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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, like the kidneys, liver, and heart, are working

  • Diagnose diseases like cancer, HIV/AIDS, diabetes, anemia, and heart disease

  • Learn whether you have risk factors for heart disease

  • Check whether medicines you’re taking are working


Blood tests are very common. When you have routine checkups, your doctor often orders 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 whether this is necessary.

During a blood test, a small amount of blood is taken from your body. It’s usually drawn from a vein in your arm using a thin needle. A finger prick also may be used. The procedure is usually quick and easy, although it may cause some short-term discomfort. Most people don’t have serious reactions to having blood drawn.

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.

However, blood tests alone can’t be used to diagnose or treat many diseases or medical problems. Your doctor may consider other factors, such as your signs and symptoms, your medical history, and results from other tests and procedures, to confirm a diagnosis. Blood tests have few risks. Most complications are minor and go away shortly after the tests are done.

What To Expect Before Blood Tests

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).

What To Expect During Blood Tests

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.

What To Expect After Blood Tests

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.

What Are the Risks of Blood Tests?

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.

What Do Blood Tests Show?

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.

Some of the most common blood tests that doctors order are:

Complete Blood Count

The CBC is one of the most common types of blood test. It's often done as part of a routine checkup.

A CBC measures many different parts of your blood (as described below). This test can help detect blood diseases and disorders. These include anemia, infection, clotting problems, blood cancers, and immune system disorders.

Red Blood Cells

Red blood cells carry oxygen from your lungs to the rest of your body. Abnormal red blood cell levels may be a sign of anemia, dehydration (too little fluid in the body), bleeding, or another disorder.

White Blood Cells

White blood cells are part of your immune system, which fights infections and disease. Abnormal white blood cell levels may be a sign of infection, blood cancer, or an immune system disorder. A CBC measures the overall number of white blood cells in your blood. A differential count looks at the amounts of different types of white blood cells in your blood.


Platelets (PLATE-lets) are blood cells that help your blood clot. They stick together to seal cuts or breaks and stop bleeding. Abnormal platelet levels may be a sign of a bleeding disorder (not enough clotting) or a thrombotic disorder (too much clotting).


Hemoglobin is an iron-rich protein in red blood cells that carries oxygen. Abnormal hemoglobin levels may be a sign of anemia, sickle cell anemia, thalassemia (thal-a-SE-me-ah), or other blood disorders. If you have diabetes, excess glucose in your blood can attach to hemoglobin and raise the level of hemoglobin A1c.


Hematocrit  is a measure of how much space red blood cells take up in your blood. A high hematocrit level might mean you're dehydrated. A low hematocrit level might mean you have anemia. Abnormal hematocrit levels also may be a sign of a blood or bone marrow disorder.

Mean Corpuscular Volume

Mean corpuscular (kor-PUS-kyu-lar) volume (MCV) is a measure of the average size of your red blood cells. Abnormal MCV levels may be a sign of anemia or thalassemia.

Blood Chemistry Tests/Basic Metabolic Panel

The basic metabolic panel (BMP) is a group of tests that measure different chemicals in the blood. These tests usually are done on the fluid (plasma) part of blood. The tests can give doctors information about your muscles, including the heart; bones; and organs, such as the kidneys and liver.

The BMP includes blood glucose, calcium, electrolyte, and kidney tests. Some of these tests require you to fast (not eat any food) before the test, and others don't.

Blood Glucose

Glucose is a type of sugar that the body uses for energy. Abnormal glucose levels in your blood may be a sign of diabetes. For some blood glucose tests, you have to fast before your blood is drawn. Other blood glucose tests are done after a meal or at any time with no preparation.


Calcium is one of the most important minerals in the body. Abnormal calcium levels in the blood may be a sign of kidney problems, bone disease, thyroid disease, cancer, malnutrition, or another disorder.


Electrolytes are minerals that help maintain fluid levels and acid-base balance in the body. They include sodium, potassium, bicarbonate, and chloride.Abnormal electrolyte levels may be a sign of dehydration, kidney disease, liver disease, heart failure, high blood pressure, or other disorders.


Kidney tests measure levels of blood urea nitrogen (BUN) and creatinine. Both of these are waste products that the kidneys filter out of the body. Abnormal BUN and creatinine levels may be signs of a kidney disease or disorder.

Blood Enzyme Tests

Enzymes are chemicals that help control different reactions in your body. There are many blood enzyme tests. This section focuses on blood enzyme tests used to check for heart attack. These include creatine kinase (CK) and troponin tests.

Creatine Kinase

When muscle or heart cells are injured, CK (a blood product) leaks out, and its levels in your blood rise. There are different types of CK. CK-MB is released when the heart muscle is damaged.High CK levels can mean that you've had muscle damage in your body. High levels of CK-MB can mean that you've had a heart attack. Doctors order CK tests (such as CK-MB) when patients have chest pain or other heart attack signs and symptoms.


This is a muscle protein that helps your muscles contract. Blood levels of troponin rise when you have a heart attack. For this reason, doctors often order troponin tests along with CK-MB tests when patients have chest pain or other heart attack signs and symptoms.

Blood Tests To Assess Heart Disease Risk

Abnormal levels of certain chemicals in the blood may mean that you’re at higher risk for heart disease. Your doctor may want to test the levels of these chemicals to assess your risk and to suggest ways to reduce it.

Lipoprotein Panel

This test can help show how high your risk is for coronary heart disease. A lipoprotein panel looks at substances in your blood that carry cholesterol.

  • Total cholesterol.

  • LDL ("bad") cholesterol. This is the main source of cholesterol buildup and blockages in the arteries.

  • HDL ("good") cholesterol. This type of cholesterol helps decrease blockages in the arteries.

  • Triglycerides. These are another form of fat in your blood.

  • A lipoprotein panel measures the levels of HDL and LDL cholesterol and triglycerides in your blood. Abnormal cholesterol and triglyceride levels may be signs of increased risk for coronary heart disease.

  • Most people will need to fast for 9 to 12 hours before a lipoprotein panel.

High-Sensitivity C-Reactive Protein

This is a fairly new test for heart disease risk. It looks at blood levels of C-reactive protein (CRP). High CRP blood levels can be a sign of inflammation.

Doctors use standard CRP tests to check for inflammation and autoimmune diseases. Your doctor may order an hs-CRP test, along with other tests, to see whether you’re at increased risk for heart disease.

However, CRP tests aren’t routinely done, because it’s still unclear how useful they are for showing heart disease risk.


High levels of this chemical in the blood can mean that you’re at higher risk for heart attack or stroke. This isn’t a routine test for heart disease risk. But some doctors may use it, a long with other tests, if they think you’re at increased risk

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