Chronic obstructive pulmonary disease (COPD) is a disease that is characterized by irreversible and progressive airflow obstruction, and is associated with high morbidity and mortality [1
]. COPD is predominantly diagnosed in adults aged well over 40 years. In developed countries cigarette smoking is the main risk factor [2
], and accelerated lung function decline is the predominant clinical and prognostic hallmark of the disease [3
]. Spirometry is recommended to assess airflow obstruction, i.e. to establish the ratio of the forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). Next, severity of obstruction is quantified by calculation of FEV1 as percentage of predicted value.
For subjects suspected of having the disease current clinical COPD guidelines recommend a fixed FEV1/FVC cutpoint < 0.70 (after administration of a bronchodilator) to decide whether or not airflow obstruction is present [1
]. However, because lung function physiologically declines with age [5
], it has recently been advocated that a correct definition of airflow obstruction should not be based on a fixed cutpoint for all ages, but should take the physiological decline into account [6
]. One suggested approach for this is to use lower limit of normal (LLN) cutpoints based on the distribution of FEV1/FVC values in an appropriate reference population, which takes gender and age differences between individuals into account [7
]. Several recent studies have shown high rates of false-positive interpretations (especially among elderly subjects) when the 0.70 fixed cutpoint is off-set against an age-specific LLN cutpoint [9
]. Because the majority of COPD patients are diagnosed and managed in primary care [15
] and primary care doctors need to differentiate between various underlying causes for the respiratory symptoms a patient presents with (i.e., asthma, COPD, congestive heart failure, and a wide range of other causes), it is especially important for them to know which cutpoint is preferred when assessing the presence (or absence) of airflow obstruction. This is even more important because in elderly patients co-morbid conditions are often present, and misattribution of a patient's symptoms (e.g., dyspnoea) to COPD could lead to inappropriate or delayed treatment.
We previously reported on the use of different criteria to diagnose airflow obstruction in subjects who present with respiratory symptoms in primary care [16
]. To date, little longitudinal research has been published to establish the course of clinical markers of COPD prognosis in relation to the recommended diagnostic criteria for airflow obstruction [17
]. An influential review acknowledged that overestimation of airflow obstruction with the fixed FEV1/FVC ratio becomes more problematic with increasing age, but also stated that the incremental benefits of changing the recommendation to use the fixed 0.70 cutpoint in COPD guidelines remain to be seen [20
]. Recent discussions illustrate that there currently is no consensus on this issue [21
] and that there is a clear need for further evidence.
The aim of the study reported in this paper was to assess lung function decline in symptomatic middle-aged and elderly subjects identified as 'obstructive' according to either the fixed 0.70 FEV1/FVC cutpoint or an age- and gender-specific LLN cutpoint for this ratio. We also investigated whether our findings and conclusions would change when different sets of prediction equations are used to calculate LLN cutpoints for FEV1/FVC.