During the test, a blood sample is drawn. If the physician needs only a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of an infant) can provide enough blood for testing. However, most allergy and asthma-related blood tests require a larger blood sample. For most blood tests, the sample is drawn from a vein in a process called venipuncture. Blood samples may be more difficult to obtain from infants, overweight people whose veins are difficult to find, elderly people whose veins tend to roll away or patients with scarred or collapsed veins due to multiple transfusions or intravenous drug use.

During a venipuncture, a health provider wraps an elastic band around the patient’s upper arm to stop blood flow through the upper veins. This keeps the lower veins full of blood and less likely to collapse, making them ideal sites for drawing a sample.

Once a precise site is chosen, the spot is swabbed with alcohol. The needle is inserted into the vein. In some cases the needle may have to be removed and inserted again to ensure it is properly placed, or if the health provider cannot obtain enough blood from the original site. Patients may feel a brief sting as the needle is inserted, but discomfort is usually minor.

Once the needle is in place, a collection tube is attached and blood flows into it. Sometimes, more than one tube will be collected. Once the required amount of blood has been obtained, the rubber band is removed. The needle also is removed from the vein and a cotton ball or gauze pad is applied to the puncture site. Direct pressure is applied to the puncture spot for several minutes to help the blood clot, and a sterile bandage is placed over the site.

The blood sample is placed in tubes, vials or is placed on specially treated paper and sent to a lab. There, the sample will be analyzed to measure the amount of immunoglobulin E (IgE) in the blood and to identify specific allergens.

There are very few risks or side effects reported with blood tests. Occasionally, bruising is reported at the injection site. This can be minimized by keeping direct pressure on the spot for several minutes after the needle has been removed.

In rare situations, the vein used to obtain the sample may become inflamed (phlebitis). Applying warm compresses to the spot several times daily can help reduce the swelling.

Patients with bleeding disorders or who are taking anticoagulants (including aspirin) may have problems with continued bleeding. Patients who experience these side effects should consult with their physician.