Principes, caractéristiques et interprétation des tests de diagnostic et de dépistage

In daily practice, the physician uses tests to confirm a diagnosis or to identify a group of individuals at risk for a certain condition; he/she carries out diagnostic and screening tests. The aim of these tests is to confirm or to disprove the existence of a condition, in giving the most precise response possible; they can be positive (identifying the individual as being sick) or negative (identifying the individual as being well). However, tests are rarely completely reliable, errors are observed, and there are false negative and false positive results. Sensitivity and specificity are important test features; they measure the proportion of test-positive persons who truly have the condition being examined and the proportion of test-negative persons who truly do not have the condition being considered. The positive predictive value (i.e. the proportion of individuals who really have the condition among those who are test-positive) and the negative predictive value (the proportion of individuals who really do not have the condition among those who are test-negative) have the particularity of being sensitive to the prevalence of the condition. In contrast to sensitivity and specificity, predictive values provide an estimation of the risk of having the condition when the test is positive and the likelihood of not having the condition when the test is negative. The objective of a screening test is to identify a subpopulation that has an elevated risk for developing the condition compared to the general population, thereby allowing them to be integrated into appropriate health care channels. Even when a test is positive, it is not certain that the individual has the condition. For that reason, a diagnostic procedure (including other diagnostic tests) may need to be done. Knowledge of these characteristics (sensitivity, specificity and predictive values) permits one to decide when additional measures should be carried out, to select those that are the most appropriate, and to be able to interpret the results correctly. It must be understood that such tests do not replace clinical findings; rather, they are complementary. Moreover, they are no more than a support, to be integrated into the framework of the diagnostic strategy.