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Accurate diagnosis is fundamental in order to avoid untreated symptoms as a result of under-diagnosis, and inappropriate treatment as a result of over-diagnosis. Both scenarios have implications both to the health of the patient and the cost of providing healthcare. National and international guidelines emphasise the importance of demonstrating variable lung function in order to confirm the diagnosis of asthma. Variability of PEF and FeV1, either spontaneously over time or in response to therapy is a characteristic feature of asthma.
See SIGN Guideline 101 (SIGN and BTS) British Guideline on the Management of Asthma 2008.
“...measurements of airflow limitation, its reversibility and its variability are considered critical in establishing a clear diagnosis of asthma” [Global Strategy for Asthma Management and Prevention. www.ginasthma.org]. One peak flow measurement (as required by the Asthma 2 indicator in the 2004/5 QOF) provides no information about variability and therefore can neither confirm, nor refute, the diagnosis.
Objective measurement of variability either spontaneously over time or in response to therapy is thus fundamental to the diagnosis of asthma, and may be conveniently achieved in primary care with serial peak flow measurements. Significant variability in peak flow is defined as a change of 20% or greater with a minimum change of at least 60 l/min ideally for three days in a week for two weeks seen over a period of time and may be demonstrated by monitoring diurnal variation, demonstrating an increase after therapy (15 minutes after short-acting bronchodilator, after six weeks inhaled steroids, two weeks oral steroids) or a reduction after exercise or when the patient next meets his/her trigger. Spirometry (>15% and 200ml change in FeV1) may still be used to confirm variability, though the limitation imposed by a surgery-based measurement means that changes over time may be missed.
It is important to recognise that while repeated normal readings in a symptomatic patient cast doubt on a diagnosis of asthma, the natural variation of the disease means that many patients with asthma will not necessarily have significant variability at any given time. Confirmation of the diagnosis may therefore require further recordings e.g. during a subsequent exacerbation. In circumstances of persisting doubt then more specialist assessment is required which may include hyper-responsiveness testing and consideration of alternative diagnoses.
It is of note that a proportion of patients with COPD will also have asthma i.e. they have large reversibility – 400mls or more on FeV1 – but do not return to over 80% predicted, and a significant smoking history. Evidence would suggest that this should not usually be more than 15% of the overall COPD population.
The practice should report the percentage of patients aged eight or over diagnosed as having asthma after 1 April 2006 with measures of variability or reversibility.
Prepared By Jean Keenan