Pulse oximetry saturation can predict prognosis of idiopathic pulmonary fibrosis

Background: In Japan, the severity staging system for idiopathic pulmonary fibrosis (IPF) has been used to determine medical care subsidies. However, this system requires invasive procedures to measure arterial oxygen tension. Recently, noninvasive and simple measurements of oxygen saturation by pulse oximetry (SpO₂) have been used for severity assessments. We propose a pulse oximetry saturation (POS) staging system consisting of SpO₂ parameters to predict prognosis.

Methods: We developed four prototype staging systems based on SpO₂ at rest and desaturation, and adopted the system with the highest C-statistic as the POS staging system. The cutoff SpO₂ values at rest were 96% and 90%, and desaturation was defined as SpO₂ < 90% at the end of the 6-min-walk test.

Results: Two-hundred and nineteen IPF patients were studied and the C-statistic values of models 1, 2, 3, and 4 were 0.633, 0.643, 0.630, and 0.673, respectively. We judged model 4 to be a superior POS staging system; it defined SpO₂ ≥ 96% at rest without desaturation as stage Ⅰ; SpO₂ ≥ 96% at rest with desaturation or SpO₂ 90%-95% at rest without desaturation as stage Ⅱ; and SpO₂ 90%-95% at rest with desaturation or SpO₂ < 90% at rest as stage Ⅲ. The hazard ratios of POS stage Ⅰ, Ⅱ, and Ⅲ were 1.00, 2.25, and 4.99, respectively. The C-statistic of the POS staging system produced from 1000 bootstrap samples was similar (0.673), suggesting good internal validation.

Conclusion: A noninvasive and simple POS staging system defined by SpO₂ can easily predict prognosis.