Background: There is a lack of epidemiological data on COPD by disease severity. We have estimated the prevalence and underdiagnosis of COPD by disease severity defined by the British Thoracic Society (BTS) and Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines. The impact of smoking was evaluated by the population attributable fraction of smoking in COPD.
Methods: A random sample of 1500 responders of the third postal survey performed in 1996 of the Obstructive Lung Disease in Northern Sweden (OLIN) Studies' first cohort (6610 subjects recruited in 1985) were invited to structured interview and spirometry. One thousand two hundred and thirty-seven subjects (82%) performed spirometry.
Results: The prevalence of mild BTS-COPD was 5.3%, moderate 2.2%, and severe 0.6% (GOLD-COPD: mild 8.2%, moderate 5.3%, severe 0.7%, and very severe 0.1%). All subjects with severe COPD were symptomatic, corresponding figures among mild COPD were 88% and 70% (BTS and GOLD), Subjects with severe BTS-COPD reported a physician-diagnosis consistent with COPD in 50% of cases, in mild BTS-COPD 19%, while in mild GOLD-COPD only 5% of cases. The major risk factors, age and smoking, had a synergistic effect on the COPD-prevalence. The Odds Ratio (OR) for having COPD among smokers aged 76-77 years was 59 and 34 (BTS and GOLD) when non-smokers aged 46-47 was used as reference population.
Conclusions: Most subjects with COPD have a mild disease. The underdiagnosis is related to disease-severity. Though being symptomatic, only a half of the subjects with severe COPD are properly labelled. Smoking and increasing age were the major risk factors and acted synergistic.