Dutch researchers looked at which patients would be diagnosed with COPD if a fixed cut-off definition of airway obstruction was used, as is recommended by NICE and other COPD guidelines, compared with a definition adjusted for age and sex.
Using spirometry data from 330 GPs in the Netherlands, they found that the fixed definition led to misdiagnosis in around a quarter of patients, with the proportion of patients being misdiagnosed increasing with age, from 15.5 per cent for those 41-50 years old, to 42.7 per cent for those aged over 81.
Overdiagnosis of airway obstruction may have adverse effects on patients who are incorrectly diagnosed and treated for COPD, the researchers point out.
Current NICE guidelines define COPD by the presence of airflow obstruction, as demonstrated by a lung function score under 70 per cent of expected for either FEV1 or forced vital capacity (FVC).
The researchers compared the diagnosis rate using this fixed score with a definition based on whether patients' FEV1 or FVC was below the lower limit of normal for their age or sex.
Dr Dermot Ryan, a GP in Leicestershire stressed that a diagnosis of COPD is really a matter of clinical judgment.
'An FEV1 of 70 per cent of expected is where we start to think about a diagnosis of COPD, but that still doesn't allow for individual variation,' he said.
'If a patient is asymptomatic, it doesn't matter what their FEV1 is. Someone with an FEV1 below 70 per cent of expected may have always had an FEV1 at that level.'
He added: 'Spirometry is relatively crude, but it is cheap, accessible and reliable, and we really don't need anything more sophisticated.'
- Eur Respir J 2008; 32: 945-52.