Original Article
Influenza-like illness criteria were poorly related to laboratory-confirmed influenza in a sentinel surveillance study

https://doi.org/10.1016/j.jclinepi.2004.08.014Get rights and content

Abstract

Objective

To analyze the most related clinical data for influenza and the utility of influenza-like illness criteria as the clinical threshold for sampling in an influenza sentinel surveillance over a 3-year period.

Methods

Sentinel physicians collected throat specimens and data from outpatients with acute respiratory infection (≤72 hours duration). Laboratory-confirmed influenza infection was compared with independent symptoms and the influenza-like illness criteria, as defined by the Classification Committee of the World Organization of Family Doctors.

Results

From 1934 patients, 359 (18.56%) yielded positive results for influenza viruses. Only 199 (55.4%) of laboratory-confirmed cases fulfilled clinical criteria of influenza-like illness: positive and negative predictive value (PPV and NPV) of 0.36 and 0.88, respectively. Fever, cough, and rhinorrhea individually correlated with influenza infections (PPV: 0.30, 0.20, and 0.20, respectively; NPV: 0.92, 0.87, and 0.85, respectively). Multivariate analysis demonstrated that the correlation of influenza infection with the presence of fever and cough was similar to the correlation between influenza infection and influenza-like illness criteria (odds ratio 2.24 vs. 2.71, respectively).

Conclusion

Influenza-like illness criteria are poorly related to laboratory-confirmed influenza. For early detection of influenza viruses in surveillance systems, a less restrictive clinical criterion (specifically, acute respiratory infection) perhaps should be followed.

Introduction

Influenza illness is a highly contagious infectious disease, annual epidemics occurring typically in the Northern Hemisphere between December and April [1]. Major changes on influenza A viruses have led to periodic emergence of pandemics, during which the rates of morbidity and mortality increase [2].

Influenza disease is subjected to surveillance worldwide by national networks that predict the epidemic threshold by reporting clinical and virological data [3]. World Health Organization encourages reinforcing influenza surveillance systems, because they provide data about the occurrence of a new influenza A virus subtype [2]. In an influenza surveillance network, patients from whom throat and/or nasal samples are collected must fulfill a clinical case definition [3]. Sample collection in most surveillance systems is made by using the clinical criteria of influenza-like illness (ILI) or acute respiratory infection (ARI) [3], the later being a less restrictive definition than the ILI criteria. In Spain, sentinel physicians follow the clinical criteria of ILI [3]. ILI has been defined by the Classification Committee of the World Organization of Family Doctors [4] (ICHPPC-2) by the presence of six of nine symptoms, though other authors have adopted the criteria of ILI as defined by the presence of fever plus two of the following four symptoms: headache, cough, sore throat, and myalgia [5], [6].

The present study analyzes the most related clinical data for influenza and the utility of ILI as defined by the ICHPPC-2 criteria in the context of the sentinel surveillance network in Andalusia (Spain) during a 3-year period.

Section snippets

Methods

As part of the sentinel surveillance network for influenza disease in Spain, throat samples and clinical data from outpatients were collected by 20 sentinel physicians (17 general practitioners and 3 pediatricians) during three periods of influenza surveillance, October 2000 to May 2001 (Period 1), October 2001 to May 2002 (Period 2), and October 2002 to May 2003 (Period 3). Inclusion criteria comprised persons of all ages with clinical evidence of ARI over a duration of ≤72 hours. ARI was

Results

A total of 1,934 patients with ages ranging between 2 months and 96 years (mean age ± standard deviation = 25.09 ± 22.99) were studied: 541 in period 1, 668 in period 2, and 725 in period 3. Patient characteristics by age groups are shown in Table 1: sex, vaccination status, and influenza virus isolations. From the 1934 throat specimens processed, 359 yielded positive results for influenza viruses (18.56%), 173 influenza A viruses (48.2% of total isolations), and 186 influenza B (51.8% of total

Discussion

The distribution of the population in the present study shows that almost 50% correspond to a pediatric population. Although the percentage is high, it is not surprising, because children are the main reservoir of respiratory viruses and attack rates are higher in this population [9], [10]. We note that the pediatric population was also attended by some general practitioners from the sentinel network (apart from the three pediatricians), which may have contributed in part to the increased

Acknowledgments

Authors are indebted to María del Mar Rodríguez del Águila, Servicio de Bioestadística del Hospital Universitario Virgen de las Nieves, Granada, for helping in the statistical analysis of the data.

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    Hemagglutination and hemagglutination-inhibition test

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    Influenza Surveillance Network in Andalusia, Spain: Carmen Petit-Gancedo, Mercedes Sánchez-Lanuza, Ángeles Oteros-Corpas, Francisco Sánchez-Prados, José Gil-Martín, Nuria Martínez-Moral, Fernando de Osma-Rodríguez, Juan Antonio Navarro-González, Mercedes Barón-Crespo, Francisca Ceínos-Vicente, Rafael Bejarano-Cielos, Dolores Rueda-Lozano, Javier Sánchez Ruiz-Cabello, José Luis Schiaffino-Cano, Dolores Corrochano-Dalí, Giordano López-Tierra, Gregorio Martos-Toribio, Manuel Zafra-Garrido, Carlos Guerra-García, José Manuel Andrade-López, Manuel Cid-Chavero, Manuel Caraballo-Ávila.

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