Influence of country-level differences on COPD prevalence

Abstract

Purpose: Studies suggest that COPD prevalence may vary between countries. We conducted an ecological study of data from COPD prevalence articles to assess the influence of differences in country-level risk factors on COPD prevalence.

Patients and methods: Our study covered English language articles published during 2003-2014. Qualified articles used spirometry to assess COPD prevalence and used representative samples from national or subnational populations. Stepwise binomial regression was used to analyze associations between study- and country-level factors and COPD prevalence.

Results: Eighty articles provided 1,583 measures of COPD prevalence for subjects in different sex, age, and smoking categories for 112 districts in 41 countries. Adjusted prevalence rates for COPD were significantly lower for Australia/New Zealand and the Mediterranean and significantly higher for Latin America, compared to North America, Southeast Asia, and Northern Europe. Country-level socioeconomic development variables had an uneven and mixed association with COPD prevalence. High elevation above sea level was shown to be a protective factor for COPD. Study-level variables for the established risk factors of sex, age, and smoking explained 64% of variability in COPD prevalence. Country-level risk factors raised the explanatory power to 72%. Approximately 28% of worldwide variability in COPD prevalence remained unexplained.

Conclusion: Our study suggests that COPD prevalence varies across world regions, even after adjustment for established risk factors. Major country-level risk factors contributing to the worldwide epidemic of COPD remain to be investigated.

Keywords: country effects; ecological study; epidemiology; lung disease; risk factors; spirometry.

Figure 1

PRISMA flow chart for articles that are identified, screened, and found eligible in the literature search.

Figure 2

Postbronchodilator COPD prevalence rates for GOLD I–IV, plotted as a function of established risk factors. Notes: Prevalence numbers for each risk factor are averaged over all other established risk factors without weighting. Panels (A) and (C) are box plots. Panels (B) and (D) are scatter plots that also show least-squares fitted logistic and linear regression curves, respectively. Abbreviation: GOLD, Global Initiative for Chronic Obstructive Lung Disease.

Figure 3

Change in fitted prevalence (in percent) with the addition of the high elevation risk factor. Notes: Elevation (in meters above sea level) is displayed on a logarithmic scale. A vertical line is shown at the 470 m threshold where prevalence begins to decline. The prevalence change is measured after all other risk factors are incorporated in the regression model except world region.

Figure 4

Odds ratios for COPD prevalence by world region, before and after adjustment for study- and country-level risks. Note: Northern Europe serves as the reference region. Abbreviations: Aus/NZ, Australia/New Zealand; LAmerica, Latin America; NAmerica, North America; NEurope, North Europe; SEAsia, Southeast Asia.