From the Guidelines
The gold standard for diagnosing asthma is spirometry with bronchodilator reversibility testing, as recommended by the most recent and highest quality study 1. This test measures lung function by having the patient forcefully exhale into a device that measures airflow. A diagnosis of asthma is supported when spirometry shows airflow limitation (FEV1/FVC ratio less than 0.7 or below the lower limit of normal) that improves significantly after administration of a bronchodilator medication. Significant reversibility is defined as an increase in FEV1 of at least 12% and 200 mL from baseline after inhaling a short-acting beta-agonist like albuterol (typically 400 mcg) 1. The European Respiratory Society clinical practice guidelines for the diagnosis of asthma in children aged 5-16 years also recommend spirometry, bronchodilator reversibility testing, and exhaled nitric oxide fraction as first-line diagnostic tests in children under investigation for asthma 1. If spirometry is normal but asthma is still suspected, additional tests may be necessary, such as bronchial challenge testing with methacholine, exercise, or eucapnic voluntary hyperventilation 1. Peak expiratory flow (PEF) monitoring over 2-4 weeks can also help establish variability in airflow limitation, but it is not recommended as the primary objective test on its own to diagnose asthma in children aged 5-16 years 1. The diagnosis should incorporate clinical history of respiratory symptoms like wheezing, shortness of breath, chest tightness, and cough that vary over time and in intensity, along with the objective measurements. This combination of symptom assessment and objective lung function testing is essential because asthma is characterized by variable airflow limitation and bronchial hyperresponsiveness. Some key points to consider when diagnosing asthma include:
- Spirometry is an essential objective measure to establish the diagnosis of asthma 1
- Bronchodilator reversibility testing can help confirm the diagnosis of asthma 1
- Exhaled nitric oxide fraction can be used as a diagnostic test for asthma 1
- Peak expiratory flow monitoring can help establish variability in airflow limitation, but it is not recommended as the primary objective test on its own to diagnose asthma in children aged 5-16 years 1
- Clinical history of respiratory symptoms is important in diagnosing asthma, but it should be used in combination with objective lung function testing 1
From the Research
Diagnosis of Asthma
The diagnosis of asthma can be challenging, and there is no single gold standard test. However, several physiological tests can help in diagnosing asthma, including:
- Spirometry: measures the forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) to assess airway obstruction 2, 3, 4
- Bronchodilator test: measures the reversibility of airway obstruction after administration of a short-acting bronchodilator 5, 4
- Bronchoprovocation testing: used to diagnose allergen- or exercise-induced asthma when spirometry results are normal 4
Spirometry in Asthma Diagnosis
Spirometry is a central test in diagnosing asthma, but it may not be sufficient to confirm or exclude the diagnosis 6. The FEV1/FVC ratio is used to assess airway obstruction, but there is no strong evidence to clearly define what constitutes a significant decrease in this ratio 4. A low FVC is defined as a value below the 5th percentile in adults or less than 80% of predicted in children and adolescents 4.
Small Airway Function Tests
Small airway dysfunction is highly prevalent in asthma, and tests of small airway function may more accurately diagnose early or mild asthma and assess the response to treatment than spirometry 6. However, more research is needed to further assess the utility of these tests in asthma diagnosis.
Bronchodilator Test
The bronchodilator test can be useful in assessing control in asthmatic patients and can predict future risk parameters 5. A cutoff point of ≥10% post-bronchodilator FEV1 reversibility was found to be related to non-control of asthma, with a sensitivity of 65.8% and specificity of 48.4% 5.