When can personal best peak flow be determined for asthma action plans?

BACKGROUND Written asthma action plans based on personal best peak expiratory flow (PEF) consistently improve health outcomes, whereas those based on predicted PEF do not. Guidelines state that personal best PEF should be assessed over 2-3 weeks during good asthma control, but it is unclear how long to wait after commencing or changing treatment. METHODS Electronically recorded spirometric data from 61 subjects with initially poorly controlled asthma from a 72 week budesonide study were analysed. For each week, average morning pre-bronchodilator PEF was calculated and personal best PEF was determined as the highest PEF in the previous 2 weeks. The time to plateau was defined as the week beyond which no further improvement occurred. RESULTS At baseline, average morning PEF was 61% predicted and personal best PEF was 87% predicted. Personal best PEF from twice daily monitoring increased to a plateau of 95% predicted (p<0.0001) after only 3 weeks of budesonide treatment. However, average morning PEF continued to improve for 3 months and "as needed" reliever use for 7 months. CONCLUSIONS Personal best PEF is a useful concept for asthma self-management plans when determined as the highest PEF over the previous 2 weeks. With twice daily monitoring, personal best PEF reaches plateau levels after only a few weeks of corticosteroid treatment.