the effect of a treatment are overestimated, especially in screening and prevention trials.