[Proposals for a strategy for evaluation and utilization of screening tests for hepatitis C].
New assays for the screening of anti-HCV antibodies are regularly proposed for registration. The evaluation of new tests is based on the measurement of their intrinsic value: sensitivity and specificity. To assess the sensitivity of a test, it is very important to be able to obtain sera samplets tested with a "Gold Standard", which is, for HCV, RNA detection. Sensitivity is defined as the ratio of the number of true positive to the number of true positive added to the number of false negative samples. The problem is to define the samples to be tested. It is methodologically impossible to use a serological test for samples selection. The Gold Standard might be performed on samples from the general population or more easily from risk groups. The estimation of the precocity is more difficult. The constitution of an acute hepatitis panel is of interest an the follow up of risk groups has to be encouraged. The estimation of the number of samples needed depends on the required precision in the sensitivity measurement. The confidence interval (CI) decreases when the number of samples tested increases and when the sensitivity is high. The important questions are opposite: if the test is positive what is the probability for the patient to be really positive? (positive predictive value PPV), and, if the test is negative, what is the probability for the patient to be non-infected? (negative predictive value: NPV). The NPV depends on the prevalence of HCV infection in the tested population and on the sensitivity of the test. PPV increases with the prevalence rate and the specificity. Regarding these results, one may suggest that donor screening assays and tests used for diagnose HCV infection in patients may be evaluated with different strategies. Regarding blood donors screening, sensitivity is the most important parameter, the best strategy is to eliminate risk groups, and then, to perform very sensitive assays on negative samples. On the other hand, for patients diagnosis, specificity (avoid false positive results) is more important. The best strategy is to select patients at risk from clinical findings and then, to perform a specific test. Screening strategies for the general population have to take into account these two different strategies.
Lunel
F
F
Service de Bactério-virologie, CHU Angers.
Rosenheim
M
M
Duneton
P
P
D001771:Blood Banks; D005069:Evaluation Studies as Topic; D006526:Hepatitis C / Q000175:diagnosis*; D006801:Humans; D008403:Mass Screening / Q000379:methods*; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity