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Asthma is largely diagnosed from the history with a patient having the classical symptoms of asthma. However, tests such as lung function tests can also be useful to help diagnose asthma and to distinguish asthma from other lung diseases. These lung function tests are also used to keep an eye on patients who have asthma to monitor their progress and during severe attacks to help determine treatment. Other tests such as blood tests may also help to identify allergies that may be making asthma worse.

Lung Function Tests

  • These are simple breathing tests which measure the amount of air you can blow out from your lungs or how quickly you can blow air out from your lungs. This may be done by blowing into a machine called a spirometer or a small hand held device called a peak flow meter. Spirometry: Spirometry is a test to measure how much air you can blow out of your lungs and how quickly you can do so. It calculates two main things. Firstly, the amount of air you can blow out in one second (called forced expiratory volume in 1 second or FEV1) and secondly the total amount of air you can blow out of your lungs in one breath (called forced vital capacity or FVC). After this, a ratio of the two values is calculated, and this is called the FEV1/FVC ratio. As you would expect in asthma if your airways are narrowed and inflamed you will not be able to blow out all the air out from your lungs and it will take you longer to do so. An FEV1/FVC value of <70% indicates that you have narrowed airways, often called obstructive lung disease. This can occur in asthma but in other lung diseases too. However, an improvement in the value after treatment to open up the airways (e.g. an inhaler) is typical of asthma. Therefore, spirometry may be repeated after using an inhaler to see if there is any improvement. It is important to remember however that as asthma symptoms are variable a normal result does not mean that you do not have asthma. However a normal result usually excludes any other lung disease from causing your symptoms.
  • Peak flow testing: Unlike spirometry which measures the amount of air you can breathe out from your lungs, peak flow testing only looks at how hard and how quickly you can blow air out from your lungs. If your airways are narrowed, as in asthma, you will not be able to blow out air as hard or as quickly as would be expected for your age and size. As asthma symptoms vary and are usually worse in the morning and night, you may be asked to keep a diary of the peak flow results .If you are asthmatic it would be typical to see a variation in the results with the worse results being in the morning and the evening.
  • Reversability testing: This is when tests such as spirometry and other lung function tests are done before and after a treatment for asthma e.g. an inhaler or steroid tablets. If a patient is asthmatic it would be expected that the results would improve dramatically after treatment.
  • Trial of treatment: Similarly doctors or nurses may prescribe asthma medications to see if your symptoms respond to treatment. If they do it is a good indication that you have asthma but if they don’t taking them would not have done you any harm.

Additional tests and Imaging Studies

  • Pulse oximetry – This measures the amount oxygen in the blood by placing a small plastic device over the end of a finger. The amount of oxygen in the body would be expected to be normal in asthma patients unless they are having a severe attack which is stopping them from breathing properly. Therefore this test may be used for a person having a severe asthma attack in to see if enough oxygen is getting around the body. It is also helpful in ruling out diseases other than asthma, as if a person was not having an acute asthma attack but had low oxygen levels, it is likely that their symptoms may be caused by another lung disease. 
  • Chest x-ray - to look for signs of infections or other lung diseases that may mimic asthma An x-ray may also be used to look for signs of asthma as the lungs are often larger than normal due to loss of elastic recoil in asthmatic lungs. Elastic recoil is the ability of the lungs to rebound back to their normal position after been stretched by breathing in. As this is often lost in asthma the tissue of the lungs may not be packed as tightly and therefore the lungs may appear “hyper-inflated”on x-ray. This is usually more common in those with severe or long standing asthma symptoms. Therefore the chest x-ray is often normal in asthmatic patients and a normal x-ray does not mean you do not have asthma.

For other lung function tests see, John Hopkins Medicine: Pulmonary Function Laboratory.

Laboratory Tests

Laboratory testing is used to help rule out conditions that cause symptoms similar to asthma, to identify allergies, and to help find and assess complications of asthma. During severe asthma attacks, testing may be used to look for problems with oxygen levels, the body's acidity and signs of infection. Tests include:

  • Allergy testing – blood tests for the allergen(s) suspected to be causing symptoms, such as dust mites, mould, pets, and pollens. This is normally reserved for patients with severe asthma or asthma not responding to treatment.
  • Blood gases – Blood gases may be used in severe asthma attacks to check for any complications from the breathing difficulty.In this test an arterial blood sample is collected to look at acidity, oxygen, and carbon dioxide levelsThis is because the difficulty in breathing can alter the levels of oxygen and carbon dioxide in the blood, the latter of which can cause dangerous pH changes in the blood stream as carbon dioxide is acidic. Therefore too much carbon dioxide or too little carbon dioxide can alter the pH of the blood. If this happens a person may need to have specific treatment to counteract this as many of the body’s normal functions do not work if the pH in the blood changes.
  • FBC (Full Blood Count) – to look for signs of infection that may be complicating asthma or mimicking the signs of asthma. Infection would typically result in a raised white cell count as these cells fight infection. In addition, eosinophils a particular type of white blood cell can be increased with inflammation in some asthma patients and may help with diagnosis.
  • Renal Function –to ensure the kidneys are working as they may be affected during an acute asthma attack especially if there are pH changes in the blood.
  • Theophylline – therapeutic drug monitoring if a patient is taking this medication levels may be checked to ensure the concentration in the blood is adequate.

Other tests that may be occasionally requested:

For additional laboratory testing that is sometimes performed, see Lung Diseases.

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