156 Laboratory Findings

Differential diagnosis of flu-like illness

Many different symptoms are described as influenza-like: fever, cough, nasal congestion, headache, malaise and myalgia. However no clear definition or uniformity in the use of the term “flu-like” exists.

During an epidemic the clinical symptoms of fever, cough, severe nasal symptoms and loss of appetite are highly predictive of influenza (Zambon 2001). However many other infections can present with influenza-like symptoms. These include viral, bacterial, mycoplasmal, chlamydial and fungal infections and also parasite infestations. Infections that could either be life-threatening also in the young and healthy, such as viral haemorrhagic fevers, or infections such as legionellosis that are life-threatening in at-risk groups such as the old-aged, can initially present with flu-like symptoms. Therefore it is important to consider a wide differential diagnosis which should be guided by the patient’s history, which includes travel, occupational exposure, contact with animals and sick individuals, history of symptoms as well as the local epidemiology of disease.

Diagnosis of suspected human infection with an avian influenza virus

Introduction

Accurate and rapid clarification of suspected cases of H5N1 infection by laboratory diagnosis is of paramount importance in the initiation and continuation of appropriate treatment and infection control measures. Isolation of virus from specimens of suspected cases of avian influenza should be conducted in specialised reference laboratories with at least Biosafety Level 3 facilities.

Specimen collection

Specimens for virus detection or isolation should be collected within 3 days after the onset of symptoms and rapidly transported to the laboratory. A nasopharyngeal aspirate, nasal swab, nasal wash, nasopharyngeal swab, or throat swab are all suitable for diagnosis. However a nasopharyngeal aspirate is the specimen of choice. In cases where patients are intubated, a transtracheal aspirates and a bronchoalveolar lavage can be collected.

At the same time, acute and convalescent serum samples should be collected for serological diagnosis (WHO 2005b).

Virological diagnostic modalities

Rapid identification of the infecting agent as an influenza A virus can be performed by ordinary influenza rapid tests that differentiate between types. However commercial rapid chromatographic methods have a sensitivity of only 70% for avian influenza compared to culture (Yuen 2005). Direct diagnosis of influenza H5N1 infection can be performed by indirect immunofluorescence on respiratory cells fixed to glass slides using a combination of influenza type A/H5-specific monoclonal antibody pool, influenza A type specific and influenza B type-specific monoclonal antibody pools as well as influenza A/H1 and an A/H3 specific monoclonal