Laboratory diagnosis of Mycoplasma pneumoniae infections: data analysis from clinical practice

An etiological diagnosis of respiratory infections caused by Mycoplasma pneumoniae is particularly challenging due to the lack of a definite standard test. This study aimed to analyse the correlation and combination of diagnostic results obtained from direct and indirect assays (Mycoplasma pneumoniae DNA by PCR and serology) in use at a first level diagnostic laboratory. Samples from patients with respiratory infections tested for M. pneumoniae during routine clinical practice were retrospectively analysed. In pediatric patients <15 years old, we documented a significantly higher proportion of IgM positive results (23.6% versus 3.9% in ≥15-year-old patients, p<0.0001) but a lower IgM specificity (false positive IgM 34.8% versus 12.2% in ≥15 years old patients, p 0.01), as verified by seroconversion. A small percentage (4%) of respiratory samples were positive for M. pneumoniae DNA regardless of age and type of sample. Assuming IgM positivity as the reference standard, PCR showed a total lack of sensitivity in patients <15 years old and 20% sensitivity in children <15 years old; specificity was 95% in both age groups. Agreement between PCR and IgM serology was slight (Cohen's kappa 0.09). According to our data, no single diagnostic test could be considered optimal for M. pneumoniae detection and improved assays are required.