Laboratory Testing

You will have further tests to assess the extent of your asthma. Each doctor will look for different things, depending on what he suspects is wrong.
Pulmonary (Lung) Function Tests
Blood Tests
Skin Tests

Pulmonary (Lung) Function Tests
If your symptoms and history are indicative of asthma, the doctor will usually perform pulmonary function tests to confirm the diagnosis and determine the severity of the disease. A device known as a spirometer is used.
The tests done on a spirometer are to see:

how quickly you can exhale – this is called the peak expiratory flow rate (PEFR) and is what the plastic peak flow metre also measures

the total amount of air you can exhale – this is called the forced vital capacity (FVC)

the greatest amount of air you can exhale in one second – this is called the forced expiratory volume in 1 second or FEV1

how the flow rate decreases between 25 and 75% of the forced expiratory volume – this is called the maximum midexpiratory flow rate (MMEF)
Narrowing of the airways during an asthma attack will decrease the PEFR and FEV1. If these measurements indicate that some degree of airway obstruction is present, the doctor may administer a bronchodilator (a drug that opens the airways) and then take measurements again. A reversal of the obstruction confirms a diagnosis of asthma.
If there are no signs of airflow obstruction but asthma is still suspected, the doctor may perform a challenge test by administering a drug (histamine or methacholine), which usually increases airway resistance in people with asthma. The challenge test is not always accurate, particularly in patients whose only asthmatic symptom is persistent coughing. It may be quite useful in ruling out occupational asthma.

You can expect some side-effects from inhaling these chemicals. Common reactions to histamine are flushed face and neck, sweating, blood pressure drop, heart rate increase, and deeper breathing. Throat irritation and coughing are possible. Methacholine side-effects are similar though usually less intense. The side effects of a challenge with these two agents are generally short-lived, although there are some patients whose reactions have lasted for hours.

Allergen inhalation tests and occupational exposure tests (such as with chemicals in the welding industry) measure the response to specific triggers. These tests are potentially dangerous as they may produce a prolonged asthma response. They must be done under a doctor’s supervision.

Other methods for inducing airway resistance are to administer cold air and do exercise. Although this so-called “cold air hyperventilation” test is very accurate for ruling out asthma, it is not sensitive enough to accurately identify adults who actually are asthmatic.

Blood Tests
Blood tests may be taken to check on the overall health of a person and they may also test for activity of the immune system. A very low immune system means that you are more likely to catch chest infections which could be causing the asthma-like symptoms. A blood test which shows an active immune system with a high count of eosinophils can indicate allergic asthma. Eosinophils indicate allergic reactions and inflammation in the airways.

Skin Tests
Skin testing is done to primarily identify offending allergens and to suggest other allergens which may be potentially cause future symptoms:
There are usually three types of skin testing:

the scratch test – A series of short, shallow scratches are made in the skin. A different suspected allergen is rubbed into each scratch.
the skin-prick test – A drop of the suspected allergen is placed on your arm and then a needle pushes through the drop into the skin on your arm.

intradermal test – A solution containing the suspected allergen is injected directly into the skin.
In each case you will have a red, itchy welt if you are allergic to the substance within 15 – 20 minutes. Within an hour the reaction should have gone. The prick and scratch test lack precision, but there is a much smaller chance of inducing anaphylaxis shock if the asthmatic has a strong reaction to the allergen.

Tests to rule out other complications:
X-rays are used to detect emphysema, lung cancer and to show collapsed lungs or very obstructed airways.

Sweat tests are taken to rule out cystic fibrosis. Children with cystic fibrosis have more salt in their sweat than children without the disease.