In a population-based cohort, the COPD severity score adds predictive value in estimating future respiratory-related hospitalizations, ED visits, and outpatient visits. Moreover, the COPD severity score changes over time, indicating that the score is not static, and these changes are themselves prospectively associated with health-care utilization, indicating that the score is responsive to change in a clinically relevant manner.
This COPD severity instrument may have several applications. It may be used as a tool to risk-adjust for disease severity within epidemiologic or outcomes research. The score may also be used to identify those at greater risk of hospitalizations or ED visits, possibly allowing resource-limited disease management programs to target patients at highest risk. Although the severity scoring instrument may be time-consuming to administer face-to-face in the context of a physician visit, the increasing availability of prior utilization data through electronic health records may assist in large-scale calculation of the score (34
). Alternatively, the score may potentially be derived from self-reported questionnaires. Importantly, the COPD severity score does not require measurement of pulmonary function, physical examination or other diagnostic testing, which may greatly facilitate its usage in population-based studies and in targeting high-resource utilizers. Finally, because the COPD severity score is responsive to change, it may be valuable, after further validation, as a measure of current clinical status or as an end-point of therapeutic interventions for COPD. We are currently planning further work on a different cohort of COPD subjects to establish the minimum clinically significant score change.
The risk nomogram assists with interpreting the COPD severity score’s association with utilization outcomes. Not surprisingly in a population-based cohort, hospitalizations and ED visits are a relatively uncommon event for subjects with average COPD severity. However, approximately 10% of the cohort had a COPD severity score greater than 15, which corresponds to a one-year hospitalization risk of about 17%. The threshold at which clinicians or disease management programs might target individuals would likely depend on resource constraints but ideally would target those above some threshold of risk (36
). To place this in context, Kaiser Permanente’s extensive disease management program involved approximately 14% of their asthmatics over a 4-year time-frame (8
). Thus it might be reasonable to choose some value of COPD severity score close to 15 to target a limited subset of the COPD population at significant risk of hospitalization.
Our approach to recruiting and following subjects with COPD resulted in both strengths and limitations. A major advantage of our population-based recruitment is the generalizability of findings to the population of COPD patients. This is particularly important if one is choosing a hospitalization-risk threshold for targeted intervention in a general population. One potential drawback of our approach is that we have relied on self-reported physician diagnosis, which may have resulted in some disease misclassification. However, our method for identifying subjects with COPD is a standard epidemiologic technique (37
). Also, as previously reported, the prevalence of COPD in the initial national sample (not including subjects surveyed in “hot-spots” for COPD) was 13.5%, which is similar to the 12.5% national COPD prevalence rate reported in the Third National Health and Nutrition Examination Survey (NHAINES III) (37
). Furthermore, the 21% prevalence rate in our cohort of self-reported CAD is close to the 19% prevalence rate of angina pectoris or prior myocardial infarction reported among older adults who attended the medical examination portion of NHANES III (39
). These findings support the validity of our results.
Another limitation is our use of self-report to identify health-care utilization, which may have resulted in recall bias. The use of self-reported events is nonetheless supported by other studies, although there is a suggestion that older persons may underreport health-care utilization (40
). Underreporting would likely cause underestimation of the effect of the COPD severity score on utilization. Loss to follow-up is an additional limitation. It is difficult to be sure of the effect of subject attrition. However, if subjects lost to follow-up had more severe COPD with higher health care utilization (e.g. at least 12 subjects did not reinterview because of death), then we would expect that subject attrition would tend to cause an underestimation of the incidence of hospitalization and possibly also the predictive ability of the COPD severity score. Regardless, subjects lost to follow-up did not appear statistically significantly different with respect to most sociodemographic factors and the COPD severity score. Furthermore, a probability-of-attrition weighted analysis did not change the results substantively, indicating that losses to follow-up did not affect our findings (data not shown). Nonetheless, the fact that African Americans were less likely to participate in the final interview may limit our ability to make strong conclusions about this racial-ethnic group.
Overall, the COPD severity score demonstrates prospective validity as a survey-based instrument for risk adjustment or the identification of high-risk cohorts. The severity score added predictive value to our estimate of future health-care utilization over and above a basic predictive model, and it did so in more than one time period. Moreover, the change in the severity score is itself associated with utilization outcomes, suggesting that the score is longitudinally responsive to changes in COPD status. Future work is needed to compare the score with other risk indices, determine a minimally clinically significant score change, evaluate the score’s predictive ability in other cohorts and for other important outcomes such as mortality, and to determine its ability to predict costs and thus its potential for use in risk-adjusting capitated payments. Nonetheless, this COPD severity score appears to have valuable potential as a disease-specific risk index for one of the leading causes of death and disability worldwide.