airway inammation. BPT becomes particularly important when other diagnostic measures, such as spirometry or assessing bronchodilator response, yield inconclu­sive results. It’s especially useful in identifying triggers related to exposure to envi­ronmental or occupational allergens. Moreover, BPT may play a signicant role in assessing the efcacy of current asthma therapy. To evaluate airway responsiveness in patients BPT could be performed directly or indirectly methods. Direct BPT involves substances like methacholine or histamine that directly act on specic receptors on the bronchial smooth muscle cells. When these substances come into contact with these receptors, they trigger the contraction of the smooth muscles, leading to airway narrowing. Methacholine is a synthetic derivative of the neu­rotransmitter acetylcholine. Since histamine is linked to greater systemic adverse effects, such as headache, ushing, and hoarseness, methacholine is more fre­quently utilized. Indirect BPT, on the other hand, involves substances or stimuli like hypertonic saline, mannitol, adenosine monophosphate, cold air, or exercise. These agents trigger the release of various mediators within the airways, causing inammation and, subsequently, airway constriction. Indirect methods are bene­cial in assessing the overall response of the airways, involving not just smooth muscle cells but also various mediators, receptors, and cells within the airway walls. Thus, indirect BPT is considered to be more specic for asthma. Methacholine is available as dry crystalline powder (Provocoline®) has 100mg vial. Methacholine solution is prepared with normal saline (0.9% sodium chloride) as the diluent of choice. The ve-breath method and a dosage regimen employing methacholine concentrations of 0.0625, 0.25, 1, 4 and 16mg/mL have been recommended by the American Thoracic Society [59]. Baseline spirometry is performed (FEV1 (forced expiratory ow in 1s) >70%) then 2mL of the rst diluted solution is given with nebulizer which has a dosimeter to patient. The patient inhales ve time from the nebulizer. The FEV1 at about 30 and 90s after the fth inhalation is measured. If the FEV1 decline is less than 20%, challenge test is continued subsequent concen­tration. If the FEV1 falls more than 20% from baseline or the highest concentration has been given methacholine challenge test is nished and considered to be posi­tive. If the FEV1 does not decrease by at least 20% after the maximum dose (i.e., 16 mg/mL), the PC20 (provocative concentration causing a 20% fall in FEV1) should be documented as “>16mg/mL”. If PC20 is greater than 16mg/mL, it is highly probable that the patient does not currently have asthma. However metha­choline challenge testing has a higher negative predictive power than positive pre­dictive power, it is more benecial in eliminating an asthma diagnosis than in establishing one [59, 60].