ACCURACY OF DIAGNOSTIC METHODS USED FOR EPIDEMIOLOGIC STUDIES OF HELICOBACTER-PYLORI

Epidemiological studies involve groups of individuals, or whole popula tions, many or most of whom are not ill, Clinical investigation has an individual perspective and precise statements need to be made about t he individual alone. Serological methods are most commonly used for po pulation-based epidemiological studies. Non-invasive epidemiological m ethods, using breath tests or the study of saliva or urine, are increa sing in use. All methods depend on accuracy in identifying presently i nfected or non-infected persons, and accuracy in defining a previously infected person. The performance of serological methods varies with t he antigens chosen, the population from which reference sera are drawn , age, ethnicity, and homologous and heterologous infection rates in t he population being studied. Much of the standardization of epidemiolo gical assays has been done in adults, which means that for children th ere is still uncertainty concerning standards and cut-off values. Beca use of differences in strains of H. pylori in different geographical a reas, antigen selection is important when geographical comparisons are made. The sensitivity and specificity of a test is not strongly affec ted by the prevalence of infection. However, as the prevalence rises i n the tested populations, the reported positive predictive value rises , and the negative predictive value falls. Depending on the patient po pulation studied, accuracy varies with changes in the prevalence, and its magnitude depends both on the sensitivity and specificity. Accurac y is therefore not a very useful measure. It is better to look at the sensitivity and specificity, and the prevalence in the study, where th ey are measured. An alternative to separating test results into two or three categories is to report likelihood ratios, which report the pro bability of a person with a particular result being truly positive com pared with the probability of a person with that result being truly ne gative. A receiver operating characteristic (ROC) curve, which describ es the effect of varying the cut-off value on the performance of a tes t, can be useful in comparing the performance of two or more different tests. The use of multiple tests to augment positive culture as a 'go ld standard', has been aided by use of polymerase chain reactions and other molecular biological methods. However, this augmentation has its limitations, since each of the additional methods may produce false p ositives. For example, polymerase chain reactions can be falsely posit ive if instruments are contaminated. There are people with more than o ne strain of Helicobacter pylori in the stomach, and, without molecula r biological efforts, or serological typing tests, the 'gold standard' does not deal with multiple infections.