Improving the diagnostic efficiency of H1N1 2009 pandemic flu: analysis of predictive clinical signs through a prospective cohort.

In late June 2009, we set up a dedicated flu-like illness outpatient consultation in the Infectious Diseases and Tropical Medicine department of Marseille university hospital to detect the new A/H1N1 pandemic influenza and to contain efficiently the A/H1N1 infected patients. For 3 months, we compiled data corresponding to a total of 307 patients who presented with a flu-like syndrome. 31 of them were positive for H1N1 pandemic flu through real-time RT-PCR (rRT-PCR); among them, 19 were positive for a rapid influenza detection test (RIDT). We report here the significant clinical characteristics of A/H1N1 pandemic flu patients compared with other flu-like illnesses, which were used to improve the predictive value of the diagnosis in the current epidemiological situation. We found that regardless of the prevalence of A/H1N1 positive cases in the suspected patients, the absence of cough rejects the diagnosis of A/H1N1 infection in 100% of cases. Among patients referred for flu-like illness, those with cough should be tested for A/H1N1 by RIDT. In the current situation, the PPV and NPV of RIDT for H1N1 reached 90.5% and 95.8 %, respectively. It is important to notice that the 2 RIDT-positive that were negative for H1N1 were seasonal H3N2 influenza indicating that specificity and PPV of RIDT for all influenza was 100%. Therefore, positive RIDT does not require rRT-PCR confirmatory test. Only negative RIDT should be tested with rRT-PCR assay. Respecting this algorithm would have saved up to 70,000 Euros ( 100.000 USD) for the 307 patients and would have resulted in a significant gain of time to transmit the laboratory results to the clinical ward.