Clinical behaviour and mortality in idiopathic vs secondary nonspecific interstitial pneumonia

Rationale: Nonspecific interstitial pneumonia (NSIP) is a subtype of interstitial lung disease which can either be idiopathic or secondary to other conditions. Idiopathic NSIP is a relatively rare entity and diagnosis should be considered carefully as it is mainly a diagnosis of exclusion. The aim of this retrospective study was to evaluate a cohort of NSIP patients with a view to identifying any clinical and mortality differences between idiopathic and secondary varieties. Methods: We screened 700 patients from our interstitial lung disease database and identified 44 cases of NSIP retrospectively. Statistical analysis was conducted to evaluate if there was a difference in demographics such as gender and ethnicity, physiological parameters including forced vital capacity, diffusing capacity, average oxygen saturations, and immunology profile between two groups. Furthermore, a difference in mortality was evaluated between idiopathic and secondary NSIP. Results: The data analysis showed that 63.6% (28 of 44) of patients had idiopathic NSIP versus 36.4% (16 of 44) of patients had secondary NSIP. Majority of the secondary NSIP patients had an underlying connective tissue disease. In the idiopathic variety, there was a male preponderance (64.2%, P = .02) which was statistically different compared to relatively equal gender divide in secondary NSIP which was statistically insignificant (male vs. female: 43.8% vs. 56.3%, respectively, P = .42). The mean age of the idiopathic group was 74 years compared to 64 years in the secondary group which was statistically different (P = .01). In both groups (idiopathic and secondary NSIP), more than two-thirds (68%) were of White British ethnicity. Immunology profile was similar across both groups with no statistical difference in IgM, IgG, or IgA levels. At the time of analysis, there were 17.9% deaths (5 of 28) in the idiopathic NSIP group versus 6.3% (1 of 16) in the secondary NSIP group but this was not statistically significant (P = .14). Similarly, there was no statistically significant difference in the forced vital capacity (P = .59), diffusing capacity (P = .88), and resting oxygen saturations (P = .28) between idiopathic and secondary NSIP varieties. Conclusion: Our analysis showed that there was a statistically significant difference in gender (male preponderance in idiopathic NSIP only) and mean age difference among both varieties. There were no statistically significant differences in the clinical features and outcomes including mortality, physiological, and immunological parameters between idiopathic and secondary NSIP. Idiopathic NSIP was more common than secondary NSIP and secondary NSIP is mostly due to underlying connective tissue disease.