The effectiveness of clinical Influenza diagnosis in a Belgian out-patient setting identifies the need for rapid diagnostic assays

In primary care, influenza diagnosis is based on physical examination associated with diagnosis of Influenza Like Ilness (ILI) and/or Acute Respiratory Infection (ARI). Diagnosis is difficult as these symptoms are induced by several respiratory pathogens. This prospective study targets Belgian patients consulting their physician with ILI and/or ARI within the first 3 days of symptom onset. Recruitment and sample collection started at the peak of 2011-2012 influenza epidemic in Klein Brabant, Belgium. Patients were questioned using a study-specific questionnaire and mid-turbinate swabs were collected and stored at -20°C for later diagnosis of influenza A by RT-PCR. We recruited 177 patients with an average age of 37 years old. The majority visited their physician within 1 day of symptom onset. The PCR influenza A positivity rate was 56% (100/177) with a median Viral load (VL) of 6.99 log RNA copies/ml. The median VL declined from 7.5 log in patients presenting within 1 day to 5.4 log in patients presenting on day 3 after symptom onset. Despite the clear difference in VL between day 1 and 3, only a minor decline from 96% to 83% was observed in proportion of diagnosed ILI patients. Compared to the PCR results, ILI had a sensitivity of 95% (95/100), a specificity of 14% (11/77) and an accuracy of 60% (106/177). Overall, ILI was a poor indicator for influenza infections even during the influenza season. Given the fast intercept of Influenza patients after symptom onset and a median VL > 6 log during the first 2 days of symptoms, complementation of clinical diagnosis with rapid diagnostic tests can improve influenza diagnosis in primary care.