Investigations
1st investigations to order
WBC count
Test
Recommended for all patients admitted to hospital with suspected pneumonia as part of FBC.[30]
WBC count may be slightly elevated (leukocytosis). Relatively low WBC counts in patients with pneumonia suggest an atypical pathogen.
Result
WBC count <13 x 10⁹/L (<13,000 per mm³)
haemoglobin
Test
Recommended for all patients admitted to hospital with suspected pneumonia as part of FBC.[30]
Haemolytic anaemia may accompany Mycoplasma pneumoniae infections.
Result
haemolytic anaemia
LFTs
pulse oximetry
urea and electrolytes
Test
Recommended for all patients admitted to hospital with suspected pneumonia.[30]
Informs about disease severity and renal function.
Result
elevated
C-reactive protein
Test
Recommended for all patients admitted to hospital with suspected pneumonia.[30]
Result
elevated level may aid in diagnosis; repeated measures may be taken to evaluate response to treatment
chest x-ray
Test
Chest x-ray is recommended for patients admitted to hospital with suspected pneumonia. Chest x-ray is not required in non-hospitalised patients unless it will help with management of the acute illness.[30]
X-ray may reveal a worse clinical picture than expected given the patient's symptoms.
Result
infiltrates
Investigations to consider
nucleic acid amplification test (NAAT)
Test
If an atypical pathogen such as Mycoplasma pneumoniae is suspected, it is best to confirm the diagnosis using a NAAT (e.g., polymerase chain reaction [PCR] on nose and throat swabs) because it may have implications for duration of therapy.[30][31][38][39][40]
NAATs such as PCR may also be warranted in the setting of an outbreak or during epidemic mycoplasma years.[30]
Result
positive for specific pathogen
serology
Test
Serology to detect antibodies against M pneumoniae (specifically IgM, IgG, and IgA) may be supportive but cannot be used to influence treatment, as convalescent serum takes 2-4 weeks after the infection to show an increase in specific antibody levels. Serology also lacks sensitivity and specificity for reasons including high prevalence of background antibodies in healthy individuals and lack of an IgM response in older adults.[9][30]
Available assay types include the complement fixation test and enzyme immunoassays.[41] The amount of titre change is dependent on the commercial assay used.
Result
elevated antibodies against specific pathogen
culture±/Gram stain
Test
Blood and sputum culture is recommended in all patients with severe pneumonia by joint American Thoracic Society/Infectious Diseases Society of America guidelines, and in all patients with moderate- and high-severity pneumonia by the British Thoracic Society.[30][31]
Nasopharyngeal viral diagnostics can be used for differential diagnosis of viral pneumonia.[3]
However, specific cultures for M pneumoniae are relatively insensitive compared to nucleic acid amplification tests. The growth rate of M pneumoniae is also slow, and there is a requirement for specialised media.[31]
Sputum Gram stain may be available in some regions; however, it will not detect M pneumoniae due to lack of a cell wall.
Result
specific pathogen growth
Emerging tests
antigen test
Test
In some regions (e.g., Japan) antigen testing is available for diagnosis; however, comparative data on sensitivity and specificity is not yet widely available.[42]
Result
positive for M pneumoniae bacterial antigen
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