Comparison of clinical, laboratory and radiological characteristics between COVID-19 and Chlamydia psittaci pneumonia: a multicenter retrospective study.

Chlamydia psittaci pneumonia (CPP) exhibits similar characteristics as of COVID-19 with respect to clustering outbreaks and onset symptoms. This study is aimed at exploring the different clinical manifestations of both pneumonias to establish a simple nomogram to distinguish them. This multicenter, retrospective, case-control study compared two independent cohorts of patients with CPP or COVID-19. The risk factors of CPP were analyzed using multivariate logistic regression, which was used to establish the nomogram. Both patients with CPP and COVID-19 exhibited similar clinical symptoms. As compared to patients with COVID-19, a higher proportion of patients with CPP had nervous system symptoms. Patients with CPP had higher inflammatory indicators, creatine kinase, and lower lymphocyte and albumin. They also had lower proportions of ground-glass opacity and bilateral lung involvement than COVID-19 patients. Furthermore, patients with CPP had higher 30 day mortality as well as higher rates of severe pneumonia, septic shock, and ICU admission. Multivariate logistic regression showed that nervous system symptoms, lymphocytes, creatine kinase, bilateral lung lesions, and ground-glass opacity were risk factors for CPP. Incorporating these five factors, the nomogram achieved good concordance index of 0.989 in differentiating CPP from COVID-19, and had well-fitted calibration curves. Despite similar clinical characteristics, nervous system symptoms, lymphocyte, creatine kinase, lesions in bilateral lungs, and ground-glass opacity may help in differentiating the pneumonias. These were combined into a clinically useful nomogram for rapid and early identification of CPP to avoid misdiagnosis and help in the decision-making process.