2.2 Diagnostic Criteria
MPP was diagnosed based on the following criteria: (1) clinical presentation (fever, cough); (2) chest imaging with infiltrates; (3) serum anti-MP IgM titer ≥1:256 (double testing) or four-fold rising titer in acute and convalescent serum specimens. In addition, real-time PCR of MP deoxyribonucleic acid (DNA) in throat swabs was positive on the acute stage in all patients. In this study, SMPP was defined as MPP with the following symptoms: (1) pulse oxygen saturation in room air ⩽92%, and (2) ⩾2/3 pulmonary lobe involvement. No other respiratory tract pathogens were detected in the enrolled patients. All patients received azithromycin therapy before sample collection and were previously healthy.