CME ReviewMicrobiological diagnostic procedures in respiratory infections: suppurative lung disease
Section snippets
Aetiology
Suppurative lung disease consists of lower respiratory tract infection involving the accumulation of purulent material – in the airways (e.g. cystic fibrosis), as focal collections (abscesses) or in the pleural space (empyema). This pathology can be either acute or chronic (Table 1). Acute suppuration can occur as a complication of a community- or nosocomially-acquired infection, and may be the result of an underlying predisposing condition, e.g. aspiration pneumonia in a child with severe
Role of microbiological investigations
The advantage of pursuing a microbiological diagnosis of the causative organism(s) is three-fold. First, while many common pathogens can be predicted by the experienced clinician (Table 2), surprises will always occur. Second, identification of pathogens allows targeted antibiotic therapy, improving bacterial eradication and clinical response,3 and reducing the development of resistance. Third, identification of some organisms carries significant prognostic implication for the patient, e.g.
Sputum
The appropriate use of sputum culture is not to make the diagnosis of a pneumonic process, but to identify the aetiological agent once a clinical diagnosis of lower respiratory infection has been made.4 The sensitivity of the Gram stain of good quality sputum cultures in adults has been estimated to be around 50–80% in community-acquired pneumonia.5 Sputum cultures are unlikely to be useful in lung abscess. Most empyemas in children are parapneumonic, which may mean sputum culture can be
Blood cultures
Blood cultures are not useful in exacerbations of chronic suppurative lung disease. They have a limited role in most acute respiratory suppuration. They are positive only in around 20% of parapneumonic empyemas,24, 25 and are rarely positive in lung abscesses, particularly primary abscesses.26 In a few notable conditions involving suppurative foci in the lung, they can be diagnostic.
Lung abscess
While surgical intervention may not be necessary for resolution of lung abscess, timely radiologically guided aspiration can be both diagnostic and therapeutic, resulting in a significant decrease in organism load, as well as providing a microbiological diagnosis. Anatomical position of the abscess will affect patient suitability for this procedure, with lesions close to the chest wall being more accessible percutaneously.35 In young children and neonates, the smaller chest cavity makes most
Serology
Serology has a limited role in suppurative lung disease. In the investigation of lung abscess or empyema, serology for S. pyogenes, Mycoplasma pneumoniae or Legionella spp may offer supportive evidence for these pathogens. While paired serum samples are generally required for a definitive diagnosis, a single high titre can support a clinical diagnosis. The antibody response in legionella infection is not detected for at least 2 weeks after onset of symptoms which limits its use in diagnosis of
Bacterial culture
Growth of almost any organism from surgically acquired specimens from sterile sites is deemed significant. Sputum and other lower respiratory tract specimens are all likely to contain some oropharyngeal flora, and are screened with selective media, in order to detect only known or suspected pathogens. Media is made selective by adding antibiotic combinations, an inhibitory substance or a metabolic substrate to produce colonies of a particular colour. Commonly used selective agars in respiratory
Conclusion
Microbiological diagnosis serves multiple purposes in suppurative respiratory disease: aiding diagnosis, directing therapy, enabling surveillance of transmissible pathogens and emerging antibiotic resistance. No single sampling technique is ideal in all situations, but correct collection and an understanding of possible shortcomings of each method is important in interpretation of laboratory results.
Key points
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A microbiological diagnosis can help focus treatment choices in both acute and chronic suppurative lung disease in children.
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Blood cultures are often negative, but can be diagnostic if positive, especially with unusual pathogens.
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Induced sputum samples can be useful in children who are too young to expectorate.
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Lower respiratory tract samples can be contaminated by upper respiratory tract colonizers, and careful interpretation of results is needed.
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Aspirating pus from suppurative collections can be
Educational Aims
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To appreciate the advantages behind pursuing a microbiological diagnosis of lower respiratory tract pathogens.
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To discuss the range of diagnostic procedures available for the investigation of lower respiratory tract suppuration, and their advantages and disadvantages.
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To review common and unusual pathogens responsible for acute suppurative lung disease in children.
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To review the strengths and weaknesses of conventional antibiotic sensitivity testing in chronic suppurative lung disease of children.
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2011, ThoraxCitation Excerpt :Lung abscesses occur as a result of the acute destruction of the pulmonary parenchyma, usually due to bacteria gaining access from the oropharynx,102 103 although less frequently they are caused by an underlying bronchial process, the evolution of CAP or a septic embolus. Anaerobic bacteria (Prevotella, Fusobacterium, Peptostreptococcus and Bacteroides) are the most frequent agents recovered from lung abscesses and account for 70-90% of all isolates.104 105 S aureus is most frequently found when there is associated empyema,106 107 and facultative pathogens include α-haemolytic streptococci, Enterobacteriaceae, S pyogenes and S pneumoniae.108 109
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