Blood tests may be ordered during a routine check up to monitor a person's health, to help diagnose the cause of a particular medical problem, or to monitor a person's response to treatment.
Specifically, blood tests can help doctors:
Some of the most common blood tests that doctors order are:
Many blood tests don't require any special preparation and take only a few minutes. Other blood tests require fasting (not eating any food) anywhere from 8 to 12 hours before the test.
Many blood tests don’t require any special preparation and take only a few minutes.
Other blood tests require fasting (not eating any food) anywhere from 8 to 12 hours before the test. Your doctor will let you know whether you need to fast for your blood test(s).
Blood usually is drawn from a vein in your arm or other part of your body using a thin needle. It also can be drawn using a finger prick.
The person who draws your blood might tie a band around the upper part of your arm or ask you to make a fist. These things can make the veins in your arm stick out more. This makes it easier to insert the needle.
The needle that goes into your vein is attached to a small test tube. The person who draws your blood removes the tube when it's full, and the tube seals on its own. The needle is then removed from your vein. If you're getting a few different blood tests, more than one test tube may be attached to the needle before it’s withdrawn.
Some people get nervous about blood tests because they’re afraid of the needle. Others may not want to see blood leaving their bodies.
If you’re nervous or scared, it can help to look away or talk to someone to distract yourself. You might feel a slight sting when the needle goes in or comes out.
Drawing blood usually takes less than 3 minutes.
Once the needle is withdrawn, you’ll be asked to apply gentle pressure with a piece of gauze or bandage to the place where the needle went in. This helps stop bleeding. It also helps prevent swelling and bruising.
After a minute or two, you can remove the pressure. You may want to keep a bandage on for a few hours.
Usually, you don’t need to do anything else after a blood test, except wait for the results. They can take anywhere from a few minutes to a few weeks to come back. Your doctor should get the results. It’s important that you follow up with your doctor to discuss your test results.
The main risks with blood tests are discomfort or bruising at the site where the needle goes in. These complications usually are minor and go away shortly after the tests are done.
Blood tests show whether the levels of different substances in your blood fall within a normal range.
For many blood substances, the normal range is the range of levels seen in 95 percent of healthy people in a particular group. For many tests, normal ranges are different depending on your age, gender, race, and other factors.
Many factors can cause your blood test levels to fall outside the normal range. Abnormal levels may be a sign of a disorder or disease. Other factors—such as diet, menstrual cycle, how much physical activity you do, how much alcohol you drink, and the medicines you take (both prescription and over-the-counter)—also can cause abnormal levels.
Your doctor should discuss any unusual or abnormal blood tests results with you. These results may or may not suggest a health problem.
Many diseases or medical problems can’t be diagnosed with blood tests alone. However, they can help you and your doctor learn more about your health. Blood tests also can help find potential problems early, when treatments or lifestyle changes may work best.
Reference: The National Heart Lung and Blood Institute
Last updated April 27, 2017