Relative survival analysis of 252 patients with COPD receiving long-term oxygen therapy

A survival analysis was conducted on patients with COPD receiving long-term oxygen therapy (LTOT) to compare two different statistical methods.

We used a multivariate crude (observed) survival model (Cox) and a multivariate relative survival model (Hakulinen). Only the latter is able to correct the survival by adjusting it to the normal life expectancy of the studied patients.

Two hundred fifty-two hypoxemic COPD patients (207 male) requiring LTOT were included. Mean PaO2 was <50 mm Hg before oxygen therapy. Mean age was >69 years (SE: 9.9). They had severe bronchial obstruction: mean FEV1 was <33% (10.6) of predicted values, with some CO2 retention: mean PaCO2 was 45.6 (7.1) mm Hg. By December 31, 1995, 189 patients had died (75%) and 13 (5%) were unavailable for follow-up.

The overall crude survival was poor: 80.9% after 1 year, 67.1% after 2 years, 34.7% after 5 years, and 7.1% after 10 years. In the crude multivariate analysis (Cox), the negative prognostic factors were age and hypercapnia. The overall relative survival (Hakulinen), corrected for life expectancy, was 82.8% after 1 year, 70.8% after 2 years, 41.5% after 5 years, and 10.25% after 10 years. In the final multivariate relative model, age was no longer significant and the only bad prognostic factor was hypercapnia with a relative risk of 1.97 (1.16 to 3.34).

This work shows the inadequacy of the Cox observed survival model when it comes to appreciating the real prognostic impact of age, because of the confusing factor associated with a normal life expectancy.