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Blood Testing

Medically reviewed by Last updated on Nov 28, 2022.

What is Blood Testing?

Harvard Health Publishing

Blood tests enable doctors to assess your health by analyzing cells, chemicals, proteins and other substances in your blood. Some tests are recommended regularly to see if blood levels of certain cells or chemicals fall within a normal range. Others are done to help diagnose health conditions, such as allergies, anemia, and diabetes.

There are two typical methods for taking a blood sample. One, called venipuncture, involves drawing a vial of blood from a vein, usually on the inner surface of your arm near your elbow. The other, called a finger stick, is done by pricking your finger with a sharp blade to obtain a small amount of blood from a capillary. The method used depends on how much blood is needed for the test you are having.

What It's Used For

Blood tests can be used to screen for a wide variety of diseases and disorders. They are also used to determine how well treatment is working for many different conditions. The kinds of problems for which doctors order blood tests include:

  • Allergies
  • Anemia
  • Chemical balances in the body
  • Blood clotting problems
  • Heart disease
  • Hormone levels
  • Infection
  • Inflammation
  • Kidney disease
  • Lead poisoning
  • Liver function problems
  • Disorders of the pancreas 
  • Thyroid disorders


No preparation is needed for a blood test. Tell your doctor about any medications you are taking, both prescription and over-the-counter. Some medications can elevate or lower levels of certain substances in the bloodstream, interfering with the accuracy of the blood test.

Many factors can affect test results, including your:

  • age
  • sex
  • race
  • current medications
  • medical history
  • overall health
  • diet
  • adherence to test-prep instructions
  • variations in lab techniques.

If you have become lightheaded or fainted when you have had a blood test in the past, tell the professional before they start drawing your blood. You should lay flat when your blood is drawn. Sit up for a few minutes after the blood test is finished. Be careful when you stand up.

How It's Done

An individual trained to draw blood will perform the procedure. If you have your blood drawn in a laboratory, the person drawing your blood will almost always be a blood-draw specialist called a phlebotomist. In the doctor’s office, the nurse, physician assistant or doctor will draw your blood.

The first step is to clean the area from which blood will be drawn, using an antiseptic solution. Blood is usually taken from a vein in the crook of the elbow or sometimes on the back of the hand. An elastic band is wrapped around the upper arm, cutting off blood flow to the arm and causing the veins to swell with blood. A sterile disposable needle is inserted into the vein and blood is drawn up through the needle into an airtight, attached plastic tube. The elastic arm band is then removed, followed by the needle. A bandage is placed over the puncture site and pressure is applied to stop the bleeding.

Your blood sample will be sent to a laboratory for analysis based on the type of tests being done. Sometimes, your doctor or a technician will perform the analysis in the office.


Results from the lab will be available in approximately 2–4 days for most types of blood tests. Some special tests may not be available for 7 – 10 days. Your doctor will receive a lab report and send the results to you within two weeks. Your doctor will contact you sooner if he or she wants you to take some action urgently.


There are almost no risks associated with having blood drawn. Depending on the accessibility of your veins, the professional drawing your blood may not be successful on the first try. Some people have small veins or veins that are not visible on the surface of the skin.

You may experience slight pain or a pinching or stinging sensation when the needle pierces the skin and enters the vein. Some people get a throbbing in the arm caused by the tight elastic band used to engorge the veins.

A little blood often seeps out of the vein where the needle was inserted, causing bruising under the skin. Bruising in the area may appear on the day following your blood test. There is no need to be concerned unless it is painful or the bruise continues to enlarge. Your body will reabsorb the blood that seeped out.

As with any procedure that breaks the skin, there is an extremely low risk of infection. Tell your doctor if the area becomes red, hot to the touch, painful, or if you develop a fever with these symptoms.

Some people feel faint or lightheaded, especially when a larger amount of blood is taken.

When to Call a Professional

Because harmful side effects are not expected, people typically need to call their doctors only to discuss laboratory results.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.