CD40LG and infection: However, the likelihood of M. pneumoniae infection can be estimated using clinical characteristics, such as age more than 5 years, prodromal fever and respiratory symptoms more than 6 days, and CRP or PCT levels that are normal or only slightly elevated, etc. These clinical characteristics have been validated in a patient cohort were M. pneumoniae infection was ascertained using a M. pneumoniae-specific IgM antibody-secreting cell (ASC) enzyme-linked immunospot (ELISpot) assay, which can discriminate between M. pneumoniae carriage and infection [70].