Prognostic value of distance-saturation product in COPD: Korean nationwide cohort study

The distance-saturation product (DSP), calculated by multiplying 6-min walk distance (6MWD) by lowest oxygen saturation, is a composite physiological marker of exercise capacity and gas exchange impairment. Although DSP has shown prognostic potential in Western COPD cohorts, its utility and optimal threshold in Asian populations remain unestablished. This study evaluated whether DSP <290 m·% predicts exacerbations and mortality in COPD, compared with the BODE index.

We analysed 1995 patients with valid 6MWD data from the Korean COPD Subgroup Study, a nationwide prospective cohort. DSP was dichotomised at 290 m·% based on restricted cubic spline analyses. Primary outcomes were 3-year acute exacerbations (AEs) and all-cause mortality. Associations were evaluated using multivariable logistic, negative binomial and Cox regression models, with the BODE index as comparator.

Patients with DSP <290 m·% had more severe symptoms, worse airflow limitation and more frequent prior-year AEs. Low DSP was independently associated with more frequent severe (adjusted incidence rate ratios (aIRRs) 2.27-2.54) and moderate (aIRRs 1.42-1.52) exacerbations, with significantly increased moderate AE risk observed only in year 1 (aOR 1.60; 95% CI 1.05-2.45). Patients with low DSP also had higher 3-year mortality (9.2% versus 2.6%, p<0.001), with a significantly increased risk of death (adjusted hazard ratio 2.74; 95% CI 1.68-4.45). DSP-based models demonstrated superior discrimination for 3-year mortality (area under the curve (AUC) 0.673) compared to the BODE index (AUC 0.636).

DSP is a simple, scalable prognostic marker in COPD patients. A threshold of 290 m·% identifies patients at higher risk of exacerbations and mortality.