Sputum Culture (Routine)

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Subject: Sputum Culture (Routine)

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Definition

  • Lower respiratory tract (LRT) infections may involve any of the anatomic areas inferior to the larynx. Infections include tracheobronchitis (large airway disease), bronchiolitis (small airway disease), and pneumonia (alveolar disease). The common etiologies depend on the site of infection, age and underlying health of the patient, season, and other factors.

  • Patients with LRT infections typically present with a constellation of symptoms of varying severity, including fever, cough, sputum production, difficulty breathing, and shortness of breath. Common bacterial pathogens are less commonly associated with coryza and rhinorrhea than are respiratory viruses and mycoplasmas. Symptoms and clinical examination may help distinguish tracheobronchitis, bronchiolitis, and pneumonia from one another.

Special Collection and Transport Instructions

  • For expectorated sputum samples, patient instruction is critical for collection of a good-quality, informative sample.

  • Specimens must be collected in sterile transport containers with tight-fitting lids.

  • First-morning specimens are usually most sensitive because of pooling of secretions during sleep.

  • Contamination is reduced for patients who brush their teeth and gargle with water or saline just before specimen collection.

  • The patient must understand that sputum from a deep cough is needed, and saliva should not be spit into the collection cup.

  • Sputum production may be improved by chest wall percussive techniques.

  • Specimens obtained by more invasive procedures, such as sputum induction, BAL, tracheal aspirate, and lung puncture, are collected by physicians or respiratory therapists trained with specific collection protocols.

  • Specimens must be transported to the laboratory as quickly as possible at room temperature.

Use

  • These cultures are used to identify bacterial pathogens responsible for LRT infections by culture of sputum. A variety of human pathogens may cause lower respiratory infections; there is a large overlap in the clinical signs and symptoms.

  • Method:

    • Gram stain examination of sputum should be performed to ensure that poor-quality specimens are not processed for routine sputum culture. A number of screening strategies have been proposed. Sputum specimens are scored on the basis of the presence and quantity of PMNs and SECs.

    • Acceptable specimens are usually inoculated onto SBA, chocolate and MacConkey agar.

    • If anaerobic infection is suspected, specialized techniques, like needle aspiration, are required to exclude contamination by the patient's endogenous flora.

  • Turnaround time

    • Cultures are incubated for 48–72 hours.

    • Additional time is required for isolation, identification, susceptibility, and other testing, as required.

Interpretation

  • Expected results: Light or rare growth of normal endogenous respiratory tract flora (or no growth).

  • Positive results: Positive cultures must be interpreted carefully in the context of Gram stain results and other laboratory findings, imaging studies, and clinical presentations.

  • Negative results: A negative culture does not exclude LRT infection. Poor specimen quality and transport conditions, or heavy contamination, may prevent the isolation of fastidious pathogens. Uncommon fastidious LRT pathogens, such as Bordetella pertussis, are not detected reliably by routine sputum culture.

Limitations

  • Noninvasive and minimally invasive techniques for specimen collection may result in contamination of the specimen with the patient's endogenous upper respiratory flora. Because LRT infections are commonly caused by a patient's flora, such contamination may compromise the interpretation of sputum cultures.

  • The sensitivity and specificity of routine sputum cultures are relatively low for diagnosis of LRT infections. Diagnoses may be improved by submission of blood cultures, urinary antigen tests (e.g., S. pneumoniae), serology, and molecular diagnostic techniques and tests for other types of LRT pathogens, such as Mycoplasma species.

  • Common pitfalls

    • Poor specimen collection and transport are the major causes for compromised information from sputum cultures. Rejection criteria may be not be applied appropriately.

 
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