Value of measuring diurnal peak flow variability in the recognition of asthma: a study in general practice.

In this study we analysed the value of measuring diurnal peak flow variability (DPV) in general practice for diagnosing asthma or chronic obstructive pulmonary disease (COPD). One hundred and eighty-two subjects, aged 18-75 yrs, with undiagnosed asthma or COPD, presenting with a persistent cough recorded a peak flow diary twice daily for 2 weeks. A diagnosis of asthma or COPD was based on the recurrence of airway symptoms in the past year accompanied by spirometric measurements and a provocative dose of methacholine causing a 20% fall in forced expiratory volume in one second. DPV was expressed as amplitude percentage highest of the day. Cut-off values of 15% and 20% (DPV15%, DPV20%) were employed and the number of days that these values were reached, was assessed. The influence of age, sex and pack-years smoking on DPV was analysed by logistic regression. The a priori probability to have asthma (n=69) or COPD (n=12) was 45% (81/182) and increased to >70% with a DPV20% for at least 3, and a DPV15% for at least 4 days. Scoring formulas for asthma (DPV15% (number of days present) + 4 (if female sex)) and for asthma and COPD combined (8x DPV15% (number of days present) + 24 (if female sex) + pack-years smoking) predicted which subjects were at risk for having asthma (or COPD). Simple formulas based on the number of days with diurnal peak flow variability at 15%, female sex and pack-years can predict which patients with persistent cough are likely to have asthma or chronic obstructive pulmonary disease.