The importance of diagnostic testing in the management of community-acquired respiratory infection during influenza season

Kuster and colleagues (Critical Care 15:R182) recently reported that during seasonal influenza febrile patients admitted to critical care with a diagnosis of pneumonia or respiratory infection are likely to have a diagnosis of influenza. Prompt identification of patients with influenza facilitates early empirical therapy and infection control. However, optimal management of patients requires confirmatory diagnostic microbiological testing and is exemplified by the case of a young male admitted at the peak of seasonal influenza.

In a recent issue of Critical Care, Kuster and colleagues [1] reported an observational study of patients admitted to critical care during periods of high infl uenza activity. A diagnosis of respiratory infection and being febrile (>38°C) were predictive of infl uenza among 126 confi rmed subjects and had a higher predictive value for pandemic 2009 infl uenza A H1N1 (pH1N1) than seasonal infl uenza. A previous study identifi ed that obesity, pregnancy, asthma, cardiac disease, immunosuppression, and diabetes are all risk factors for pH1N1 [2]. Such fi ndings are important for focusing early optimal empiric therapy in addition to infection control measures during peaks of infl uenza activity. Nevertheless, it is important to investigate alternative diff erential diagnoses and pursue a defi nitive microbiological diagnosis when possible, particularly as bacterial co-infection occurs in almost a fi fth of pH1N1 respiratory infections [3].
During the fi rst 3 weeks of the 2010/2011 infl uenza season, 17 patients were admitted with proven infl uenza infection to critical care in our institution. A 28-year-old previously fi t man presented at the end of this period with a 5-day history of dyspnea and cough. Th e patient was orientated, febrile (38°C), hypoxic with an arterial oxygen saturation (SaO 2 ) of 88% to 90% on high-fl ow oxygen, and tachypnoeic. He was intubated within 3 hours of hospital admission for severe hypoxic respiratory failure. His leukocyte count was 12 × 10 9 /L, and his C-reactive protein level was elevated (355 mg/L). A chest radiograph on admission demonstrated four quadrant infi ltrates ( Figure 1). Microbiological samples included a throat swab and non-directed bronchoalveolar lavage (NBL) for respiratory viruses and blood for serology. Guideline-concordant antibiotics for community-acquired pneu monia (co-amoxiclav and clarithromycin) were initiated [4]. Oseltamivir was also administered as the clinical presentation was typical of critically ill patients admitted in the previous 3 weeks with confi rmed infl uenza.
Th e NBL and throat swab did not confi rm infl uenza by reverse transcription-polymerase chain reaction. In the context of the 2010/2011 infl uenza season and the high index of clinical suspicion, it was assumed that these results represented a false negative [5] and oseltamivir was continued for 10 days. Th e patient required mechanical ventilation for 5 days. All other antimicrobials were discontinued after 7 days as the patient had improved, and he was discharged home on day 9. Subsequently, serology for infl uenza became available and was negative but did confi rm a diagnosis of Mycoplasma pneumoniae infec tion, for which the patient had received appropriate therapy.
Pattern recognition of symptoms and signs is a fundamental aspect of how clinicians formulate a diagnosis.  Identifying features associated with a disease entity is an important aspect of improving this process but requires support from appropriate diagnostic tests that confi rm or refute a diagnosis. Th e case described appeared typical of infl uenza infection during a period of intense infl uenza activity, but the serological results for Mycoplasma illustrate the importance of confi rmatory diagnostic testing.

Stefan P Kuster and Allison McGeer
We thank Holmes and colleagues for their interest in our article. We agree that the consideration of various diff erential diagnoses is a critical priority for clinicians, particu larly those working in intensive care units (ICUs). As the case of Holmes and colleagues demonstrates, there is good evidence that 'no specifi c symptom or combination of symptoms is diagnostic of [infl uenza]' [6]; in addition, a signifi cant proportion of community-acquired pneumonia episodes requiring ICU care are polymicrobial [7].
Th e aim of our study was not to fi nd symptom complexes that were diagnostic of infl uenza -a quixotic quest -but rather to identify populations in whom the probability of infl uenza was high enough that infl uenza testing, empiric antiviral therapy, or empiric infection control precautions (or a combination of these measures) was warranted [1]. Th e results of our study are intended to sensitize clinicians to ordering infl uenza testing in addition tonot instead of -other diagnostic procedures.

Competing interests
The authors declare that they have no competing interests.