bronchial challenge test
A bronchial challenge test deliberately provokes the airways to see whether they overreact. The patient inhales rising doses of a triggering agent while spirometry is repeated after each step, watching for the airways to clench down — a controlled way to expose a twitchiness that may be invisible when the patient feels well.
The most common agent is methacholine, which directly stimulates airway muscle to contract; alternatives include mannitol or exercise, which provoke indirectly. The test is positive when FEV1 falls by a defined amount (typically 20 percent) at or below a threshold dose, indicating airway hyperresponsiveness.
Challenge testing is most useful for ruling asthma out: a negative test in someone with typical symptoms makes asthma unlikely, thanks to its high sensitivity. A positive result is supportive but not unique to asthma — it can also occur in COPD, allergic rhinitis or after a viral infection — so it is interpreted in context. The test is done under close supervision with a bronchodilator on hand to reverse any reaction.
A patient with a normal spirogram but a nagging cough undergoes a methacholine challenge; her FEV1 drops 20 percent at a low dose, confirming airway hyperresponsiveness and supporting cough-variant asthma.
A challenge test can unmask asthma hidden behind a normal spirogram.
Challenge testing is reserved for cases where spirometry and reversibility are inconclusive, because provoking the airways carries some risk and is contraindicated in severe baseline obstruction.