Background: Cryptogenic organizing pneumonia (COP) is an idiopathic form of organizing pneumonia. The exact incidence, prevalence and pathogenesis are unknown. This retrospective study aims to reveal the prevalence of COP in hospitalized population and to investigate the odds ratio (OR) of COP in connective tissue disease (CTD) of interest including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), graft-versus-host disease (GVHD), and gastroesophageal reflux disease (GERD).

Methods: We used datasets from the National Inpatient Sample (NIS) from 2012 to 2014. The NIS is the largest publicly available inpatient database in the United States (U.S.) and contains data from approximately 8 million hospital stays each year, representing a 20% stratified sample of all US non-federal hospitals, and is sponsored by the Agency for Healthcare Research and Quality and the Healthcare Cost and Utilization Project (HCUP). We included hospital encounters for patients with the diagnosis of cryptogenic organizing pneumonia using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code 516.36, 279.5, 714.0, 710.0 and 530.81 were used to identify COP, GVHD, RA, SLE, and GRED, respectively. Relationship among COP, CTD of interest, GVHD and GERD were determined by utilizing logistic regression analysis using STATA. P-value <0.001 was used as the significance threshold.

Results: There was an estimate of 6,595 admissions with a discharge diagnosis of COP identified from 2012 to 2014 (7.29 per 100,000 adult admissions), among which 51.6% were female and 74.4% were Caucasian. The mean age of COP admissions was 63.5 [95% confidence interval (CI): 62.7-64.5]. 91.9% of COP admissions happened in large urban teaching hospitals (63.0% large hospitals, 91.9% urban location, and 67.0% teaching hospitals). After adjusting age, gender, race, income, comorbidities, hospital bed size, region, insurance, teaching status, and location, the OR of COP in admissions with a discharge diagnosis of GVHD was 27.54 [95% confidence interval (CI) 17.12- 44.29, p<0.001], with RA 2.33 (95% CI 1.74- 3.11, p<0.001), with SLE 2.26 (95% CI 1.32- 3.85, p<0.001), and with GRED 1.07 (95% CI 0.91-1.26, p 0.43).

Conclusions: Our study is the first population-based retrospective study of COP in hospitalized population in the U.S. Our findings showed that admissions with a diagnosis GVHD, RA, or SLE had a higher chance to have a diagnosis of COP, suggesting a possible relationship between COP and GVHD, COP and connective tissue diseases. There was no statistically significant relationship between COP and GERD.