Development and validation of a prognostic index for health outcomes in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a debilitating and progressive disease. The severity of the condition has typically been characterized by a single physiological measurement: forced expiratory volume in 1 second, which has been shown to be prognostic for mortality.

To develop a prognostic tool for COPD that is sensitive not only to mortality but also to other important drivers of health status and cost, data were obtained from a pooled analysis of 12 randomized controlled trials and 3 main outcomes were chosen: mortality, hospitalization, and number of exacerbations. Cox models were employed for the time-to-event data (death or hospitalization), and a negative binomial model was used for calculating the count data (exacerbations). From these models, 3 specific indexes were developed on a 100-point scale, and 1 composite index was obtained as a mean of the specific indexes. One-third of the data was reserved for validation purposes.

All indexes provided good discrimination among tertiles in the training and validation samples. The composite index had a performance very similar to that of the specific index in both the training and validation samples: the overall C statistic was estimated as 0.71 for both mortality and hospitalization. Each 10-point change in the composite index corresponds to an increase of 54% in the hazard ratio of death, 57% in the hazard ratio of hospitalization, and 21% in the incidence rate of exacerbations.

A composite index for COPD prognosis (the COPD Prognostic Index) has been validated in data not used in its development and is capable of predicting not only mortality, but also hospitalizations and exacerbations. All factors included in the index are straightforward to obtain, which should make the index suitable for use in primary as well as secondary care settings.