What is the best test to rule out asthma?

From the Guidelines

Methacholine challenge testing is the best test to rule out asthma.

Key Points

• Methacholine challenge testing has a high negative predictive power, making it more useful in excluding a diagnosis of asthma than in establishing one 1.
• A negative methacholine challenge result is commonly defined as non-response to the highest concentration (a PC20 > 8–25 mg/ml), and this can rule out asthma with reasonable certainty in patients who have had asthma symptoms during the previous 2 weeks 1.
• The optimal diagnostic value of methacholine challenge testing occurs when the pretest probability of asthma is 30–70% 1.
• Some factors to consider before accepting a negative test as ruling out asthma include airway responsiveness suppression due to intensive antiinflammatory medications, the season for aeroallergen exposure, and occupational asthma due to a single antigen or chemical sensitizer 1.
• Other tests, such as spirometry and exercise challenge testing, may also be useful in diagnosing asthma, but methacholine challenge testing is the most effective in ruling out the condition 1.
• The European Respiratory Society clinical practice guidelines recommend methacholine bronchial challenge tests as a diagnostic tool for asthma in children aged 5-16 years, with a focus on the PC20 or the delivered dose of methacholine resulting in a 20% fall in FEV1 (provocative dose (PD20)) 1.

Important Considerations

• Methacholine challenge testing should only be performed by a trained individual, and patients should be informed of the risks and benefits of the test 1.
• The test results should be interpreted in the context of the patient's medical history, physical examination, and other diagnostic tests, such as spirometry and peak flow meter measurements 1.

From the Research

Diagnostic Tests for Asthma

The diagnosis of asthma is based on the presence of symptoms and objective tests of lung function. Several diagnostic tests can be used to rule out asthma, including:

• Spirometry with bronchodilator reversibility testing 2
• Methacholine challenge test 3, 4
• Peak flow measurement 2
• Fractional exhaled nitric oxide measurement 2, 5
• Skin prick tests (SPT) with common aeroallergens 3
• Measurements of serum total IgE and blood eosinophil counts 3

Characteristics of Diagnostic Tests

The characteristics of these diagnostic tests vary:

• The methacholine challenge test has a sensitivity of 96.5% and specificity of 78.4% 3, and a sensitivity of 77% and specificity of 96% in asthmatic patients receiving controller medications 4
• Spirometry with bronchodilator reversibility testing is the mainstay of asthma diagnostic testing for children and adults 2
• The reversibility test has the highest correlation with a positive result and asthma 3
• The methacholine challenge test has the highest correlation with a negative result 3

Factors Affecting Diagnostic Test Results

Several factors can affect the results of diagnostic tests for asthma, including:

• Racial characteristics: the sensitivity of the methacholine challenge test is lower in white than in African American participants 4
• Atopic characteristics: the presence of atopy is associated with a positive test result 4
• Controller medications: the use of controller medications can affect the results of diagnostic tests 4
• Smoking status: adult smokers with intermittent respiratory symptoms may have either asthma or chronic obstructive pulmonary disease (COPD) 5