Inaccurate ascertainment of morbidity and mortality due to influenza in administrative databases: a population-based record linkage study.

David J Muscatello,William D Rawlinson,Janaki Amin,Sarah Thackway,Vitali Sintchenko,Anthony T Newall,Robin Gilmour,C Raina Macintyre,Martyn Kirk

doi:10.1371/journal.pone.0098446

Abstract

BackgroundHistorically, counting influenza recorded in administrative health outcome databases has been considered insufficient to estimate influenza attributable morbidity and mortality in populations. We used database record linkage to evaluate whether modern databases have similar limitations.MethodsPerson-level records were linked across databases of laboratory notified influenza, emergency department (ED) presentations, hospital admissions and death registrations, from the population (∼6.9 million) of New South Wales (NSW), Australia, 2005 to 2008.ResultsThere were 2568 virologically diagnosed influenza infections notified. Among those, 25% of 40 who died, 49% of 1451 with a hospital admission and 7% of 1742 with an ED presentation had influenza recorded on the respective database record. Compared with persons aged ≥65 years and residents of regional and remote areas, respectively, children and residents of major cities were more likely to have influenza coded on their admission record. Compared with older persons and admitted patients, respectively, working age persons and non-admitted persons were more likely to have influenza coded on their ED record. On both ED and admission records, persons with influenza type A infection were more likely than those with type B infection to have influenza coded. Among death registrations, hospital admissions and ED presentations with influenza recorded as a cause of illness, 15%, 28% and 1.4%, respectively, also had laboratory notified influenza. Time trends in counts of influenza recorded on the ED, admission and death databases reflected the trend in counts of virologically diagnosed influenza.ConclusionsA minority of the death, hospital admission and ED records for persons with a virologically diagnosed influenza infection identified influenza as a cause of illness. Few database records with influenza recorded as a cause had laboratory confirmation. The databases have limited value for estimating incidence of influenza outcomes, but can be used for monitoring variation in incidence over time.

Highlights

Influenza, an acute viral disease of the respiratory tract, is a major threat to public health
More than two thirds (68%) of persons with a virological notification had an associated any-cause emergency department (ED) presentation, 57% had an any-cause hospital admission, and 1.6% died
Just over one half (52%) of the virological notifications were in persons with both a hospital admission and ED presentation (Table S1 in File S1)

Summary

Introduction

An acute viral disease of the respiratory tract, is a major threat to public health. Annual seasonal influenza type A and B epidemics cause substantial burden of illness, death and cost to society [5]. Under-ascertainment of severe outcomes of influenza infections in administrative databases of health outcomes has long been recognised. Ambiguity of influenza’s role in illness recorded in databases of health outcomes arises from its lack of pathognomonic features, that is, lack of characteristic symptoms. Certain influenza-like symptoms such as fever with cough and fatigue are reasonable predictors of uncomplicated influenza infection during epidemic influenza activity. Counting influenza recorded in administrative health outcome databases has been considered insufficient to estimate influenza attributable morbidity and mortality in populations. We used database record linkage to evaluate whether modern databases have similar limitations

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Results

Discussion

Conclusion