Guideline definitions for airflow obstruction are meaning GPs are misdiagnosing many middle-aged and older people with chronic obstructive pulmonary disease (COPD), shows a Dutch study.
Dr Tjard Schermer of the Nijmegen Medical Centre and colleagues compared outcomes when two recommended but different definitions for airflow obstruction based on the forced expiratory volume in one second to forced vital capacity ratio (FEV1/FVC) were used in patients with no previous diagnosis of chronic respiratory disease.
All 14,056 individuals (53% female, 69% current or former smokers) had been referred for a diagnostic spirometry test by their GP.
NICE and all other guidelines recommend a fixed cut-off point of 0.70 for the FEV1/FVC ratio to decide on whether or not airflow obstruction is present, regardless of the age or sex of the person being measured.
The investigators decided to compare this with an age and sex specific ‘lower limit of normal’ cut-off point for the FEV1/FVC ratio.
Using diagnostic spirometry tests data from the population, they calculated a sensitivity of 97.9% for the fixed cut-off relative to the lower limit of normal cut-off point for COPD, a specificity of 91.2%, a positive predictive value of 72% and a negative predictive value of 99.5%.
The older patients were, the more pronounced the discrepancy between the two definitions became. In patients 50 or older that were current or ex-smokers, at least 25% were misdiagnosed with airflow obstruction when the fixed cut-off point definition was used.
‘Although the negative predictive value of the fixed ratio definition was very high, the positive predictive value of the fixed ratio was insufficient,’ said the authors. ‘As spirometry is used as a diagnostic test to verify obstruction (instead of excluding it), these predictive values are insufficient.’
They conclude: ‘Although diagnosis of chronic obstructive pulmonary disease obviously requires more than a spirometric test (i.e. symptoms, smoking history and additional diagnostic tests), a definition of airflow obstruction that is based on lower limits of normal from an appropriate reference population would diminish the rate of false positive interpretations. The individual, as well as the societal, burden of chronic obstructive pulmonary disease is sufficiently large to warrant critical appraisal of the main criterion on which the diagnosis of this disease is based.’
The research is published in this month’s issue of the European Respiratory Journal.