Diagnostic Use
Identification of the causative organism of pneumonia can be difficult, and in practice is often not achieved. This may lead to inappropriately broad spectrum antibiotic treatment.
A Gram stain and culture of a well collected sputum can be very useful if a predominant organism is present. Streptococcus pneumoniae is the most common cause of bacterial pneumonia.
Urine:
The S.pneumoniae antigen test performed on urine is able to detect infection with S.pneumoniae. In adults, the test has a sensitivity for detecting pneumococcal pneumonia of about 50 – 60%. The specificity is 97 – 100%. The sensitivity is increased in bacteraemic patients. The specificity of the test in adults allows for more directed therapy; however the specificity of the test in children is poor due to nasopharyngeal colonisation. Note that this test typically remains positive for 3-4 weeks post acute infection in adults and longer in children so must be interpreted in the clinical context.
CSF:
The S.pneumoniae antigen test can be performed on CSF as an aid to the identification of the causative organism in patients with meningitis. Sensitivity has been reported as > 95% (21/22 patients) and specificity as 99 – 100% (497/497 patients).