Influence of country-level differences on COPD prevalence

Figures & data

Figure 1 PRISMA flow chart for articles that are identified, screened, and found eligible in the literature search.

Table 1 Number of countries and qualified articles for each world region

World region	Number of countries	Number of articles
Australasia	2	6
Latin America	5	3
Mediterranean	10	18
North America	2	11
Northern Europe	14	30
Southeast Asia	5	17
Other	3	3
Total	41	88

Notes: Some countries were covered by several articles, so the number of articles exceeds the number of countries. The total number of articles exceeds 80 because some qualified articles studied multiple countries.

Table 2 Stepwise binomial regression results for the prevalence outcome measure

Variable	Adjusted odds ratio (95% CI)	P-value
GOLD I–IV (indicator)	2.067 (2.035–2.100)	<0.001
Postbronchodilation (indicator)	0.926 (0.904–0.948)	<0.001
GOLD I–IV × postbronchodilation interaction (indicator)	0.683 (0.665–0.702)	<0.001
Intercept	0.0854 (0.0842–0.0865)

Abbreviation: GOLD, Global Initiative for Chronic Obstructive Lung Disease.

Table 3 Fitted binomial regression estimates of mean COPD prevalence rates (%) for the two GOLD levels according to whether postbronchodilation was used or not

Fitted mean prevalence rate (%)	No bronchodilation	Bronchodilation
GOLD II–IV	7.9% (n=199)	7.3% (n=434)
GOLD I–IV	15.0% (n=294)	10.0% (n=656)

Note: Sample sizes shown are numbers of records (total =1,583).

Abbreviation: GOLD, Global Initiative for Chronic Obstructive Lung Disease.

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.

Table 4 Results for stepwise regression of COPD prevalence on successive groups of country-level variables

Variable	Adjusted odds ratio (95% CI)	P-value
Country-level established risk factors
Smoking prevalence (men, %)	0.994 (0.993–0.995)	<0.001
Smoking prevalence (women, %)	1.012 (1.011–1.013)	<0.001
Working-age men (15–64, %)	0.922 (0.913–0.931)	<0.001
Working-age women (15–64, %)	1.061 (1.047–1.075)	<0.001
Seniors (men, >64, %)	0.954 (0.935–0.973)	<0.001
Seniors (women, >64, %)	0.968 (0.954–0.981)	<0.001
Socioeconomic risk factors
Literacy (standardized)	1.013 (1.002–1.025)	0.024
Gross national income per capita (standardized)	1.059 (1.049–1.069)	<0.001
Health system performance (WHO ranking, standardized)	0.933 (0.922–0.944)	<0.001
Environmental risk factors
CO2 emissions per capita (standardized)	0.985 (0.977–0.993)	<0.001
Vehicles per capita (standardized)	1.009 (1.001–1.018)	0.030
Physical geography risk factors
Latitude (north or south, truncated to nearest degree)	0.996 (0.996–0.997)	<0.001
High elevation (log-elevation above 470 m)	0.566 (0.541–0.593)	<0.001
Effects of world region
Australia/New Zealand	0.761 (0.725–0.798)	<0.001
Latin America	1.102 (1.066–1.138)	<0.001
Mediterranean	0.952 (0.928–0.976)	<0.001
North America	1.020 (1.003–1.036)	0.018
North Europe	Reference
Southeast Asia	0.983 (0.968–0.998)	0.023
Other	1.492 (1.384–1.608)	<0.001

Notes: The regression analysis has already taken into account the effects of the COPD prevalence outcome variables and the study-level established risk factors in the first two regression steps before stepping in the groups of country-level variables. At each step, the regression function includes an offset covariate calculated in the preceding step. The offset therefore captures the regression effects of all risk factors that have entered in earlier steps.

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.

Table 5 Stepwise contributions of groups of risk factors to explaining variability in COPD prevalence rates

Risk factor groups	Variables added to the regression model	Total deviance	Pseudo R^2
Prevalence outcome measures (null regression model)	Prevalence severity, post-bronchodilation	64,671	0.000
Study-level established risks	Sex, age, smoking status, smoking amount	23,170	0.642
Country-level established risks	Sex, age, smoking status	19,671	0.696
Socioeconomic risks	Literacy, gross national income, health system performance	19,409	0.700
Environmental risks	CO2 emissions, vehicles	19,395	0.700
Physical geography risks	Elevation, latitude	18,706	0.711
World regions (full regression model)	World region	18,397	0.716

Notes: Pseudo R² is the proportion of total deviance explained. Deviance would be 0 if the regression model provided an exact fit to 1,583 prevalence rates from the 80 study articles.

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.