Step-by-step diagnostic approach
Diagnosis involves identification of typical signs and symptoms, and confirmatory tests.
Recurrent episodes of dyspnoea, chest tightness, wheezing, or coughing typically occur.
The patient's medical history may help to identify allergen exposures that worsen the patient's asthma, for example, episodes may be exacerbated by exposure to irritants such as tobacco smoke or fumes from chemicals, such as bleach. Attacks may occur seasonally or upon exposure to cats in allergic patients. Exercise can also make the symptoms worse.
More severe asthmatics have night-time symptoms, waking them up from sleep. In severe exacerbations, patients are continuously short of breath, and may use accessory muscles of respiration.
The examination may be normal in patients with bronchial asthma. Examination of the nasal passages may reveal nasal polyposis or nasal congestion. Chest auscultation may reveal expiratory wheezes.
With more severe asthma, the wheezes may be audible without the use of a stethoscope. In patients with severe exacerbations, the lung examination may be silent.
For patients presenting for the first time, CXR, FBC, and differential are indicated in the initial workup, to exclude other pathologies. For patients presenting with an acute exacerbation, these may also be performed if complicating factors are suspected from history and exam.
Pulmonary function testing (PFT), including forced expiratory volume at 1 second (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio showing airflow obstruction can help diagnose asthma. The diagnosis of asthma is confirmed by demonstrating reversibility of airflow obstruction (usually defined as improvement in FEV1 by 12% and 200 mL) to short-acting bronchodilator.
Peak expiratory flow rate (PEFR) monitoring demonstrating diurnal variability (defined as [highest daily PEFR - lowest daily PEFR]/[highest daily PEFR] can help diagnose asthma. The diagnosis of asthma is supported if PEFR varies by at least 20% for 3 days in a week over several weeks or PEFR increases by at least 20% in response to asthma treatment. It is also useful as an alternative to spirometry in an acute setting and can be readily performed as an outpatient or in the home to monitor disease progress for monitoring progress. However, PEFR does not always reflect the level of obstruction of the lung as accurately as the FEV1 and FEV1/FVC ratio.
Allergy testing is indicated in patients with a possible allergic component to their disease, including skin prick testing and serum IgE levels. These tests can reliably determine sensitivity to inhalant allergens to which the patient is exposed. Allergy testing is recommended for patients with persistent asthma requiring regular preventer therapy. It may also be considered in patients with asthma and allergic rhinitis to clarify whether allergens are contributing to disease. If allergy is not present there is no need to consider anti-allergy measures.
Challenge tests to diagnose asthma are divided into: direct (using agents that directly constrict airway smooth muscle (i.e., histamine or methacholine) and indirect (methods or agents that activate mast cells to release mediators such as histamine and leukotrienes to constrict airway smooth muscle, e.g., exercise, eucapnic hyperventilation, inhaled hypertonic saline, mannitol or adenosine monophosphate) challenges. These direct and indirect challenges reflect the baseline fixed (airway remodelling) and episodic variable (inflammatory) components of airway hyper-responsiveness, respectively.  They may be considered if spirometry and PEFR do not show reversibility and variability.
Exhaled nitric oxide levels can be used to monitor a patient over time and in combination with sputum eosinophilia, has a high sensitivity and specificity; however, neither is a standard test in the UK or US at present.   Click to view diagnostic guideline references.