Asthma Diagnostic Testing

In the older child, lung function tests, such as peak expiratory flow rate (PEFR) tests, spirometry measurements, lung volume measurements (body plethysmography), bronchodilator response tests, and bronchial challenges, can confirm or exclude a diagnosis of asthma.

Most of these testing options require patient participation and cooperation, making them impractical for the younger pediatric patient outside of a research setting where infant pulmonary function testing and ausculatory bronchial challenges can be completed.

For the older child, PEFR testing is an easy and affordable modality to test lung function. The PEFR represents the maximum expiratory flow rate after the patient has inhaled to total lung capacity. The peak flow meter is not only helpful in monitoring day to day lung function and variability, but also can be a tool to help guide treatment intervention.

Additionally, the peak flow meter can be used in the office or home to document response to bronchodilator therapy. Spirometry is a critical tool in evaluating a patient for asthma and can be adequately completed by patients as young as 5 to 6 years.

The flow-volume loop can demonstrate obstruction in the asthmatic patient when compared with the loop seen in a patient without asthma. The forced expiratory volume in the first second (FEV1) has been the main measure of airflow obstruction.

According to previous guidelines, mild asthma is characterized by an FEV1 greater than 80%, moderate asthma by an FEV1 of 60% to 80%, and severe asthma by an FEV1 of less than 60%. Unfortunately, the FEV1 in pediatric patients likely underestimates the extent of the disease. The ratio of FEV1 to forced vital capacity is likely a better marker of obstruction in the pediatric patient.

According to the standard set by the American Thoracic Society, a diagnosis of asthma is supported when a reversal of airflow obstruction after bronchodilator therapy is 12% or more. Additionally, the degree of reversibility may help the clinician determine asthma severity and correlate with underlying airway inflammation.

Bronchial challenges, which can establish a diagnosis of asthma, include exercise and cold-air challenges as well as challenges with pharmacologic agents, such as methacholine, adenosine monophosphate, and histamine. These challenges typically involve a change in lung function (FEV1) of 20% or more after exposure to the trigger followed by reversal with bronchodilator medication.

A challenge using methacholine is considered a ‘‘gold standard’’ for confirming an asthma diagnosis with high sensitivity and specificity. Additionally, a negative methacholine challenge makes a diagnosis of asthma unlikely. Finally, formal evaluation of lung volumes, whether by helium dilution or body box plethysmography, can demonstrate hyperinflation and air trapping.

Radiographic studies such as chest radiographs and chest CT scans of pediatric patients with recurrent respiratory symptoms are not routinely obtained during well periods. However, when they are obtained, they may show hyperinflation, flattened diaphragms, and/or bronchial thickening.

The usefulness of radiographic studies during acute wheezing episodes has been debated. Such studies are usually reserved for patients with significant tachypnea, localized findings on auscultation, associated fever, or significant hypoxemia. Additionally, radiographic studies can be useful to exclude alternate diagnoses, such as the presence of a foreign body or an anatomical abnormality.

Confirmation of allergic sensitizations, either by percutaneous skin-prick testing or radioallergosorbent testing of serum, can lend support to the diagnosis of asthma as well as identify potential triggers of recurrent respiratory symptoms.

In the preschool child, sensitization is usually only seen to perennial allergens (pet dander, dust mite, mold, and cockroach) and food allergens, while the older child may be sensitized to seasonal and/or perennial allergens.

Other laboratory results that suggest atopy include an elevated eosinophil count or an elevated serum IgE level, although these testing options are much less specific for asthma. An elevated eosinophil count is considered a minor criterion for predicting persistence as defined by the API and modified API.

The differential diagnosis for the pediatric patient with recurrent respiratory symptoms is quite extensive, but should be narrowed by a thorough history and physical. Additionally, the clinician should focus on common conditions, such as asthma, allergies, and gastroesophageal reflux disease, before focusing on less likely causes.

In the preschool child with recurrent respiratory symptoms that have been unresponsive to conventional therapies, other causes should be considered, including tracheomalacia, airway compression due to congenital anomalies, tracheo-esophageal fistulas, and foreign body aspiration.

In the older patient, vocal cord dysfunction can mimic asthma symptoms, but does not respond to bronchodilator or steroid therapies. Diagnosis can be suspected by a history of inspiratory wheezing and truncation of the inspiratory loop on spirometry. The diagnosis is confirmed by visualization of paradoxical movement of the vocal cords during inspiration.

In all pediatric patients with difficult-to-control respiratory symptoms or with concomitant poor growth, cystic fibrosis should be considered. Cystic fibrosis typically presents with symptoms of failure to thrive and evidence of pancreatic insufficiency (malabsorption or diabetes) in combination with upper-and lower-respiratory symptoms.

Cystic fibrosis diagnosis can be confirmed by an abnormal sweat chloride and/or demonstration of a cystic fibrosis DNA mutation. A history of recurrent bacterial infections along with respiratory symptoms may prompt consideration for a primary immune deficiency or dysfunction.

Patients with primary immunodeficiency typically present with infections that are not limited to the respiratory tract or have other abnormal findings on physical examination. However, when considering this diagnosis, radiographic and immune laboratory studies should be obtained.