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What You Need to Know About Blood Testing
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

What You Need to Know About Blood Testing

Blood testing, or blood work, is a common medical procedure used to measure substances in your blood to help diagnose conditions, monitor chronic diseases, and check how well your organs are functioning.

What are blood tests?

Blood tests are used to measure or look at cells, chemicals, proteins, or other substances in your blood. Blood work is often included as part of a regular checkup. Blood tests are also used to:

  • Help diagnose certain diseases and conditions.
  • Monitor a chronic (long-lasting) disease or condition, such as diabetes or high cholesterol.
  • Find out if treatment for a disease is working.
  • Check how well your organs are working, including your liver, kidneys, heart, and thyroid.
  • Help diagnose bleeding or clotting disorders.
  • Find out if your immune system is having trouble fighting infections.

What are the different types of blood tests?

There are many different types of blood tests. Common ones include:

  • Complete blood count (CBC): This test measures different parts of your blood, including red and white blood cells, platelets, and hemoglobin. A CBC is often included as part of a regular checkup.
  • Basic metabolic panel (BMP): This group of tests measures certain chemicals in your blood, including glucose, calcium, and electrolytes.
  • Blood enzyme tests: An enzyme is a protein that speeds up certain chemical reactions in your body. Common types include troponin and creatine kinase tests, which are used to find out if you've had a heart attack and/or if your heart muscle is damaged.
  • Blood tests to check for heart disease: These include cholesterol tests and a triglyceride test.
  • Blood clotting tests: Also known as a coagulation panel, these tests can show if you have a disorder that causes too much bleeding or too much clotting.

What happens during a blood test?

A health care provider will need to take a sample of your blood, known as a blood draw. When taken from a vein, it is called venipuncture.

During venipuncture, a lab professional, known as a phlebotomist, will take a blood sample from a vein in your arm using a small needle. A small amount of blood is collected into a test tube or vial. You may feel a little sting when the needle goes in or out, but the process usually takes less than five minutes.

Other ways to perform a blood test include:

  • A finger prick test: This involves pricking your fingertip to obtain a small amount of blood. It is often used for at-home test kits and rapid tests.
  • A heel stick test: Most often done on newborns, a provider cleans the baby's heel with alcohol, pokes it with a small needle, and collects a few drops of blood.
  • Arterial blood test: This test measures oxygen levels. Because blood from arteries has higher oxygen levels than blood from a vein, the sample is taken from an artery in the wrist, arm, or groin. It is often drawn by a specially trained provider and may cause a sharp pain.

Will I need to do anything to prepare for the test?

You don't need any special preparations for most blood tests. For some, you may need to fast (not eat or drink) for several hours. You may also need to stop taking certain medicines, so tell your provider about everything you take. Do not stop taking any medicines unless your provider tells you to.

Are there any risks to the test?

There is very little risk to having a finger prick test or venipuncture. During venipuncture, you may have slight pain or bruising at the site, but symptoms usually go away quickly. Risks to a baby during a heel stick are minimal, though a small bruise may form. Collecting blood from an artery is more painful than from a vein, and you should avoid lifting heavy objects for 24 hours after the test.

Is there anything else I should know about blood testing?

Blood testing provides important health information, but it does not always give enough data for a diagnosis. Your provider may consider your symptoms, medical history, and the results of other tests to understand your blood test results.

Frequently Asked Questions

Yes, many medications, vitamins, and supplements can interfere with blood test accuracy. For example, biotin (Vitamin B7) supplements can significantly skew thyroid and cardiac marker results, and certain antibiotics or steroids can alter white blood cell counts or liver enzymes. Always provide your healthcare provider with a complete list of all prescription drugs, over-the-counter medications, and supplements you are taking before your blood draw so they can interpret the results correctly or advise you to pause certain supplements temporarily.
Routine tests like a Complete Blood Count (CBC) or Basic Metabolic Panel (BMP) are often processed quickly, with results available within 24 to 48 hours, and sometimes immediately if done in-office. More complex tests, such as specific hormone panels, autoimmune markers, or genetic tests, can take several days to a few weeks. If you are waiting for results related to an acute condition, such as a suspected heart attack (troponin test), results are usually prioritized and available within a few hours.
No, fasting is not required for all blood tests. You typically need to fast for 8 to 12 hours before a lipid panel (cholesterol test) or a fasting glucose test to ensure accurate measurements of fats and sugar in your blood. However, tests like a Complete Blood Count (CBC) or a Basic Metabolic Panel (BMP) usually do not require fasting. Always follow the specific instructions provided by your healthcare provider or the lab, as eating or drinking (other than water) can alter the results of certain tests.
single slightly abnormal result is often not cause for immediate alarm, as "normal ranges" are statistical averages and can vary between different laboratories. Your provider will look at the results in the context of your overall health, symptoms, and previous test history. They may recommend repeating the test in a few weeks to see if the level stabilizes or changes. It is important not to self-diagnose based on a single number; instead, schedule a follow-up appointment to discuss the clinical significance of the result with your doctor.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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