Background/Purpose

            Chronic obstructive pulmonary disease (COPD) is increasingly recognized as an inflammatory condition. We aimed to identify the risk of newly recorded COPD among patients with incident GPA cases compared to the control population using a combination of physician billing codes and hospitalization data covering the entire province of British Columbia, Canada.

Methods

            Our data includes all health professionals and hospital visits covered by the comprehensive provincial medical services plan (1990-2010) and all dispensed medication (1996-2010), for all BC residents.

            We conducted a retrospective matched cohort study among new cases with GPA meeting a pre-defined criteria as follows: a) diagnosis of GPA (ICD-9-CM 446.4) in adults on at least two visits within a two-year period between 1996 and 2010 by a non-rheumatologist physician; b) diagnosis of GPA on at least one visit by a rheumatologist or from hospitalization; c) absence of a prior GPA diagnosis between January 1990 and December 1995 (to ensure incident GPA cases). Ten controls matched by birth year, sex and calendar year of follow-up were selected from the general population. Incident COPD cases were identified using a validated algorithm (first ICD-9-CM: 491, 492, 496, 493.2, or ICD-10-CM J43 or J44) from hospitals or death certificates. We estimated incidence rate ratios (IRRs) by comparing GPA cases with age-, sex- and entry-time-matched comparison cohorts. Multivariable Cox-regression models adjusting for confounders were also used. Sensitivity analyses were conducted to assess for unmeasured confounders (e.g. smoking).

Results

   Among 512 patients with incident GPA, (mean age 57.7, 54% female) 34 developed COPD.  This translated to a 20.25 incident rate (IR) per 1000 person-years in the GPA cohort compared to 4.10 IR / 1000 person-years in the control population.  The age-, sex- and entry-time matched RR was significantly increased in the GPA cohort (See table). The risk of developing COPD was the highest in the first year following diagnosis of GPA (17 fold). The results also remained statistically significant after adjusting for the potential impact of unmeasured confounders (adjusted RRs ranging between 5.03 and 5.58 in all sensitivity analyses).

Conclusion

To our knowledge, this is the first general population-based study that shows a six-fold increase in risk of COPD in patients with GPA. The highest incidence of COPD in the first year following diagnosis supports the role of immune mediated inflammation in the pathogenesis of COPD.

Table: Risk of Incident COPD according to GPA Status