Further investigation with an intermediate probability of asthma

Patients with airways obstruction

Tests of peak expiratory flow variability, lung volumes, gas transfer, airway hyper-responsiveness and airway inflammation are of limited value in discriminating patients with established airflow obstruction due to asthma from those whose airflow obstruction is due to other conditions.Patients may have more than one cause of airflow obstruction, which complicates the interpretation of any test. In particular, asthma and chronic obstructive pulmonary disease (COPD) commonly coexist

• offer patients with airways obstruction and intermediate probability of asthma a reversibility test and/or a trial of treatment for a specified period:
  • if there is significant reversibility, or if a treatment trial is clearly beneficial treat as asthma
  • if there is insignificant reversibility and a treatment trial is not beneficial, consider tests for alternative conditions

Patients without airways obstruction

In patients with a normal or near-normal spirogram it is more useful to look for evidence of airway hyper-responsiveness and/or airway inflammation.These tests are sensitive so normal results provide the strongest evidence against a diagnosis of asthma

• in patients without evidence of airways obstruction and with an intermediate probability of asthma, arrange further investigations before commencing treatment
  
  
• tests of airway responsiveness have been useful in research but are not yet widely available in everyday clinical practice
  • most widely used method of measuring airway responsiveness relies on measuring response in terms of change in FEV1 a set time after inhalation of increasing concentrations of histamine or methacholine. The agent can be delivered by breath-activated dosimeter, via a nebuliser using tidal breathing, or via a hand held atomiser
    • response is usually quantified as the concentration (or dose) required to cause a 20% fall in FEV1 (PC20 or PD20) calculated by linear interpolation of the log concentration or dose-response curve
      
      
• eosinophilic airway inflammation can be assessed non-invasively using the induced sputum differential eosinophil count or the exhaled nitric oxide concentration (FENO)
  • a raised sputum eosinophil count (>2%) or FENO (>25 ppb at 50 ml/sec) is seen in 70-80% of patients with untreated asthma
    • neither finding is specifi c to asthma: 30-40% of patients with chronic cough and a similar proportion of patients with COPD have abnormal results
    • there is growing evidence that measures of eosinophilic airway inflammation are more closely linked to a positive response to corticosteroids than other measures even in patients with diagnoses other than asthma

Consider performing chest X-ray in any patient presenting atypically or with additional symptoms or signs. Additional investigations such as full lung function tests, blood eosinophil count, serum IgE and allergen skin prick tests may be of value in selected patients.

Reference:

• (1) British Thoracic Society (BTS)/Scottish Intercollegiate Guidelines Network (SIGN) 2011. British Guideline on the Management of Asthma. A national clinical guideline.