Before testing, screening questions were asked to determine medical safety exclusions (ie, those who had undergone chest or abdominal surgery within 3 weeks or had experienced heart problems [myocardial infarction or heart attack, angina or chest pain, and congestive heart failure]) within 6 weeks before attending the MEC. Airflow obstruction was defined as stage 1 according to the following Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines: FEV1, < 80% of predicted (based on gender, age, and height); FEVj/FVC ratio, < 70%.4 As children aged < 7 years did not have spirometry measurements recorded in NHANES III, estimates of airflow obstruction were weighted to the sample of participants aged > 8 years for whom spirometry had been measured.
Statistical Analysis
The prevalence for each condition per gender and age group is presented for the United States and the United Kingdom separately. Values have been extrapolated to the total population in each country using the NHANES III sample weights for the United States and using the Office of National Statistics data for the United Kingdom. Each graphic representation in Figures 2 to 5 was calculated by applying simple proportional euclidean geometry, with the area of the circle representing the exact population size. The areas of intersection of circles represent the percentage of overlap of two or three OLD conditions. Analyses were conducted using a statistical software package (SAS, version 8.0; SAS Institute; Cary, NC).
Results
The descriptive characteristics of NHANES III and GPRD participants with OLD are presented in Table 1. Patients were stratified into seven mutually exclusive disease groups by age and sex for each study population. Patients with asthma, in whom diagnoses had been made with or without other OLD conditions, were younger than COPD patients. The combination of asthma with chronic bronchitis, but not with emphysema, was associated with younger age.
In the US NHANES III total population, the prevalence of current asthma was 5.5%, the prevalence of current chronic bronchitis was 3.2%, and the prevalence of ever having had emphysema was 1.5%. In the UK GPRD total population, the prevalence of current asthma was 2.3%, the prevalence of current chronic bronchitis was 0.5%, and the prevalence of current emphysema was 0.5% (Fig 2). The asthma-only group was the largest group of OLD patients, accounting for 4.3% and 2.2%, respectively, of the United States and United Kingdom general populations; that is, 50.3% and 79.4%, respectively, of all OLD patients. Seventeen percent of OLD patients in the United States had more than one OLD condition, and 2.8% had all three conditions simultaneously. The UK figures were 19.1% and 3.6%, respectively. The areas of intersection among the three OLD conditions differed by data source and country (Fig 2).