Several blood tests are recommended for diagnosing asthma. These include:
Eosinophil count: Eosinophil is a kind of white blood cell. Normally, there are about four eosinophils for every hundred white blood cells. In case of asthma, this number may be higher.
IgE antibody levels: High IgE antibody levels in the blood indicate that allergies may be triggering the current condition of asthma. IgE antibody levels vary at different ages. Infants have very low levels of IgE. It gradually increases till about thirty years of age. After forty, IgE antibody levels tend to decrease. IgE antibody test has several disadvantages. It does not help detect specific allergens and cannot indicate if your current symptoms are due to allergens.
Arterial blood gases: This is not a routine test for asthma but is recommended in case of an emergency admission due to an asthmatic attack. It measures the amount of oxygen and carbon dioxide in the blood of the arteries. Normally the blood is collected from the arteries of the wrist. Asthma interferes with the normal exchange of gases in the lungs and can therefore cause an imbalance of the gases in the blood. Measurement of arterial blood gases can help detect severe asthma that requires emergency treatment.
Pulse oximetry: This test measures the amount of oxygen in the arteries. It is a very simple test and does not require pricking with a needle. A clip is attached to a fingertip and is also attached to a monitor that displays the amount of oxygen in the blood. Although pulse oximetry is not as accurate as arterial blood gas measurement, it is an easy method to detect if there is need for oxygen supplements. It can also measure oxygen levels continuously without interfering with other modes of treatment.