Other Diagnostic Tests

• Spirometry, an objective measure of pulmonary function, can be used to assist in confirming the diagnosis of asthma.

• Useful pulmonary function tests include the forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC). The following values support a diagnosis of asthma:

• Decreased FEV1/FVC relative to predicted values demonstrates airway obstruction. The ratio may be normal between exacerbations.

• 12% or greater (at least 200 mL) improvement in FEV1, after an inhaled bron-chodilator demonstrates a reversible obstruction. A 2-to 3-week course of oral corticosteroids may be necessary to demonstrate reversibility in airway obstruction.

• A decrease in FEV1, of 15% or more after an exercise test is diagnostic for exercise-induced asthma.

• Assessment of diurnal variation in peak expiratory flow (PEF) may be useful in patients who have asthma symptoms and normal spirometry.

• When spirometry is equivocal, a 20% or greater decrease in FEV1 after the administration of methacholine is diagnostic for asthma. A negative bronchoprovocation test with methacholine may help rule out asthma.

• A positive allergen test may help guide nonpharmacologic therapy but is not diagnostic for asthma.

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