Blood Tests

A blood test is one of the most common tests healthcare providers use to monitor your overall health or help diagnose medical conditions. You may have a blood test as part of a routine physical examination or because you have certain symptoms.


What are blood tests?

Blood tests are common medical tests. You may have a blood test as part of a routine physical examination or because you have certain symptoms.

There are many different blood tests. Some tests focus on your blood cells and platelets. Some evaluate substances in your blood such as electrolytes, proteins and hormones. Others measure certain minerals in your blood.

Regardless of why you’re having a blood test, it’s important to remember that blood tests help healthcare providers diagnose health issues. But blood test results aren’t diagnoses. An abnormal blood test result may not mean you have a serious medical condition.

When would I need to have a blood test?

Your blood plays a big role in your overall health and contains a lot of information about what may be going on in your body. That’s one reason why blood tests are a common medical test. A healthcare provider may do a blood test because:

  • It’s time for your regular physical. During a checkup, your provider may order blood tests to check on your overall health. They may order a blood test that evaluates many parts of your blood, such as a complete blood count (CBC), basic metabolic panel (BMP) or a comprehensive metabolic panel.
  • Your provider recommends screening tests. Screening tests are done before you have any symptoms. They may recommend screening tests if you’re at risk of developing certain conditions, such as cancer. For example, if you’re at risk for developing coronary artery disease, your healthcare provider may order several blood tests to evaluate that risk.
  • You don’t feel well. If you have specific symptoms, your provider may order blood tests to determine what’s causing them. For example, if you have symptoms that may be signs you’re pregnant, your provider will do a pregnancy test. The blood test looks for a specific hormone your body only ever makes when you’re pregnant.
  • You have a medical condition that happens when certain genes change (mutate). Depending on your situation, your blood cells and platelets may show information about the specific changes. Understanding which genes changed may help your provider plan your treatment.
  • You’re receiving treatment for a medical condition. Your provider may use regular blood tests to see if treatment is working.
  • You may have inherited certain genetic mutations that cause medical conditions. Your provider may take blood samples for genetic analysis so you know if you’re at risk of developing a specific condition.

What do blood tests show?

In a broad sense, a blood test shows changes in your body. Blood test results don’t show a complete picture. Instead, they’re a kind of snapshot. After seeing that snapshot, your provider may do other blood tests to get a closer view. Here’s a glimpse of what your healthcare provider may see with blood tests:

  • The tests show if your blood is working as it should. For example, your red blood cells carry oxygen throughout your body. A blood test may show you have low red blood cell levels (anemia). If healthcare providers look at your cells under a microscope, they may see your red blood cells are larger than normal or shaped differently than normal red blood cells. These differences may be signs of blood disorders or blood cancers.
  • They show if you have normal levels of enzymes and electrolytes. Enzymes are proteins that help speed up the chemical reactions that build up and break down substances in your body. Electrolytes do several things, such as helping your body regulate chemical reactions and maintaining the balance between fluids inside and outside your cells.


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What are the most common blood tests?

There are many different blood tests. Some tests — such as complete blood count tests, basic metabolic panels, complete metabolic panels and electrolyte panels — check on several different elements in your blood at the same time. Other blood tests look for very specific elements in your blood.

What does a complete blood count (CBC) test show?

This is the most common blood test that includes several specialized tests. CBC tests:

What does a basic metabolic panel show?

A basic metabolic panel (BMP) measures several substances in your blood. Healthcare providers use BMPs to evaluate your overall health and screen for or monitor health issues. A BMP may include a:

  • Blood glucose test: Screens for diabetes.
  • Calcium blood test: Checks to make sure you have appropriate levels of calcium, a mineral that helps with many of your body’s functions.
  • Blood urea nitrogen (BUN) test: Measures the amount of urea, a waste product that passes through your kidneys. BUN tests show the amount of urea nitrogen in your kidneys.
  • Creatine kinase (CK) test: Screens for a waste product your muscles produce. High CK levels may be a sign of injured or damaged muscles.
  • Sodium levels.
  • CO2 blood test: Measures the amount of bicarbonate in your blood. This test detects carbon dioxide.
  • Serum potassium test: Measures potassium levels. Potassium supports your heart, nerve and muscle function and your metabolism.
  • Chloride blood test: Checks on chloride, an electrolyte that helps keep your body fluids and acids balanced.
  • Globulin blood test: Measures how much of this protein your liver produces.

What does a comprehensive metabolic panel show?

Comprehensive metabolic panels (CMP) include all the blood tests done as part of a basic metabolic panel. Additional blood tests include:

  • Albumin blood test: Albumin is a protein in your blood plasma. This test checks on kidney and liver function.
  • Alanine transaminase (ALT): Healthcare providers use this test to assess liver health.
  • Alkaline phosphatase (ALP): High levels of this enzyme may indicate liver disease or certain bone disorders.
  • Ammonia levels: Blood tests will show the amount of ammonia in your blood. High ammonia levels may be a sign of liver and kidney damage.
  • Bilirubin blood test: Bilirubin is a substance in your liver’s bile. Too much bile in your blood may be a sign of liver issues.
  • Aspartate transferase: Sometimes called AST, this test measures the amount of the enzyme aspartate transferase in your blood. Providers use this test to assess liver health.

What does an electrolyte panel show?

Electrolytes are minerals in your blood. Imbalance with electrolytes may be a sign of issues with your heart, kidneys or your lungs. An electrolyte panel includes all electrolyte tests in BMPs and CMPs. Additional electrolyte levels tested include magnesium and anion gap. Magnesium supports your brain, heart and muscles. Anion gap tests check the acid-base balance in your blood.

What blood tests do healthcare providers use to help diagnose specific conditions?

While the various blood and electrolyte panel tests provide a lot of information, there are disease-specific blood tests that help providers diagnose and treat specific conditions.


An allergy blood test checks your blood for increased levels of immunoglobulin E (IgE) antibodies. The test can help detect allergies to foods, pets, pollen or other irritating substances.

Autoimmune diseases

Autoimmune diseases happen when your immune system accidentally attacks your body instead of protecting it from intruders like viruses, parasites and cancer. Your provider may order the following blood tests:

  • Antinuclear antibody test: Antinuclear antibodies (ANA) are antibodies that mistakenly attack your immune system. Large amounts of ANA in your blood may be a sign of certain autoimmune disorders.
  • CE complement blood test: Providers may use this test to diagnose and monitor autoimmune disorders like rheumatoid arthritis or lupus.
  • C-reactive protein (CRP) test: Your liver makes and releases this protein. High C-reactive protein levels may be a sign of inflammatory conditions, including some autoimmune diseases.
  • Erythrocyte sedimentation rate (ESR): ESR tests help detect inflammation.
  • Peripheral blood smear (PBS): This is a technique healthcare providers use to examine your red and white blood cells and your platelets under a microscope.

Cancer/Noncancerous blood disorders

Healthcare providers may use several different tests to diagnose and treat cancer, blood cancer and noncancerous blood disorders.

Cancer blood tests

Blood tests for cancer fall into four basic categories — complete blood count, tumor markers, blood protein testing and circulating tumor tests. CBC, tumor markers and circulating tumor tests may help detect some solid tumors. Blood in your poop (stool) or pee (urine) may also be a sign of cancer.

Complete blood count (CBC)

A CBC measures red and white blood cell and platelet levels. Abnormally high or low blood cell or platelet levels may be a sign of some types of cancer.

Tumor markers

Tumor markers are substances made by cancerous cells or your body’s normal cells in response to cancer. Tumor marker blood tests include:

Circulating tumor test

The circulating tumor test is a relatively new blood test for cancer. This test looks for cancerous cells that have broken away from a tumor and into your bloodstream. Currently, it can help monitor certain types of cancer, such as breast, prostate and colorectal cancers. Scientists are still developing the technology.

Other blood tests

Healthcare providers may use the same tests to diagnose blood cancer or noncancerous blood disorders:

Some blood tests don’t involve providing blood samples, such as:

  • Fecal occult blood test (FOBT): FOBTs screen for colorectal cancer by looking for blood in your poop (stool).
  • Urinalysis: Healthcare providers may use this test to detect blood cells in your pee (urine).

Endocrine system disorders

Your endocrine system is made of organs called glands. Glands produce hormones. Healthcare providers may use blood tests to diagnosis conditions affecting parts of your endocrine system. Common blood tests include:

Heart disease

Some blood tests evaluate your risk of developing heart disease:

  • Cardiac blood tests: Your healthcare provider may order these tests if you’re at risk of having a heart attack or developing heart disease.
  • Arterial blood gas (ABG) test: This test measures oxygen levels and carbon dioxide levels, among other things. Healthcare providers may do this test to diagnose acute heart failure and cardiac arrest.


Specialized blood tests

Your healthcare provider may recommend specialized blood tests, including:

  • Ammonia levels: Blood tests will show the amount of ammonia in your blood. High ammonia levels may be a sign of liver and kidney damage.
  • Blood alcohol content (BAC): This test measures the amount of alcohol in your system.
  • Ferritin: You may have a ferritin test if your CBC tests show you don’t have enough iron.

When should I have a blood test?

That depends on your situation and your overall health. Most healthcare providers recommend annual physical examinations that may include a complete blood count test. In general, providers recommend tests based on what they know about you. For example, they may recommend regular blood glucose tests if you have overweight (a Body Mass Index or BMI over 25) or obesity (a BMI greater than 30).


Test Details

What should I do to prepare for my blood test?

That depends on the kind of test you’re having. For example, some blood tests require you to fast for several hours before the test. You may be asked not to drink any liquids apart from a few sips of water. Most blood tests don’t require fasting, but it’s a good idea to ask your healthcare provider what to avoid before your blood test. Other steps may include:

  • If you don’t need to fast before your blood test and you’re able to drink water, try to drink as much as you can before your test. Being well-hydrated may make it easier for healthcare providers to obtain blood.
  • Use moisturizer on your arms. It may make it easier for your provider to insert the needle and obtain blood.
  • Boost your blood pressure right before your blood test by doing some gentle exercise while waiting to be called in for your test.

What happens during blood tests?

Phlebotomists — healthcare providers with special training in drawing blood — do blood tests. During the blood test process:

  • You’ll sit in a chair with an armrest where you can stretch your arm out in front of you.
  • If you’re wearing a long-sleeve shirt, you’ll roll up your sleeve past your elbow.
  • The phlebotomist will wipe an antiseptic liquid in the bend of your arm (on the other side of your elbow) and put an elastic band around your arm (above your elbow).
  • They may ask you to make a fist to encourage blood flow.
  • They’ll insert a needle into your arm. This typically happens very quickly. You may feel the needle go into your skin. This is called venipuncture.
  • The blood flows into a tube that’s sealed and sent to a lab for analysis. The phlebotomist may need to take several samples of your blood, depending on the blood tests your healthcare provider ordered.
  • Once the phlebotomist has obtained enough blood, they’ll remove the stretchy band that’s strapped around your arm and removes the needle.
  • Then, they’ll put a bandage on the area where the needle went into your skin.

Are there different ways to do blood tests?

All blood tests involve obtaining blood samples. Venipuncture (from a vein) is the most common procedure. Other procedures are:

  • Finger stick: Your provider pricks one of your fingers with a needle to collect a tiny amount of blood. The blood sample is saved on a special strip of paper that’s sent to a lab for analysis.
  • Heel stick: All babies born in the U.S. have blood tests by pricking their heel with a needle to obtain a blood sample.
  • Arterial blood gas test: In this test, providers take blood from one of your arteries instead of a vein.

How much blood is taken during blood tests?

That depends on the kind of blood test. On average, a complete blood count (CBC) test may take as much as 30 milliliters (mL) of blood. It may sound like a lot of blood, particularly if you’re watching your blood flow into several sample tubes. But it’s not — the average adult has 4,500 to 5,700 milliliters of blood in their body.

Do blood tests hurt?

They can, depending on the kind of blood test you have. It’s important to remember that phlebotomists receive training on how to obtain blood samples quickly and without causing pain.

That said, tests that take blood from an artery tend to hurt more than tests that take blood from a vein. And with venipuncture, taking blood from a vein may hurt a bit if the phlebotomist has trouble inserting the needle into your vein. Let your phlebotomist know if you have any discomfort. They’ll try different ways to obtain samples of your blood.

I’m always anxious about having blood tests. What can I do to relax?

Many people feel anxious about blood tests. Some ways to cope include:

  • Understanding why you’re having specific blood tests.
  • Taking deep breaths as the needle goes into your arm.
  • Looking away so you don’t see the needle enter your arm.
  • Finding a way to distract your attention, such as silently counting to 10.

What happens after my blood test?

Your provider will put a bandage on the spot where the needle went in. Depending on the blood test, they may recommend you rest for a minute or so before standing up and leaving.

Results and Follow-Up

When will I know my test results?

That depends on the blood test and your provider’s preferences. Your provider likely will explain how you’ll receive results. Some blood test results are available within a few hours. Others, like genetic test results, typically take longer.

Some healthcare organizations offer online access to test results. But your provider may prefer to discuss your results in a telephone conversation or in person.

My healthcare provider wants to talk to me about my test results. Does that mean something’s wrong?

Not necessarily. If your tests were part of your routine medical checkup, your healthcare provider may want to review results with you. They may have recommendations about ways you can improve your health. If you’ve received treatment for a medical condition, your provider may want to discuss your test results in detail and put the results in context.

A note from Cleveland Clinic

It’s probably fair to say blood tests are last on most people’s list of fun things to do. But blood tests are an essential tool healthcare providers use to monitor your overall health or diagnose medical conditions. You may have a blood test as part of a routine physical examination or because you have certain symptoms.

Regardless of why you’re having a blood test, it’s important to remember that while blood tests help providers diagnose health issues, they aren’t diagnoses. An abnormal blood test result may not mean you have a serious medical condition. If your healthcare provider recommends blood tests, they’ll be glad to explain why they recommend the test and what the test may show.

Medically Reviewed

Last reviewed on 12/06/2022.

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