When normal spirometry isn’t the end
Asthma is intermittent by nature. Between attacks the airways can look completely normal, so a single spirometry in clinic may be reassuring while the symptoms are real. The bronchial challenge test flips the logic: instead of waiting for an attack, it gently provokes the airways and watches whether they overreact — testing for airway hyperresponsiveness, the twitchy-airway trait that underlies asthma.
The classic version gives stepwise rising doses of an inhaled trigger — often methacholine — with spirometry after each step. Twitchy airways respond with bronchospasm and the FEV1 falls. The test reports the dose needed to drop FEV1 by 20% (the PC20 or PD20): a low dose means very reactive airways, a high dose means they are calm. Other forms use exercise, cold dry air, or mannitol to provoke the same response.
Safety, and putting the whole track together
Because the test deliberately narrows the airways, it is done carefully in a lab equipped to reverse the effect at once. A rescue short-acting beta-agonist reopens the airways within minutes, and people leave with their breathing back to baseline. It is avoided when starting FEV1 is already very low or when there are other safety concerns, since there would be too little reserve to spare.
A specialised use is suspected occupational asthma, where symptoms track with the workplace. Comparing peak flow or airway reactivity during work versus time away — sometimes with a specific challenge to the suspected agent — can link the lungs to the job. This is the pattern-thinking the whole track has been building toward: each test answers a specific question rather than declaring a verdict.
- Start with spirometry: is there obstruction (low FEV1/FVC) or possible restriction (low FVC, normal ratio)?
- If obstructed, test bronchodilator reversibility to lean toward asthma versus COPD.
- If restriction is suspected, confirm with lung volumes and read the DLCO for the gas-exchange surface.
- If spirometry is normal but asthma is still suspected, use the bronchial challenge test to confirm or exclude it.