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What Are Blood Tests?

Blood tests help doctors check for certain diseases and conditions. They also help check the function of your organs and show how well treatments are working.

Specifically, blood tests can help doctors:

  • Evaluate how well organs—such as the kidneys, liver, thyroid, and heart—are working
  • Diagnose diseases and conditions such as cancer, HIV/AIDS, diabetes, anemia (uh-NEE-me-eh), and coronary heart disease
  • Find out whether you have risk factors for heart disease
  • Check whether medicines you're taking are working
  • Assess how well your blood is clotting

Overview

Blood tests are very common. When you have routine checkups, your doctor may recommend blood tests to see how your body is working.

Many blood tests don't require any special preparations. For some, you may need to fast (not eat any food) for 8 to 12 hours before the test. Your doctor will let you know how to prepare for blood tests.

During a blood test, a small sample of blood is taken from your body. It's usually drawn from a vein in your arm using a needle. A finger prick also might be used.

The procedure usually is quick and easy, although it may cause some short-term discomfort. Most people don't have serious reactions to having blood drawn.

Laboratory (lab) workers draw the blood and analyze it. They use either whole blood to count blood cells, or they separate the blood cells from the fluid that contains them. This fluid is called plasma or serum.

The fluid is used to measure different substances in the blood. The results can help detect health problems in early stages, when treatments or lifestyle changes may work best.

Doctors can't diagnose many diseases and medical problems with blood tests alone. Your doctor may consider other factors to confirm a diagnosis. These factors can include your signs and symptoms, your medical history, your vital signs (blood pressure, breathing, pulse, and temperature), and results from other tests and procedures.

Outlook

Blood tests have few risks. Most complications are minor and go away shortly after the tests are done.

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