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:
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 hr 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.
Types of blood test
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, also called erythrocytes, 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, also called leukocytes, 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 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, 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 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.
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.
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.
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.
The test gives information about your:
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 indicate a higher risk for heart attack or stroke. This isn't a routine test for heart disease risk, but it may be used, along with other tests, if an increased risk is suspected.
What to expect with blood tests
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).
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 min.
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.
Risks of bone marrow 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 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.
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 components of the complete blood count (CBC). Some of the normal ranges are different for 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.
|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
|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
This table shows the ranges for blood glucose levels after 8 to 12 hours of fasting (not eating). It shows the normal range and also the abnormal ranges that are a sign of prediabetes or diabetes.
|Plasma glucose results (mg/dL)*
|99 and below
|100 to 125
|126 and above
*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
|240 mg/dL and above
|LDL cholesterol level
|Less than 100 mg/dL
|Near optimal/above optimal
|190 mg/dL and above
|HDL cholesterol level
|HDL cholesterol category
|Less than 40 mg/dL
|Major risk factor for heart disease
|The higher, the better
|60 mg/dL and above
|Protective against heart disease