Combined pharyngeal and nasal swab specimens were collected from 100 subjec
ts who presented with a flu-like illness (fever >37.8 degreesC plus 2 of 4
symptoms: cough, myalgia, sore throat, and headache) of <72 hours' duration
at 3 different clinics in the province of Quebec, Canada, during the 1998-
1999 flu season. The rate of laboratory-confirmed influenza infection was 7
2% according to cell culture findings and 79% according to the results of m
ultiplex reverse-transcription polymerase chain reaction (RT-PCR) analysis
(85%, influenza AH3; 15%, influenza B). All subjects for whom these results
were discordant (negative culture and positive PCR) presented with a tempe
rature <greater than or equal to>38.2 degreesC as well as 3 or 4 of the sym
ptoms in the clinical case definition. Stepwise logistic regression showed
that cough (odds ratio [OR], 6.7; 95% confidence interval [CI] 1.4-34.1; P
=.02) and fever(OR, 3.1; 95% CI, 1.4-8.0; P =.01) were the only factors sig
nificantly associated with a positive PCR test for influenza. The positive
predictive value, negative predictive value, sensitivity, and the specifici
ty of a case definition including fever (temperature of >38 degreesC) and c
ough for the diagnosis of influenza infection during this flu season were 8
6.8%, 39.3%, 77.6%, and 55.0%, respectively.