Chronic obstructive pulmonary disease (COPD) is now recognized as the most rapidly growing health problem worldwide. It ranks as the fourth most common killer and is the only disease in the top 10 whose rank is rising, although it is difficult to accurately define the prevalence of this disease in the general population. In effect, the epidemiological surveys that explore the prevalence of COPD depend upon the proper recognition and diagnosis of COPD by both the study participants and their health care providers; this tends to bias the estimates toward there being fewer cases than do actually exist. 
Most patients with COPD remain, in fact, asymptomatic or have stable symptoms, such as a morning cigarette cough, that are denied by the patient for 20 to 30 years before approximately 50% of the normal forced expiratory volume (FEV) is lost. Thus, a clinician cannot rely on clinical symptoms and signs or even radiographic abnormalities to define the presence of mild-to-moderate COPD. Nonetheless, symptoms and examination and smoking history are used to diagnose COPD in general practice, although a diagnosis made on this basis may be inaccurate due to the range of presentations from asymptomatic (while there may or may not be significant lung impairment on objective testing) through to multiple nonspecific symptoms, such as shortness of breath, cough, wheeze, and sputum production, which overlap with other respiratory disorders, not in the least of which is asthma. 
It is obvious that precise and early diagnosis is essential, because COPD is mainly an irreversible and progressive disease with serious systemic complications and a subclinical phase to these problems. Diagnosis of COPD can be established only by objective measurement, preferably using spirometry. Spirometric screening and monitoring of smokers at high risk in primary health care can, in fact, identify those most susceptible to developing COPD while the disease is in an early phase.
Furthermore, spirometry plays a central role not only in confirming the diagnosis and in monitoring disease progression, but also in allowing the classification of disease severity that relies heavily on spirometry measures. All guidelines report scales of severity of the disease, which are based primarily on the level of reduction of forced expiratory volume in 1 s (FEV₁). The Global Initiative for Chronic Obstructive Lung Disease and the American Thoracic Society/European Respiratory Society criteria classify COPD into four stages: stage 1 (FEV₁ ≥80% predicted), stage 2 (FEV₁ 50 to <80% predicted), stage 3 (FEV₁ 30 to <50% predicted), and stage 4 (FEV₁ <30% predicted). In addition, an "at-risk" stage, called GOLD stage 0, consists of patients with exposure to risk factors, chronic respiratory symptoms (cough, sputum, or dyspnea), and normal lung function. 
Most guidelines make the generally reasonable assumption that the lower the FEV₁ as a percentage of prediction, the greater the pathologic and symptomatic severities. Unfortunately, this approach has several limitations, although the GOLD stages have been shown to be reasonable predictors of mortality in cohorts followed for up to 11 years. For example, it does not account for the heterogeneity of the disease and does not necessarily differentiate between "normal" aging of the lung and disease. Fortunately, the availability of quantitative computer tomography scanning may provide a necessary stimulus to create greater knowledge into studies of the mechanisms of emphysema and air wall thickness. A prerequisite is that lung density measurement can be standardized and validated against traditional clinical outcome variables, and if proven it may be a new measurement that is objective, specific, and sensitive. 
The incapacity of differentiating between "normal" aging of the lung and disease is not the only limitation of spirometry. It is well known, for instance, that the degree of airflow limitation, as measured by FEV₁, does not necessarily correlate strongly with patient-reported clinical outcomes such as health status, dyspnea, or exercise performance. This is because other pathophysiological (eg, dynamic hyperinflation of the lungs) and psychological (eg, coexisting anxiety) influences also affect these outcomes.
 Health status or health-related quality of life has been recognized as one of the more important outcomes in clinical studies, and several guidelines emphasized that improving quality of life is one of their goals. The major factors that influence the health status of patients with COPD are dyspnea and their psychological status, whereas physiological parameters such FEV₁ generally do not contribute very much to impairments in health status. Consequently, since even mild breathlessness can impair the health status of patients with mild COPD, medical interventions for relieving dyspnea, such as inhaled bronchodilators, may be justified in the early stages of COPD. This contrasts with guideline recommendations. 
The above-mentioned reasons justify why the assessment of a patient suffering from COPD must include lung function parameters other than FEV₁ and also measures of dyspnea, functional and health status, exercise tolerance, and breathlessness after exercise. 
A more practical approach may be to use a multidimensional tool such as the BODE index, a 10-point scale combining measures of body mass index (B), the degree of airflow obstruction (O) and dyspnea (D), and exercise capacity (E) into one measure, thus encompassing the pulmonary as well as the systemic effects of COPD. Another approach may be to consider various combinations of COPD outcomes and determine their associations using a principal components analysis (PCA). BODE has been used as a predictor of the risk of death, and several studies now suggest that it might a useful indicator of disease progression and treatment effect. In the case of PCA, it offers a more comprehensive means of evaluating COPD patients than does relying simply on the degree of airflow limitation; however, there is little evidence available at this time to recommend it as a useful method in COPD research.