Laboratory Tests and Ancillaries
Pulmonary Function Testing
Asthma Pediatric_Diagnostics 1
Spirometry
Spirometry is the preferred diagnostic method in asthma. This measures airflow limitation and determines reversibility. All measurements should be done before and after administration of inhaled short-acting bronchodilator. Spirometry is generally valuable in children ≥5 years of age. Some children cannot correctly execute the required maneuvers until age 7 years.
Forced vital capacity (FVC) is a measure of the maximal volume of air exhaled from the point of maximal inhalation. The volume of air exhaled during the first second of this maneuver is called forced expiratory volume in 1 second (FEV1). FEV1/FVC appears to be a more sensitive measure of severity of impairment. An increase in FEV1 ≥12% after administration of a bronchodilator indicates reversible airflow limitation.
Peak Expiratory Flow (PEF) Measurements
Peak expiratory flow (PEF) measurements are important in the diagnosis and monitoring of asthma. If spirometry is unavailable, PEF can confirm the presence of variable expiratory airflow limitation. An average daily diurnal PEF variability of >13% indicates excessive variability in lung function.
Bronchodilator Response
Bronchodilator response determines reversibility of airflow limitation in response to treatment. These measures change in spirometry or PEF reading after administration of bronchodilator (eg 200-400 mcg Salbutamol or equivalent).
Other Tests
There are several lung function tests that do not rely on the patient's cooperation or the ability to perform the required maneuvers. Examples of these tests include impulse oscillometry, specific airway resistance, and measurements of residual volume. These may be valuable in children 2-5 years of age. These are not evaluated as diagnostic tests for asthma. These are commonly used in research studies and specialist centers.
Biomarkers of Type 2 Inflammation
The use of biomarkers is considered if spirometry or PEF are unavailable, or if the patient is tested as negative. In patients with typical symptoms of asthma, elevated results can support the diagnosis of asthma, but lower levels do not rule out asthma.
Blood Eosinophil Count
Results greater than or equal to the upper limit of normal (ULN) for a given population supports the diagnosis, but lower levels in patients with typical symptoms of asthma do not rule out asthma. In patients with severe asthma, ≥150/µL suggests the presence of type 2 inflammation while ≥300/µL is the usual threshold for eligibility to receive type 2-targeted biological therapy.
Fractional Concentration of Exhaled Nitric Oxide (FeNO) Measurement
Fractional exhaled Nitric Oxide (FeNO) measurement is associated with increases in eosinophilic levels. An increase in FeNO >4 weeks after an upper respiratory tract infection (URTI) in preschool children with recurrent symptoms may help in the diagnosis and in predicting intranasal corticosteroid use by school-age children. Further studies are needed to prove the use of FeNO measurement as a guide for adjusting asthma treatment. FeNO level is considered high in the following adolescents:
- Inhaled corticosteroid treatment-naive: >50 ppb
- With a history of medium-dose inhaled corticosteroid: ≥25 ppb
- With a history of high-dose inhaled corticosteroid: ≥20 ppb
Allergy Tests
Examples of allergy tests include serum total immunoglobulin (IgE) and allergen-specific IgE. Atopy, which is the most likely cause of respiratory symptoms in allergy-induced asthmatic patients, may be tested by skin prick testing or by measurement of specific IgE (sIgE) in serum. The presence of food-specific IgE, and/or atopic dermatitis increases the risk of sensitization to inhaled allergens and may be predictive of developing asthma. Specific IgE measurement may be preferred for patients who are uncooperative, with widespread skin disease, or if history suggests anaphylaxis risk.
In vivo Test
A skin prick test is an example of an in vivo test.
In vitro Test
An IgE panel test or radioallergosorbent test (RAST) may be done if an in vivo test cannot be performed (eg cases of severe dermatitis). This is performed if current antihistamine therapy cannot be discontinued or if there is a known possibility of a life-threatening reaction to food or inhalant.
Asthma Diagnosis in Children ≤5 Years
Objective measurements of lung function may be difficult in this age group. Atopy is a major risk factor for subsequent development of asthma in this age group, and it also predicts severity once asthma develops. To help establish a diagnosis of asthma, a diagnostic trial of asthma medications, in addition to a thorough medical history and physical exam, may be useful. Consider asthma if >3 episodes of reversible bronchial obstruction have been noted within the last 6 months. Patients may have virus-induced asthma, which is common in this age group.
Imaging
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Chest radiography may be used to rule out other pathologies and structural abnormalities.