Provocative tests.
Inhalation of an M-cholinostimulator – methacholine – causes bronchospasm in most patients with bronchial asthma , a history of bronchial asthma, and also in patients with allergic diseases of the upper respiratory tract . Bronchospasm with inhalation of methacholine is observed in less than 10% of healthy individuals. It may be due to a recent upper respiratory tract infection (within the last 6 weeks), recent flu vaccination , exposure to allergens and air pollutants. False negative reactions to methacholine are noted during a provocative test during treatment with bronchodilators .
FEV1 is determined before the test and after 2-3 minutes. after inhalation of saline and various doses of mitacholine. The decrease in FEV1 after inhalation of saline should not exceed 10%.
A test with methacholine is considered positive if FEV1 is reduced by more than 20% compared with the control (inhalation of saline). Inhalation of methacholine – 1-5 deep breaths – begins with a concentration of 0.075 mg / ml, doubling it every 5 minutes. until a positive reaction occurs. In the absence of a reaction to the maximum concentration of methacholine – 25 mg / ml – the sample is considered negative. In bronchial asthma, bronchospasm usually occurs when metacholine is inhaled at a concentration of 16 mg / ml.
Often, histamine is used instead of methacholine to conduct a provocative test . The test is carried out in the same way as with methacholine. The initial concentration of histamine is 0.03 mg / ml, the maximum is 10 mg / ml.
Other provocative tests include exercise, inhalation of dry cold air, inhalation of a hypotonic sodium chloride solution.
If, based on the anamnesis data, it is not possible to identify a relationship between asthma attacks and the action of allergens detected by skin tests and in determining specific IgE, provocative tests are performed with these allergens. It should be noted that these tests are technically complex, require time and can provoke both an immediate and delayed attack of bronchial asthma.
Provocative tests with methacholine, histamine and allergens should be performed only by a doctor and only in the interictal period, when FEV1 is at least 75% of the normal value.
Provocative tests with allergens are carried out in the same way as a test with methacholine.
A provocative test begins with 5 breaths of the allergen in a dilution of 1: 1,000,000, then the concentration is successively increased: 1: 500000, 1: 100000, 1: 50000, 1: 10000, 1: 5000, 1: 1000, 1: 500 and 1: 100. The interval between allergen inhalations should be less than 10 minutes. The test is considered positive if inhalation causes a decrease in FEV1 by at least 20% (relative to the initial level), which persists for 10-20 minutes. If there is no reaction during inhalation of an allergen at a dilution of 1: 100, the sample is considered negative.
For one study, you can conduct a test with only one allergen. If the patient’s condition allows, the day before the study, bronchodilators , cromolyn and H1-blockers are canceled . Nedocromil and astemizole should be withdrawn sooner. The most pronounced inhibition of the reaction during provocative tests is caused by beta-adrenostimulants . Corticosteroids can inhibit a delayed response to an allergen.
If there is no immediate reaction to an allergen, the patient is warned that a delayed attack of bronchial asthma may occur 4-12 hours after a provocative test. With a pronounced immediate reaction, the patient is observed for at least 12 hours.