What is the investigation of choice to diagnose Mycoplasma pneumoniae (M. pneumoniae) in infants?

Diagnostic Testing for Mycoplasma pneumoniae in Infants

Complement fixation tests with a rise in paired titers is the gold standard for diagnosing Mycoplasma pneumoniae infection in infants, while IgM ELISA testing is recommended as a practical first-line test that reaches diagnostic levels during the second week of illness. 1

Primary Diagnostic Methods

Serological Testing

• Complement fixation tests showing a rise in paired titers remains the gold standard for diagnosing M. pneumoniae infection 1
• IgM ELISA testing reaches diagnostic levels during the second week of disease and is more practical for initial diagnosis 1
• When using IgM testing alone, sensitivity is approximately 62.2% with specificity of 85.5% 2
• For optimal results, both acute and convalescent serum samples should be collected to demonstrate a rise in antibody titers 1, 3

Molecular Testing

• PCR testing of respiratory samples offers rapid diagnosis with good sensitivity 2, 4
• Sample types for PCR testing in order of decreasing yield:
  • Sputum (69% positivity in young adults) 5
  • Nasopharyngeal aspirate (50% positivity) 4, 5
  • Throat swabs (37.5% positivity) 5
• In infants who cannot produce sputum, nasopharyngeal aspirate is the preferred specimen for PCR testing 4, 6

Combined Diagnostic Approach

• Combining PCR of respiratory samples with IgM serology increases diagnostic sensitivity to 95% 3
• This combined approach is particularly valuable in infants, where serological responses may be limited 6

Special Considerations for Infants

• Infants under 12 months may have limited antibody responses to M. pneumoniae infection 6
• PCR testing may detect M. pneumoniae DNA in asymptomatic carriers, with higher bacterial loads (≥10² CCU/ml) correlating with symptomatic infection 6
• Cold agglutinin testing has limited value in diagnosing M. pneumoniae in young children 1

Diagnostic Algorithm

1. Collect nasopharyngeal aspirate for PCR testing 4, 6
2. Obtain acute serum sample for IgM ELISA 1, 3
3. If initial tests are negative but clinical suspicion remains high, collect convalescent serum sample 2-3 weeks later 1, 3
4. Consider additional testing in complicated cases or treatment failures 1

Common Pitfalls

• Bacterial culture has low sensitivity (47%) and is not recommended as a primary diagnostic method 3
• Urinary antigen detection tests are not recommended due to high false-positive rates 1
• Cold agglutinin testing has limited diagnostic value, especially in young children 1
• Nasopharyngeal bacterial cultures do not reliably indicate lower respiratory tract infection 1

In summary, while complement fixation with paired titers remains the gold standard, a practical approach for infants with suspected M. pneumoniae infection is to combine PCR testing of nasopharyngeal aspirates with IgM ELISA serology for optimal diagnostic yield.