Hypercapnia, Prognostic Nutritional Index and Length of Stay in Acute Exacerbation of COPD: A Two-Variable Admission...

Background/Objectives: Established AECOPD prognostic tools (DECAF, BAP-65, PEARL) predict mortality or readmission rather than length of stay (LOS), and no admission-based instrument specifically targets prolonged hospitalization. We tested whether admission PaCO₂ and the Prognostic Nutritional Index (PNI), reflecting ventilatory failure and nutritional-immune reserve, are independently associated with prolonged LOS and examined their interaction. Methods: In this single-center retrospective cohort, 213 adults hospitalized exclusively for AECOPD were analyzed after excluding concomitant pneumonia, pulmonary embolism, decompensated heart failure, and in-hospital deaths. Prolonged hospitalization was pre-specified as LOS > 7 days. Multivariable logistic regression evaluated admission PaCO₂ (per +10 mmHg) and PNI (per +5 units) with a PaCO₂ × PNI interaction; continuous LOS was modeled by Gamma regression. Discrimination was compared with DECAF using DeLong's test. Results: Prolonged hospitalization occurred in 83 patients (39.0%). Admission PaCO₂ was independently associated with prolonged LOS (OR 1.52, 95% CI 1.25-1.88; p < 0.001), and PNI showed a borderline association (OR 0.84, 95% CI 0.71-1.00; p = 0.049); their interaction was significant but exploratory (OR 1.16, 95% CI 1.02-1.32; p = 0.025). In Gamma regression, PaCO₂ (RR 1.18 per 10 mmHg) and PNI (RR 0.92 per 5 units) remained associated with LOS. The two-variable model achieved an AUC of 0.682, showing discrimination similar to DECAF in this cohort (AUC 0.695; DeLong p = 0.76), with optimism-corrected AUC 0.672 and calibration slope 0.96. Within moderate hypercapnia (PaCO₂ 45-60 mmHg), the prolonged-LOS rate was 44.4% in low-PNI versus 15.6% in high-PNI patients. Conclusions: In this single-center retrospective cohort of AECOPD patients surviving to discharge, admission PaCO₂ and PNI were jointly associated with prolonged hospitalization, reflecting acute ventilatory burden and nutritional-immune reserve. Using only two admission inputs, the framework showed discrimination similar to DECAF without meaningful reclassification gain (IDI -0.02; NRI 0.02). Given only moderate discrimination (AUC ~ 0.68), external validation is required before clinical use, with the main practical value likely in complementary stratification within moderate hypercapnia.