Potential misclassification of causes of death from COPD.

Little is known about causes of death in chronic obstructive pulmonary disease (COPD) and the validity of mortality statistics in COPD. The present authors examined causes of death using data from the Copenhagen City Heart Study. Of the 12,979 subjects with sufficient data from the baseline examination during 1976-1978, 6,709 died before 2001. Of these, 242 died with COPD as cause of death. Among subjects with at least severe COPD at baseline, only 24.9% had COPD as cause of death and, in almost half of the cases where COPD was listed as cause of death, the subject had a normal forced expiratory volume in one second /forced vital capacity ratio at baseline. In COPD patients, having COPD on the death certificate was associated with chronic mucus hypersecretion (CMH) at baseline, an odds ratio (OR) of 3.6 (95% confidence interval 1.7-7.7), and being female (OR 2.7 (1.3-5.6)). In subjects without COPD, CMH and smoking were predictors of COPD as underlying cause of death, ORs 2.3 (1.5-3.7) and 2.2 (1.4-3.6), respectively. It was concluded that chronic obstructive pulmonary disease is underreported on death certificates, that biases in the use of chronic obstructive pulmonary disease as cause of death can be assessed, and that possible "over-diagnosis" of chronic obstructive pulmonary disease on death certificates in subjects unlikely to have significant disease should initiate caution when using causes of mortality in chronic obstructive pulmonary disease epidemiology.