Sensitivity, specificity and predictive values of health service based indicators for the surveillance of influenza A epidemics.

BACKGROUND

The Regional Influenza Surveillance Group (GROG) is a French surveillance network set up in 1984. It collects virological specimens and health service based indicators on a weekly basis. Our aim was to assess the predictive value of the health service based indicators for the detection of influenza A epidemics.

METHODS

Virological data were used as a gold standard for defining the epidemics. For each health service based indicator, a statistical threshold was used as a test for the identification of epidemic weeks. Finally, an epidemiological criterion was defined in order to improve the specificity and the speed of detection of outbreaks.

RESULTS

Health service based indicators have a positive predictive value of around 0.80. They also advance the detection of outbreaks by between 1 and 4 weeks.

CONCLUSIONS

These indicators are easy to collect and are useful for the surveillance of influenza epidemics. Such a system is the prerequisite for the rational use of preventive tools.