CME Review
Microbiological diagnostic procedures in respiratory infections: suppurative lung disease

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Summary

Pursuing a microbiological diagnosis in suppurative lung disease can enable focused antibiotic therapy, identify pathogens of potential concern for infection control, and in some cases delineate a specific pathological process. Suppurative lung disease can be categorized as acute or chronic. Acute disease most commonly consists of lung abscess or parapneumonic empyema. The vast majority of chronic suppurative lung disease in childhood is due to cystic fibrosis. Samples from the respiratory tract offer the most useful information for diagnosis and management, but adjunctive information can also be obtained from serological methods, blood cultures and molecular techniques. The quality of respiratory tract samples is vital to aid accurate interpretation of results, and this varies according to the technique of collection. Antibiotic sensitivity testing is of particular importance in an era of evolving antibiotic resistance and can be problematical in cystic fibrosis.

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

  • •

    A microbiological diagnosis can help focus treatment choices in both acute and chronic suppurative lung disease in children.

  • •

    Blood cultures are often negative, but can be diagnostic if positive, especially with unusual pathogens.

  • •

    Induced sputum samples can be useful in children who are too young to expectorate.

  • •

    Lower respiratory tract samples can be contaminated by upper respiratory tract colonizers, and careful interpretation of results is needed.

  • •

    Aspirating pus from suppurative collections can be

Educational Aims

  • •

    To appreciate the advantages behind pursuing a microbiological diagnosis of lower respiratory tract pathogens.

  • •

    To discuss the range of diagnostic procedures available for the investigation of lower respiratory tract suppuration, and their advantages and disadvantages.

  • •

    To review common and unusual pathogens responsible for acute suppurative lung disease in children.

  • •

    To review the strengths and weaknesses of conventional antibiotic sensitivity testing in chronic suppurative lung disease of children.

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