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, anaemia, and heart disease
• Learn whether you have risk factors for heart disease
• Check whether medicines you’re taking are working

Overview

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.

Outlook

Blood tests have few risks. Most complications are minor and go away shortly after the tests are done.

Types of Blood Tests

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

• Full blood count (FBC)
• Blood chemistry tests
• Blood enzyme tests
• Blood tests to assess heart disease risk

Full Blood Count

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

A FBC measures many different parts of your blood (as described below). This test can help detect blood diseases and disorders. These include anaemia, 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 anaemia, 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 FBC 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

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).

Haemoglobin

Haemoglobin is an iron-rich protein in red blood cells that carries oxygen. Abnormal haemoglobin levels may be a sign of anaemia, sickle cell anaemia, thalassaemia, or other blood disorders.

If you have diabetes, excess glucose in your blood can attach to haemoglobin and raise the level of haemoglobin A1c.

Haematocrit

Haematocrit is a measure of how much space red blood cells take up in your blood. A high haematocrit level might mean you're dehydrated. A low haematocrit level might mean you have anaemia. Abnormal haematocrit 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 anaemia or thalassaemia.

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

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

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.

Kidneys

Kidney tests measure levels of blood urea and creatinine. Both of these are waste products that the kidneys filter out of the body. Abnormal urea 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.

Troponin

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.

The test gives information about your:

• 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.

Homocysteine

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.

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) 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.

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.

Full Blood Count

The table below shows some normal ranges for different components of the full blood count (FBC). 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.

Blood Glucose

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.

Lipoprotein Panel

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.

Key Points

• 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.
• Blood tests are very common. When you have routine checkups, your doctor often orders blood tests to see how your body is working.
• Some of the most common types of blood tests that doctors order are full blood count (FBC), blood chemistry tests, blood enzyme tests, and blood tests to assess heart disease risk.
  • A FBC can detect blood diseases and disorders.
  • Blood chemistry tests measure different chemicals in the blood. These tests give doctors information about nerves, muscles (including the heart), bones, and organs such as the kidneys and liver.
  • Blood enzyme tests measure the amounts of enzymes in your blood. These tests can help diagnose a heart attack.
  • Blood tests to assess heart disease risk measure chemicals in your blood that show whether you're at increased risk for heart disease.
• 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.
• During a blood test, 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. Drawing blood usually takes less than 3 minutes.
• 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. After a minute or two, you can remove the pressure.
• 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.
• Your doctor should get the results of your blood test(s) and discuss them with you. Blood tests show whether the levels of different substances in your blood fall within a normal range. Your doctor should discuss any unusual or abnormal results with you.