Surveillance of influenza in Wales: interpreting sentinel general practice rates using contemporaneous laboratory data. Opportunities and limitations.

OBJECTIVES

To evaluate the opportunities and limitations of using laboratory data to enhance sentinel general practice surveillance of influenza.

DESIGN

Descriptive study of active sentinel surveillance of clinically diagnosed influenza in general practice and passive total population surveillance of laboratory reports of influenza A, influenza B, Mycoplasma pneumoniae, and respiratory syncitial virus infections.

SETTING

Wales.

SUBJECTS

Total sentinel practices population (currently 228,130); population of Wales (2,913,000, 1994 mid-year estimate).

MAIN OUTCOME MEASURES

Simplicity, flexibility, acceptability, sensitivity, predictive value positive, representativeness, and timeliness of a surveillance system. Rate of influenza and other respiratory infections.

RESULTS

Sentinel general practice surveillance of influenza in Wales is simple, flexible, acceptable, timely, representative, and relatively sensitive. Current laboratory surveillance is complex and less timely than sentinel practice surveillance but is complete and has a relatively high positive predictive value. For the period January 1993 to September 1996, peaks in rates of influenza reported by sentinel practices during winters 1993/94 and 1995/96 were temporally associated with increased rates of laboratory confirmed influenza A and respiratory syncitial virus, whereas the peak in 1994/95 was associated with increased rates of laboratory confirmed influenza B, M pneumoniae, and respiratory syncitial virus.

CONCLUSIONS

Timely laboratory data can add value to influenza data already obtained from sentinel general practice surveillance. However continuous audit is essential to resolve the possible limitations of either surveillance system.