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
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.
Blood tests have few risks. Most complications are minor and go away shortly after the tests are done.
Some of the most common blood tests are:
- A complete blood count (CBC)
- Blood chemistry tests
- Blood enzyme tests
- Blood tests to assess heart disease risk
Complete Blood Count
The CBC is one of the most common blood tests. It's often done as part of a routine checkup.
The CBC can help detect blood diseases and disorders, such as anemia, infections, clotting problems, blood cancers, and immune system disorders. This test measures many different parts of your blood, as discussed in the following paragraphs.
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 diseases. 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 CBC with differential looks at the amounts of different types of white blood cells in your blood.
Platelets (PLATE-lets) are blood cell fragments that help your blood clot. They stick together to seal cuts or breaks on blood vessel walls 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 (HEE-muh-glow-bin) 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 (hee-MAT-oh-crit) 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 measures 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, and electrolyte tests, as well as blood tests that measure kidney function. Some of these tests require you to fast (not eat any food) before the test, and others don't. Your doctor will tell you how to prepare for the test(s) you're having.
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 an important mineral 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.
Blood tests for kidney function measure levels of blood urea nitrogen (BUN) and creatinine (kre-AT-ih-neen). 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 chemical 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 troponin and creatine (KRE-ah-teen) kinase (CK) tests.
Troponin is a muscle protein that helps your muscles contract. When muscle or heart cells are injured, troponin leaks out, and its levels in your blood rise.
For example, blood levels of troponin rise when you have a heart attack. For this reason, doctors often order troponin tests when patients have chest pain or other heart attack signs and symptoms.
A blood product called CK-MB is released when the heart muscle is damaged. High levels of CK-MB in the blood can mean that you've had a heart attack.
Blood Tests To Assess Heart Disease Risk
A lipoprotein panel is a blood test that can help show whether you're at risk for coronary heart disease (CHD). This test looks at substances in your blood that carry cholesterol.
A lipoprotein panel gives information about your:
- Total cholesterol.
- LDL ("bad") cholesterol. This is the main source of cholesterol buildup and blockages in the arteries. (For more information about blockages in the arteries, go to the Diseases and Conditions Index Atherosclerosis article.)
- HDL ("good") cholesterol. This type of cholesterol helps decrease blockages in the arteries.
- Triglycerides. Triglycerides are a type of fat in your blood.
A lipoprotein panel measures the levels of LDL and HDL cholesterol and triglycerides in your blood. Abnormal cholesterol and triglyceride levels may be signs of increased risk for CHD.
Most people will need to fast for 9 to 12 hours before a lipoprotein panel.
Blood Clotting Tests
Blood clotting tests sometimes are called a coagulation (KO-ag-yu-LA-shun) panel. These tests check proteins in your blood that affect the blood clotting process. Abnormal test results might suggest that you're at risk of bleeding or developing clots in your blood vessels.
Your doctor may recommend these tests if he or she thinks you have a disorder or disease related to blood clotting.
Blood clotting tests also are used to monitor people who are taking medicines to lower the risk of blood clots. Warfarin and heparin are two examples of such medicines.
What To Expect With Blood Tests
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) for 8 to 12 hours before the test. Your doctor will tell you how to prepare 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 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. Doing this can make the veins in your arm stick out more, which 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 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 needles. 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 was inserted. This helps stop bleeding. It also helps prevent swelling and bruising.
Most of the time, you can remove the pressure after a minute or two. 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. Results 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 of blood tests are discomfort and 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 certain group. For many tests, normal ranges vary depending on your age, gender, race, and other factors.
Your blood test results may fall outside the normal range for many reasons. Abnormal results might be a sign of a disorder or disease. Other factors—such as diet, menstrual cycle, physical activity level, alcohol intake, and medicines (both prescription and over the counter)—also can cause abnormal results.
Your doctor should discuss any unusual or abnormal blood test results with you. These results may or may not suggest a health problem.
Many diseases and medical problems can't be diagnosed with blood tests alone. However, blood tests 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.
Result Ranges for Common Blood Tests
This section presents the result ranges for some of the most common blood tests.
NOTE: All values in this section are for adults only. They don't apply to children. Talk to your child's doctor about values on blood tests for children.
Complete Blood Count
The table below shows some normal ranges for different parts of the complete blood count (CBC) test. Some of the normal ranges differ between men and women. Other factors, such as age and race, also may affect normal ranges.
Your doctor should discuss your results with you. He or she will advise you further if your results are outside the normal range for your group.
|Test||Normal Range Results*|
|Red blood cell (varies with altitude)||Male: 5 to 6 million cells/mcL|
Female: 4 to 5 million cells/mcL
|White blood cell||4,500 to 10,000 cells/mcL|
|Platelets||140,000 to 450,000 cells/mcL|
|Hemoglobin (varies with altitude)||Male: 14 to 17 gm/dL|
Female: 12 to 15 gm/dL
|Hematocrit (varies with altitude)||Male: 41% to 50%|
Female: 36% to 44%
|Mean corpuscular volume||80 to 95 femtoliter†|
* Cells/mcL = cells per microliter; gm/dL = grams per deciliter.
† A femtoliter is a measure of volume.
This table shows the ranges for blood glucose levels after 8 to 12 hours of fasting (not eating). It shows the normal range and the abnormal ranges that are a sign of prediabetes or diabetes.
|Plasma Glucose Results (mg/dL)*||Diagnosis|
|70 to 99||Normal|
|100 to 125||Prediabetes|
|126 and above||Diabetes†|
* mg/dL = milligrams per deciliter.
† The test is repeated on another day to confirm the results.
The table below shows ranges for total cholesterol, LDL ("bad") cholesterol, and HDL ("good") cholesterol levels after 9 to 12 hours of fasting. High blood cholesterol is a risk factor for coronary heart disease.
Your doctor should discuss your results with you. He or she will advise you further if your results are outside the desirable range.
|Total Cholesterol Level||Total Cholesterol Category|
|Less than 200 mg/dL||Desirable|
|200–239 mg/dL||Borderline high|
|240 mg/dL and above||High|
|LDL Cholesterol Level||LDL Cholesterol Category|
|Less than 100 mg/dL||Optimal|
|100–129 mg/dL||Near optimal/above optimal|
|130–159 mg/dL||Borderline high|
|190 mg/dL and above||Very high|
|HDL Cholesterol Level||HDL Cholesterol Category|
|Less than 40 mg/dL||A major risk factor for heart disease|
|40–59 mg/dL||The higher, the better|
|60 mg/dL and above||Considered protective against heart disease|
The National Heart, Lung, and Blood Institute (NHLBI) leads or sponsors many studies aimed at preventing, diagnosing, and treating heart, lung, blood, and sleep disorders.
Related Health Topics
- Laboratory Tests (MedlinePlus)
- For People of African, Mediterranean, or Southeast Asian Heritage: Important Information about Diabetes Blood Tests (National Institute of Diabetes and Digestive and Kidney Diseases)