Serological diagnosis of Mycoplasma pneumoniae infection: a complicated puzzle

Background. Mycoplasma pneumoniae has a prominent role in respiratory tract infections, especially in children and young adults. Serological methods, in particular complement fixation test (CFT) and enzyme immunoassays (ELISA), are most widely used for the diagnosis. However, a reliable diagnosis remains difficult to achieve. Study Design. The population study was subdivided in 2 groups. Group 1) 52 serum samples from blood donors; group 2) 50 serum samples from 50 patients (23 women, 27 men) affected by atypical pneumonia. The mean age was 29.2 years (1 to 85 years) and 48% was under 20 years.The following commercial products were used: Chorus M. pneumoniae ELISA IgG and IgM, Chorus CFT total Ig (DIESSE, Siena, Italy). Results. Group 1) 67.3% of blood donors resulted positive for IgG and 19.2% positive or equivocal for IgM. Group 2) specific IgG were detected in 70% of the patients and IgM resulted positive or equivocal in 64%. Since the number of IgG positive subjects was similar in the 2 groups, the CFT test was added to better define acute stage of infection.The CFT test resulted positive in 92.3% of the IgG+IgM+, and in 41.7% of the IgG+IgM- serum samples, respectively. Conclusions. High titers of IgG do not necessarily identify acute or recent infection, as previously suggested. The CFT test showed a good correlation with ELISA test in IgG+IgM+ serum samples. Detection of IgM antibodies is still the most reliable test to define acute infection, especially if confirmed by a positive CFT test.