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Investigations

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Authoring team

Airway inflammation measures

Fractional exhaled nitric oxide

offer a FeNO test to adults (aged 17 and over) if a diagnosis of asthma is being considered. Regard a FeNO level of 40 parts per billion (ppb) or more as a positive test
consider a FeNO test in children and young people (aged 5 to 16) if there is diagnostic uncertainty after initial assessment and they have either:
- normal spirometry or
- obstructive spirometry with a negative bronchodilator reversibility (BDR) test.
  Regard a FeNO level of 35 ppb or more as a positive test.
be aware that a person's current smoking status can lower FeNO levels both acutely and cumulatively. However, a high level remains useful in supporting a diagnosis of asthma

Lung function tests

Spirometry

is the preferred initial test (if available) to assess the presence and severity of airflow obstruction
offer spirometry to adults, young people and children aged 5 and over if a diagnosis of asthma is being considered (2)
FEV1 and FVC are measured during a forced expiratory manoeuvre using a spirometer
- FEV1 is largely independent of effort and highly repeatable
- >400 ml increase in FEV1 post-bronchodilator highly suggestive of asthma in adults
FEV1/FVC ratio is more useful since many lung diseases may result in reduced FEV1
- regard a forced expiratory volume in 1 second/forced vital capacity (FEV1/FVC) ratio of less than 70% (or below the lower limit of normal if this value is available) as a positive test for obstructive airway disease (obstructive spirometry).
less applicable in acute severe asthma
- a normal spirogram/spirometry in an asymptomatic patient does not exclude the diagnosis of asthma, repeated measurements of lung function are often more informative than a single assessment

Bronchodilator reversibility (BDR)

offer a BDR test to adults (aged 17 and over) with obstructive spirometry (FEV1/FVC ratio less than 70%). Regard an improvement in FEV1 of 12% or more, together with an increase in volume of 200 ml or more, as a positive test.
consider a BDR test in children and young people (aged 5 to 16) with obstructive spirometry (FEV1/FVC ratio less than 70%). Regard an improvement in FEV1 of 12% or more as a positive test.

Peak expiratory flow (PEF) variability

simple and widely available and can be used in a wide variety of circumstances including acute severe asthma
more useful in the monitoring of patients with established asthma than in making the initial diagnosis
PEF is effort-dependent and not repeatable
monitor peak flow variability for 2 to 4 weeks in adults (aged 17 and over) if there is diagnostic uncertainty after initial assessment and a FeNO test and they have either:
- normal spirometry or
- obstructive spirometry, reversible airways obstruction (positive BDR) but a FeNO level of 39 ppb or less.
  Regard a value of more than 20% variability as a positive test.
consider monitoring peak flow variability for 2 to 4 weeks in adults (aged 17 and over) if there is diagnostic uncertainty after initial assessment and they have:
- obstructive spirometry and
- irreversible airways obstruction (negative BDR) and
- a FeNO level between 25 and 39 ppb.
  Regard a value of more than 20% variability as a positive test.
monitor peak flow variability for 2 to 4 weeks in children and young people (aged 5 to 16) if there is diagnostic uncertainty after initial assessment and a FeNO test and they have either:
- normal spirometry or
- obstructive spirometry, irreversible airways obstruction (negative BDR) and a FeNO level of 35 ppb or more.
  Regard a value of more than 20% variability as a positive test.

Airway hyperreactivity measures

Direct bronchial challenge test with histamine or methacholine

offer a direct bronchial challenge test with histamine or methacholine to adults (aged 17 and over) if there is diagnostic uncertainty after a normal spirometry and either a:
- FeNO level of 40 ppb or more and no variability in peak flow readings or
- FeNO level of 39 ppb or less with variability in peak flow readings.
  Regard a PC20 value of 8 mg/ml or less as a positive test.
consider a direct bronchial challenge test with histamine or methacholine in adults (aged 17 and over) with:
- obstructive spirometry without bronchodilator reversibility and
- a FeNO level between 25 and 39 ppb and
- no variability in peak flow readings (less than 20% variability over 2 to 4 weeks).
  Regard a PC20 value of 8 mg/ml or less as a positive test.
if a direct bronchial challenge test with histamine or methacholine is unavailable, suspect asthma and review the diagnosis after treatment, or refer to a centre with access to a histamine or methacholine challenge test.

Chest radiology

undertaken after treatment of acute asthma attack has been initiated
consider in any patient presenting atypically or with additional symptoms or signs
in children, chest X-rays should be reserved for severe disease or clinical clues suggesting other conditions and need not be part of the initial diagnostic workup (1,3,4)

Do not offer the following as diagnostic tests for asthma:

skin prick tests to aeroallergens
serum total and specific IgE
peripheral blood eosinophil count
exercise challenge (to adults aged 17 and over).

Use skin prick tests to aeroallergens or specific IgE tests to identify triggers after a formal diagnosis of asthma has been made (2).

Reference:

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