In addition, an “at risk” stage (formerly known as GOLD Stage 0) consists of patients with chronic respiratory symptoms (cough, sputum, or dyspnea) and normal lung function. Although this stage has been removed from the 2006 GOLD update because of data suggesting this stage may not progress to GOLD Stage 1 and higher COPD (Vestbo and Lange, 2002; WHO, 2008), people with symptoms and normal lung function have a lower quality of life and a higher risk of hospitalizations and mortality in follow-up investigations (Mannino et al., 2006; Stavem et al., 2006).
As noted above, this classification strategy may miss some patients with disease and overestimate the extent of disease in others. In addition, surveillance of disease typically depends on using information from administrative data sets, requiring the use of diagnostic and procedure codes to infer the presence of disease. This can be particularly problematic when looking at mortality related to COPD because most people with severe COPD who die have their death attributed to another cause (Mannino et al., 2006), and most people who die with a diagnosis of COPD listed on their death certificate do not have this attributed as the underlying cause of death. Therefore, the contribution of this chronic lung disease to observed mortality patterns and trends is underestimated.
COPD is a common chronic disease. Most estimates of COPD place its prevalence in the adult population at 5 to 10 percent, although these estimates vary by the specific criteria used. Data from the Third National Health and Nutrition Examination Survey (NHANES III), the most recent national health survey that included spirometry, showed a prevalence of COPD in adults of 6.8 percent (Mannino and Buist, 2007). Over 50 percent of people with evidence of COPD have never been diagnosed with this disease. This proportion is even higher among people with mild disease, which is most amenable to intervention (Mannino and Braman, 2007).
COPD is responsible for about 700,000 hospitalizations annually in the United States. In recent years, the hospitalization rate among women has increased and is now similar to the rate among men. In 2009, more than 137,000 adults in the United States died from COPD (Kochanek et al., 2011). Age-adjusted mortality rates per 100,000 vary dramatically by state, from a low of 27.1 in Hawaii to a high of 93.6 in Oklahoma (CDC, 2008).
COPD has an enormous financial burden, with estimated direct medical costs in 1993 of $14.7 billion. The estimated indirect costs related to morbidity (loss of work time and productivity) and premature mortality is an additional $9.2 billion, for a total of $23.9 billion. By 2002 the direct and indirect costs were estimated at $32.1 billion (Mannino and Buist, 2007). The overwhelming risk factor for COPD is cigarette smoking. Other important risk factors include a history of asthma; occupational exposures to dusts, gases, vapors, and fumes; exposure to biomass smoke; and respiratory infections such as tuberculosis. In the developing world, exposures to biomass smoke and respiratory infections are particularly important (Buist et al., 2007). Comorbid diseases include cardiovascular disease, osteoporosis, lung cancer, and depression. In addition, diseases such as pneumonia and pulmonary hypertension are often complications of COPD (Decramer et al., 2008; Holguin et al., 2005).
The classification of chronic respiratory disorders is often based on the pattern of physiologic impairment, either obstructive or restrictive, as measured with pulmonary function tests. Obstructive disorders, asthma, and COPD are the most common chronic respiratory diseases. The restrictive disorders are heterogeneous, including diffuse parenchymal lung diseases (e.g., idiopathic pulmonary fibrosis) and disorders that impair chest movement (e.g., morbid obesity, neuromuscular diseases). The focus of this review is on COPD, which provides an example of how surveillance throughout the life span may contribute to the prevention and control of chronic respiratory diseases.