Sputum microscopy and culture
Last updated: Thursday, 18, October, 2007
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Sputum, not saliva, is required.
Specimen obtained by:
If an unusual pathogen is suspected the laboratory should be informed (eg, Burkholderia pseudomallei, Legionella spp and Nocardia spp may require longer incubation of cultures and/or the use of specialised media).
Gram stain for bacteria and fungi; DFA for viruses (eg, RSV, influenza virus), Legionella sp, Pneumocystis jiroveci; acid fast stains for mycobacteria.
Mycobacterial culture; prolonged culture for fungus; virus detection, culture.
Nucleic acid detection after amplification may be used for detection of Legionella spp, Pneumocystis jiroveci, mycobacteria, mycoplasma, Chlamydia trachomatis in bronchial washes or lavage specimens.
Sputum examination is not justified for the investigation of acute bronchitis and is only of limited value for acute exacerbations of chronic bronchitis.
Investigation of pneumonia, particularly if severe or in an immunocompromised patient.
Investigation of exacerbations of respiratory symptoms in patients with cystic fibrosis and bronchiectasis.
Testing for Pneumocystis jiroveci in immunocompromised patients. (Bronchoscopy with bronchoalveolar lavage is usually required.)
Investigation of suspected infection with Legionella spp, Cryptococcus neoformans, Nocardia spp, Burkholderia pseudomallei.
Diagnosis of suspected tuberculosis (see also Mycobacteria testing ) and fungal infection.
Virus detection, culture is seldom required.
The Gram stain may be used to screen sputum specimens before culture is performed. If squamous epithelial cells predominate the specimen is considered to be heavily contaminated with saliva and culture may not be performed as the yield is predictably low and misleading. If the specimen contains few or no epithelial cells it is cultured and the appearance of organisms on Gram stain is reported.
If Streptococcus pneumoniae, Haemophilus influenzae or Moraxella catarrhalis predominate on culture with a consistent Gram stain they are likely to be significant pathogens in an appropriate clinical setting (eg, pneumonia).
The isolation of Staphylococcus aureus from sputum usually reflects colonisation of the airway, but in certain circumstances it may be pathogenic (eg, post influenza pneumonia, respiratory burns, pneumonia secondary to septicaemia).
Carroll KC. J Clin Microbiol 2002; 40: 3115-3120.