Blood Tests

Blood tests are used to check for certain diseases and conditions. They are used for patients both in and out of the hospital to diagnose conditions, see how well treatments are working, and make sure that treatments are not harming the patient. The diagnosis of many common diseases can be confirmed or assisted by a simple blood test, and blood tests are a very common first step in a doctor's investigation of a patient's complaint.

Description

Blood being drawn from a vein. Source: Wikimedia Commons.

Blood tests are a major part of many routine medical examinations.

During a blood test, a small amount of blood is taken from a vein, usually in the forearm, elbow, or hand using a thin needle and syringe or vacuum container system. The containers contain small amounts of chemicals that preserve the blood. A finger prick also may be used for some tests. The procedure is usually quick and easy, although it may cause some short-term discomfort and in some cases can require more than one attempt, particularly when many blood tests have recently been performed. The amount of blood taken is usually small, although it may seem like a lot to the patient.

Lab workers process 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. Since there are so many different blood tests available, many of which are expensive, the blood is only tested for a range of likely conditions or diseases that is specified by the ordering doctor. Blood tests alone cannot be used to diagnose or treat many diseases and medical problems—for example, only very specific types of cancer can be diagnosed by blood tests. It is often in combination with other factors—such as signs and symptoms, previous medical history, and results from other tests and procedures—that doctors use blood tests to help confirm a diagnosis.

Why Blood Tests Are Done

Blood tests show whether the levels of different substances in the blood fall within a normal range.

For many blood substances, the normal range is the range of levels seen in 95% of healthy people in a particular group. For many tests, normal ranges are different depending on an individual's age, gender, race, and other factors.

Many factors can cause blood test levels to fall outside the normal range. Abnormal levels may be a sign of a disorder or disease, but often require interpretation by a physician in association with other factors such as the patient's complaints and physical exam. Other factors such as diet, menstrual cycle, physical activity, alcohol or drug consumption, and medicine use (both prescription and over-the-counter) also can cause abnormal levels. Abnormal levels do not automatically mean that a patient has a disease.

Many diseases or medical problems cannot be diagnosed with blood tests alone. However, they can help the patient and his/her doctor determine whether to perform further tests or treatments. Blood tests also can help find potential problems early, when treatments or lifestyle changes may work best.

Types

A huge number of blood tests are available to doctors, many of them for use in very specific circumstances. However, certain widely used blood tests give a more general picture of how the body is performing. Some of the most common blood tests that doctors order are:

• 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 types of blood test. It is often done as part of a routine checkup.

A CBC measures the numbers and sizes of the different type of living cells in the 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 the lungs to the rest of the 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 the part of the immune system that fights infection and disease. Abnormal white blood cell levels may be a sign of infection, blood cancer, or an immune system disorder.

A total WBC count measures the overall number of white blood cells in the blood. A differential count looks at the amounts of different types of white blood cells in the blood, which can help to distinguish a viral infection from a bacterial infection.

Platelets

Platelets are blood cells that allow the blood to 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

Hemoglobin is an iron-rich protein in red blood cells that carries oxygen. Abnormal hemoglobin levels may be a sign of bleeding, anemia (including the congenital anemias sickle cell disease and thalassemia), and other blood disorders.

In diabetes, excess glucose in the bloodstream can attach to hemoglobin and raise the level of hemoglobin A1c ("glycosylated hemoglobin"). This particular test is not part of a routine CBC and must be ordered separately.

Hematocrit

Hematocrit is a measure of how much space red blood cells take up in the blood. Usually this is a little below half of the blood, and is a slightly higher proportion on average in men than women. A high hematocrit level might mean the person is dehydrated or that he or she lives at high altitude. A low hematocrit level can indicate anemia or bleeding. 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 the red blood cells. Abnormal MCV levels may be a sign of anemia, alcohol abuse, and certain dietary deficiencies.

Blood chemistry tests/basic metabolic panel

The basic metabolic panel (BMP) is a group of tests that measures different chemicals in the blood. It is also termed the Chem 7, Chem 8, or Chemistry Panel. These tests usually are done on the fluid (plasma) part of blood. The tests can give doctors information about many organs in the blood, although they are most specific for the kidneys.

The BMP includes blood glucose, calcium, electrolytes (sodium, potassium, chloride, bicarbonate), and kidney tests (blood urea nitrogen and creatinine).

Blood glucose

Glucose is a type of sugar that the body uses for energy. Since it is contained within food, patients being tested for diabetes must fast for at least eight hours to stop recent food intake from interfering with the test. An abnormally high glucose level in a patient who has fasted is a sign of diabetes or metabolic syndrome.

Calcium

Calcium is one of the most important minerals in the body, since it makes up a large proportion of the bones, and is essential for muscle function (including heart muscle). 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 (salts) that help maintain fluid levels and acid-base balance in the body and allow electrical signals to be conducted between cells. They include sodium, potassium, bicarbonate, and chloride. Their levels depend on intake, output, and movement outside the bloodstream. These processes are controlled mainly by the kidney, but also may involve the respiratory, gastrointestinal, and the hormonal (endocrine) systems.

Abnormal electrolyte levels may be a sign of dehydration, kidney disease, liver disease, heart failure, high blood pressure, or other disorders. They are also often affected by medical treatments. Certain electrolytes, in particular potassium, are very important since high levels of potassium can be rapidly fatal.

Kidney tests

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 and are often elevated in patients who are dehydrated.

Blood enzyme tests

Enzymes are very large protein molecules which help control different reactions in the body by catalytically converting one substance to another. These enzymes may be contained in very high levels in certain organs. For example, in the storage cells of the liver there are many enzymes that convert glucose to its storage product glycogen, while in muscles there are many enzymes that break down sugar to create energy for movement. As a rule, when an organ is damaged, its cells begin to swell and may leak, allowing enzymes that are not usually present in the bloodstream to be detected in the blood using blood tests. There are a huge number of blood enzyme tests. By way of example, this section focuses on blood enzyme tests used to check for heart attack, or myocardial infarction.

These include creatine kinase (CK) and troponin tests.

Creatine kinase

Creatine kinase (CK or CPK) helps to convert chemical energy to movement and is present in very high levels in muscle cells. When muscle or heart cells are injured, CK leaks out, and its levels in the blood rise. Since CK is present in both normal muscles and the heart muscle, an elevated CK does not necessarily indicate damage to the heart and often means that there is damage to the muscles in the rest of the body. It is not a specific test. In fact, marathon runners often have very high CK levels after a race, and usually they have not had a heart attack. Fortunately, the CK in the heart is slightly different from the CK in the other muscles (skeletal muscle). If CK is found to be elevated and heart attack is suspected, the specific heart muscle "isozyme," which is called CK-MB, may be tested and if elevated indicates that heart muscle may have been damaged. High CK-MB therefore may indicate heart attack. The test is often repeated once or more over several hours or a day, as it can take time for damaged heart cells to release CK-MB into the bloodstream.

Doctors order CK tests (such as CK-MB) after patients have chest pain or other heart attack signs and symptoms. They are also ordered when muscle damage is suspected, since bad muscle damage can cause the kidneys to stop functioning normally.

Troponin

Troponin is another muscle protein that helps the muscles contract. Blood levels of troponin rise when a person has a heart attack; however, unlike CK, troponin is very specific to the heart and is not usually elevated in other conditions. 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. The test is often repeated once or more over several hours or a day, as it can take time for damaged heart cells to release troponin into the bloodstream.

Blood tests to assess heart disease risk

Some people have a higher risk of certain diseases than others due to their diet, lifestyle, and genetics. Abnormal levels of certain chemicals in the blood may be used to distinguish people at low risk of certain diseases from people who are at high risk. Testing the levels of these chemicals is often necessary to assess risk of disease and to suggest ways to reduce it. This is called risk stratification. The most widely known blood tests used for risk stratification are cholesterol and lipid panel tests.

Lipid panel

This test can help show how high an individual patient's risk is for coronary heart disease. A lipoprotein panel looks at cholesterol levels together with the structures in the blood that transport cholesterol, so called lipoproteins. Each component of the test is necessary to give a full assessment of the risk each patient has of developing blockages in the arteries to the heart and therefore for suffering a heart attack.

The test gives information about:

• 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 by transporting cholesterol away from the fatty atheromatous plaques which develop in diseased arteries.
• Triglycerides. These are another form of fat in the blood.

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

Just like with glucose tests, cholesterol is contained in food and the accuracy of the test can be affected by the diet. Most people will therefore need to fast for 9 to 12 hours before a lipoprotein panel.

Preparation

Many blood tests don't require any special preparation and take only a few minutes.

Other blood tests can require preparation by the patient—for example, by fasting for glucose tests and lipid panel tests, or by treatment with certain drugs, like the cosyntropin test for Addison's disease.

How Blood Tests Are Done

Blood is usually drawn from a vein in the arm or another easily seen or felt vein using a thin needle. It also can be drawn using a finger prick. In order to make the vein stand out and to increase the speed of blood flow, the person who draws the blood ("phlebotomist") might tie a band around the upper part of the arm. This is called a tourniquet. Once within the vein, the needle is attached to a small test tube. The phlebotomist then removes the tube when it is full and the tube seals on its own. The needle is then removed from the vein. If several tests are required, more than one test tube may be attached to the needle before it is withdrawn. This only requires a single needlestick to get many different samples of blood. On certain occasions, it may be necessary to take blood from several different areas of the body one after another, particuarly when a patient is thought to have a serious infection.

Some people get nervous about blood tests because they are afraid of the needle, so called "needle phobia." Others may not want to see blood leaving their bodies. Fainting due to a vasovagal reaction is not uncommon during or after blood tests.

Drawing blood usually takes less than three minutes once a suitable vein is found.

Once the needle is withdrawn, the patient should apply gentle pressure with a piece of gauze or bandage to the place where the needle went in for at least two minutes. This stops bleeding, reduces pain after the blood test, and prevents swelling and bruising.

Blood test results can take anywhere from a few minutes to a few weeks to come back.

Benefits

Blood tests can help:

• Evaluate how well organs (such as the kidneys, liver, and heart) are working
• Distinguish between different causes of common complaints, for example abdominal pain or jaundice
• Diagnose diseases like cancer, HIV/AIDS, diabetes, anemia, and heart disease
• Learn whether a patient has risk factors for conditions such as heart disease or stroke
• Follow the progress of known diseases and check whether medicines used to treat them are working
• Check whether medicines used to treat patients are causing damage to other organs, for example the liver, kidneys, heart, or muscle

Risks

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. They are minimized by applying adequate pressure at the site of the needle stick after the test is done. Serious reactions to having blood drawn are very uncommon.

Side Effects

The main side effects of blood tests are pain at the site where blood is drawn and bruising at the site. Bruising can be avoided by holding firm pressure over the site for several minutes.

Some patients experience vasovagal reactions when they have blood drawn. This is a situation where a person briefly loses consciousness after having a fear reaction to a procedure or event.

Effectiveness

The effectiveness of different blood tests varies. Multiple factors can determine the likelihood that a test will be accurate or useful for a given patient.

Alternatives

Depending on the condition being treated, many alternatives may be available. This could include tests on other body fluids, such as urine, or other diagnostic tools.

Research

Much medical research is aimed at finding better blood tests that are able to detect diseases such as cancer without needing expensive tests or surgeries.

A perfect test is one that is very likely to predict disease (specific), detects disease in all the patients who have disease and does not miss any cases (sensitive), and is cost effective. Although many blood tests have been used for many years, many very useful new tests have been recently adopted by doctors, for example B-type natriuretic peptide (BNP) for heart failure, D-dimer for deep vein thrombosis, and high-sensitivity C-reactive peptide (CRP), for heart disease.

Due to a heavy investment of time and money, it is reasonable to expect blood tests for diseases such as cancers or other chronic diseases to improve and advance in the future.

External Links

American Association for Clinical Chemistry: Lab Tests Online

National Phlebotomy Association: Home Page

National Cancer Institute: Screening and Testing to Detect Cancer