Approach

Mycoplasma pneumoniae may be suspected as a pathogen in pneumonia, because it is one of the most common pathogens of pneumonia in the community. The diagnosis may be made clinically, although blood counts, blood biochemistry, pulse oximetry, and chest x-ray are usually performed in patients with severe disease.

Diagnostic testing to confirm M pneumoniae as the infecting pathogen remains controversial. In general, microbiological investigations are not recommended unless disease is moderate or severe/the patient is hospitalized, as results do not usually change management.[30][31] Microbiological investigations may also be warranted in the setting of an outbreak or during epidemic mycoplasma years.[30]

According to Japanese Respiratory Society (JRS) guidelines, the following six factors increase the likelihood of atypical pneumonia such as that caused by M pneumoniae:[32]

  • <60 years of age

  • Absence of or only minor underlying disease

  • Paroxysmal cough

  • Abnormal findings on chest auscultation

  • Absence of sputum or presence of an identifiable etiologic agent by rapid diagnostic testing

  • Peripheral white blood cell count <10,0000/mm³.

A study that validated the JRS guidelines found that if four or more of these factors were present, M pneumoniae was easily discriminated from the other pathogens with high sensitivity (88.7%) and moderate specificity (77.5%).[33] These findings have not yet been validated in populations outside of Japan.

Clinical evaluation

The first step in suspected pneumonia is usually to evaluate the patient based on a detailed history and a physical exam. In M pneumoniae infection, patients may present with symptoms including an unresolved persistent cough, low-grade fever, headache, hoarseness, rash, and, rarely (5%), bullous myringitis. Recent exposure to infected individuals and living in a close community may raise suspicion of M pneumoniae infection. Younger people and smokers may be at increased risk.

Laboratory evaluation

All patients admitted to hospital with suspected pneumonia should have the following investigations:[30]

  • Complete blood coun

  • Liver function tests

  • Pulse oximetry

  • Measurement of urea and electrolytes

  • Measurement of C-reactive protein (CRP).

In M pneumoniae infection, a complete blood count may reveal leukocytosis and hemolytic anemia. Elevated liver enzymes and urea are associated with severe disease.

Imaging

Patients admitted to hospital with suspected pneumonia should have a chest x-ray to confirm the diagnosis. Chest x-ray is not required in nonhospitalized patients unless it will help with management of the acute illness.[30]

In M pneumoniae infection, infiltrates may be seen, which may provide a worse clinical picture than expected, given the patient's symptoms.

Results from small studies suggest that computed tomography (CT) lung scans may improve the diagnosis of pneumonia; however, evidence remains insufficient to routinely recommend CT scans for this purpose.[30][34][35]

Microbiological investigations

The decision to perform microbiological investigations should be based on clinical presentation, local etiological considerations, and local antimicrobial stewardship processes. In general, testing is limited to patients with moderate or severe disease, including those who are hospitalized.[30][31][36] Microbiological investigations may also be warranted in the setting of an outbreak or during epidemic mycoplasma years.[30]

Please consult relevant local guidelines for further guidance on microbiological testing in your region.

Nucleic acid amplification tests (NAATs)

  • Use of NAATs, specifically polymerase chain reaction (PCR)-based assays, have become increasingly widespread for the detection of M pneumoniae owing to their high sensitivity and quick turnaround times compared to culture and serology-based methods.[9][37]

  • If an atypical pathogen such as M pneumoniae is suspected, it is best to confirm the diagnosis using PCR of a sputum or other respiratory sample (e.g., nose and throat swabs) because it may have implications for duration of therapy.[30][31][38][39][40]

  • In the US, five different commercial assays are approved for this purpose: four of these detect multiple respiratory pathogens (M pneumoniae, Chlamydia pneumoniae, and Legionella species), with one assay specific to M pneumoniae.[37]

Culture with or without Gram stain

  • Blood and sputum culture is recommended in all patients with severe pneumonia by the Infectious Diseases Society of America and the American Thoracic Society, 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]

  • Specific cultures for M pneumoniae are relatively insensitive compared to NAATs. The growth rate of M pneumoniae is also slow, and there is a requirement for specialized media.[30][31]

  • Sputum Gram stain may be available in some regions; however, it will not detect M pneumoniae due to lack of a cell wall.

Serology

  • Serology to detect antibodies against M pneumoniae (specifically IgM, IgG, and IgA) may be useful as an adjunctive test to help support the diagnosis. However, caution is needed when interpreting findings due to risk of false-positive results.[9][30]

  • Convalescent serum takes 2-4 weeks after the infection to show an increase in specific antibody levels, and so serology cannot be used to influence treatment.

  • Serology lacks sensitivity and specificity for reasons including high prevalence of background antibodies in healthy individuals and lack of an IgM response in older adults.[37]

  • Available assay types include the complement fixation test and enzyme immunoassays; performance of these assays varies significantly between laboratories.[41]

Antigen testing

  • 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]

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