According to Asthma + Lung UK, severe asthma diagnosis is often a complex process as it involves numerous considerations before your doctor can confirm you have the condition. Normally, it begins with an appointment with your doctor to determine if you've been working with the correct prescription. If yes, and your condition is still advancing, you may have severe asthma. Your doctor may advise you to speak with a severe asthma expert or clinical psychologist, allergist, specialist asthma nurse, pharmacist, physiotherapist, or physiologist depending on your medical history and dynamics of your health issue. There, the specialist will want to know how the problem interrupts your normal life and your loved ones, too. Afterward, you might do some tests. 

Another article on Asthma + Lung UK says that some of the tests to expect include spirometry test (a test to see the amount of air you can exhale in a given period), gas transfer test (which determine oxygen levels going to your blood from the lungs), lung volume test (for determining your lung size), and bronchial challenge test (for challenging your airways on purpose). Your specialist may also recommend the Asthma Control Questionnaire, the RCP 3 Questions, or the Asthma Control Test to know about your symptoms and how well you're regulating them. Moreover, the diagnosis may also include blood tests, CT (computerized tomography) scans, chest x-rays, sputum induction tests, nasal endoscopy, and fractional exhaled nitric oxide (FeNO) tests.

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