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Insights into COPD Comorbidities from the OLIN Study and Other Large Databases

In this Journal, Lindberg et al. describe the prevalence of comorbidities (non-pulmonary chronic illnesses and conditions) in a well-characterized cohort of 993 COPD cases and 993 ageand gender-matched controls without airflow obstruction. All were recruited as part of the OLIN (Obstructive Lung Disease in Northern Sweden) study, a large longitudinal project that has collected data on over 50,000 participants since 1985 (1). The strength of the OLIN project is that it is as close to a population-based cohort of COPD patients as you will find anywhere, so it reflects the clinical characteristics of COPD among persons in the general population, as opposed to COPD patients recruited for highly selected clinical trial populations that usually exclude comorbidities. Both cases and controls had spirometry and a detailed medical interview that included questions about specific chronic diseases and conditions. The COPD patients were stratified by GOLD-COPD severity classifications I-IV (mild through very severe), and the controls were stratified by those with normal lung function (N = 776) or those with restrictive lung function (N = 217). Among those with even mild COPD, increased prevalences of cardiovascular disease, intermittent claudication, and chronic rhinitis were found as compared to controls. The prevalence of comorbidities increased with COPD severity, and as expected, the adjusted odds ratios for most other smoking-related comorbidities were also substantially elevated. Interestingly, those with restrictive lung function, which is a population about which we have relatively little epidemiologic information, also had substantially increased comorbidities including diabetes, hyperlipidemia, and cardiovascular diseases. As interest in the pathophysiology and treatment of COPD is increased, so too has interest in how to stratify the clinical characteristics of COPD patients so that we can identify the genetic factors that determine the response to airborne exposures, a process known as phenotyping. The traditional paradigm of stratifying COPD by the severity of airflow obstruction is useful and well validated, but it also has many notable limitations. It is not uncommon to find persons with severe airflow obstruction who are relatively