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.