Patients need to take very few preparatory steps prior to most blood testing. The patient may need to reduce or stop certain medications at some point prior to the test. Additionally, food intake as well as exercise may be temporarily restricted or suspended. Alcohol and caffeine should be avoided prior to a blood test.

On the day of the test, the procedure will be explained and the patient will have the opportunity to ask questions. The medical professional will also ask questions about the patient's medical history to determine if the patient is taking any medications that will interfere with the test's accuracy or has any history of clotting problems.

During the test, a blood sample is drawn. If the physician only needs a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of a baby) 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 drug use. If blood must be taken from an artery instead of a vein (e.g., during an arterial blood gas study), it is usually drawn from a small artery located on the inside or the top side of the wrist. 

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 then inserted. 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 bandage is placed over the site. Following the withdrawal of blood, patients may resume medications and food intake according to their physician's orders. 

Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing.

Results are usually returned within 24 hours, depending on which tests need to be performed. Based on the test results, additional blood tests may be ordered.

Test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active healthy people.

When someone has a disease or health problem, his or her blood test results may be higher or lower than normal ("outside of the normal range"). When a physician sees that a blood test is outside of the normal range, he or she may order a repeat test to verify the results or additional tests to determine the underlying causes behind the abnormality.