Diagnostic Tests Explained
The main tests doctors use to diagnose asthma and check how well your lungs are working.
Doctors use different tests to diagnose asthma, mainly lung function tests (also called pulmonary function tests) and allergy tests. Below are the most common and important ones, explained in brief.
Spirometry
A simple, quick test that shows how well your lungs are working. It is done with your medical team and needs very little preparation.
🗓️Before the test
- You may need to pause certain medicines, but only if your asthma team tells you to. Never stop on your own.
- Smoking and vaping affect your lungs and your results, so they are best avoided anyway.
- No need to fast, but avoid a large meal right before the test.
🫁During the test
- Sit upright in a chair.
- A small clip goes on your nose so you breathe only through your mouth.
- You put a mouthpiece in your mouth, connected to the spirometer.
- The doctor guides your breathing, usually normal breaths, then a deep breath in and fast exhales.
- The spirometer records how much air you can blow out and how fast.
- The test may be repeated after an inhaler that opens the airways, so the two results can be compared.
⭐Why it is important
- To diagnose asthma.
- To see how narrowed your airways are.
- To monitor how well treatment is working.
What spirometry measures
- How much air you can breathe in and out in total, at a steady rate. This is your vital capacity (VC).
- The most air you can blow out as fast as possible. This is your forced vital capacity (FVC).
- How much you can blow out in the first second, called FEV1.
- How FEV1 and FVC compare, shown as a ratio.
- The fastest speed you can blow air out, called peak expiratory flow (PEF), or peak flow.
Watch how spirometry is done and how to prepare
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Allergy Testing
Doctors use different tests to find out if allergies are causing or worsening asthma symptoms. These tests are always carried out by your medical team. The main types are skin tests, challenge tests, and blood tests.
💉Skin Test (Prick Test)
- The most common type of allergy test.
- A tiny amount of allergen is scratched onto the top layer of the skin (e.g., forearm, back); if you react, the spot may turn red, itchy, or slightly swollen.
- Many allergic reactions appear within the first 15 minutes.
🧪Challenge Test
- Exposure to a possible allergen in a controlled medical setting, always supervised by your medical team.
- You inhale, swallow, or have the allergen placed on your skin.
- You are then observed for signs of an allergic reaction.
🩸Blood Test
- A small blood sample is sent to a lab to check for signs of an allergic reaction.
- The lab measures immunoglobulin E (IgE), which is linked to allergic reactions.
- People with allergy-related asthma often have higher IgE levels.
Before allergy testing
- You may need to pause some medicines (like antihistamines), but only if your team tells you to. Never stop on your own.
- Your team may ask you to skip caffeine and hard exercise beforehand, as these can affect results.
- Smoking and vaping affect your lungs and your results, so they are best avoided anyway.
Video explaining allergy tests
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Bronchoprovocation Testing
These tests check how responsive your lungs are. They are always done in a supervised medical setting and help doctors see whether your lungs react normally or show signs of being overly sensitive (hyperresponsive), and how strong that reaction is.
Before the test
- You may need to pause some medicines (like antihistamines), but only if your team tells you to. Never stop on your own.
- Your team may ask you to skip caffeine and heavy exercise beforehand, as these can affect results.
- Smoking and vaping affect your lungs and your results, so they are best avoided anyway.
🏃Indirect Test
- Does not use allergens. Instead, it triggers the airways in other ways to check for narrowing.
- Common methods include exercise, mannitol, or hyperventilation.
- These work indirectly: they release mediators that make the airway muscles tighten.
- These mediators can trigger inflammation and cause the airways to narrow, a sign of hyperresponsiveness.
🔬Direct Test
- Uses substances like methacholine or histamine that act directly on airway muscle cells.
- Spirometry is done first to measure your baseline lung function.
- You inhale a small amount of the substance, then spirometry is repeated to measure how much your airways narrow (bronchoconstriction).
- When the test is over, you get medication to help reopen the airways and make breathing easier again.
Test results
If the spirometry reading drops by a certain percentage from the starting value, the test is considered positive. The exact percentage depends on which agent or method is used. This shows that the lungs are hyperresponsive, a common sign in asthma.
Exhaled Nitric Oxide
A quick test, done with your medical team, that measures inflammation in the airways. Inflammation can cause swelling and narrowing, which are common in asthma.
How the test works
- You take a deep breath in, then a slow exhale into a tube.
- The tube is connected to a computer that measures the amount of nitric oxide in your breath.
- The doctor or technician may ask you to repeat the breathing several times to ensure an accurate result.
- The test is short and usually takes no longer than 15 minutes.
Results
In asthma, nitric oxide levels are often higher than normal because inflamed airways produce more nitric oxide.
Peak Flow Meter
A small handheld tool you can use at home to track changes in your asthma over time. Your medical team will show you how to use it. Since different models of peak flow meters exist, it is important to learn the proper technique from your medical team to ensure reliable results.
📏What it is and does
- A small, handheld tool.
- Quickly measures how fast you can exhale after a deep breath.
- Often used to track changes in asthma and can help detect worsening symptoms early.
- Sometimes used in diagnosis, with readings taken twice a day for about two weeks.
🫁How to use it
- Take a deep breath in, then blow into the mouthpiece as hard and fast as possible.
- Sit or stand upright during the test.
- This is usually done 3 times, and you record the highest reading as your daily peak flow.
- Results vary based on factors such as age, height, and sex.
📝Sometimes used for daily monitoring
- Write down the peak flow result (phone or asthma journal).
- Note the use of any reliever inhaler or other medication.
- Record any asthma symptoms.
Worried about a symptom? Talk to your medical team. TeenHealthInsight is health education, not a substitute for medical advice, diagnosis, or treatment. Never change your medication without speaking to your asthma team first.