Blood Tests


As the body’s main carrier of nutrients, waste products and oxygen, the blood offers physicians a very important tool for diagnosing and monitoring health. Many forms of heart disease can be detected by various types of blood tests. During a blood test, a medical professional will take some blood, either by using a finger prick (or heel in the case of a baby) to obtain just a few drops or by using a needle to obtain a larger quantity. In adults, blood may be taken from a vein (venipuncture) or artery (arterial puncture). The blood may then be tested for many different factors. For example, the physician may need to know the number of red or white blood cells present, the levels of various blood fats,or the amount of oxygen or carbon dioxide circulating in the blood. Obtaining a blood sample takes only about five minutes, and most patients find the procedure to be virtually painless. For most blood tests, there is very little preparation, and people can generally return to their usual daily activities after testing.

About blood tests

A blood test uses a sample of blood from an artery or a vein to detect and measure various factors in the blood. In some cases, it may be just as important to establish that a condition is “normal” as to establish that a condition is “abnormal.” Blood tests serve a number of purposes when used with people who have cardiovascular conditions. These purposes include: To help diagnose heart disease or identify risk factors (e.g., abnormal cholesterol or triglyceride levels) To help establish that a heart attack has occurred and measure the extend of damage to the heart by measuring the blood levels of certain cardiac enzymes To monitor the patient’s progress during treatment Blood, which is part of the circulatory system, contains a variety of cells suspended in a light-colored fluid known as plasma.

Plasma composes more than half of the blood volume. Dissolved in the plasma are many substances, such as electrolytes, nutrients, vitamins, clotting factors, hormones, enzymes and antibodies. There are three types of blood cells present in the plasma: red blood cells (carry oxygen to the tissues), white blood cells (fight invading organisms) and platelets (involved in blood clotting). An analysis of various blood components can provide the physician with essential information about its microscopic contents, including: Blood cells Nutrients, vitamins and minerals Proteins, sugars and fats Chemicals Hormones Antigens (foreign proteins that stimulate the formation of antibodies) Antibodies (cells that identify foreign proteins for attack by white blood cells or, in the case of autoimmune diseases, mistakenly identify the patient’s own cells as foreign invaders) Blood gases (e.g., oxygen or carbon dioxide that is dissolved in the blood) enzymes Clotting factors Because blood carries all of these substances to various parts of the body, examining its contents can help to evaluate the health of major organs and organ systems, including the heart, lungs and respiratory system. For example, blood tests can determine how well the liver, kidney and lungs are functioning and whether there is any infection or inflammation.

Cardiovascular function tests

The most common blood tests used in the diagnosis and management of cardiovascular disease include: Antistreptolysin-O test. Streptolysin is a protein produced by streptococcal bacteria. In response to infection, the body mounts an immune response that relies on antibodies to identify the streptolysin. High levels of antibodies for streptolysin, called antistreptolysin-O antibodies, may indicate the presence of an infection from strep, such as endocarditis (an inflammation of the lining of the heart or valves) or rheumatic fever. Arterial blood gases. This test takes a sample of blood from an artery in the side of a wrist (which carries oxygen-rich blood) instead of a vein in the crook of the arm (which carries oxygen-poor blood). The sample gives information about levels of oxygen (which should be high) and carbon dioxide (which should be low). Blood fat profile.

Also called a lipid panel, this test measures fats and fat-like substances in the blood that, if abnormally high, have been associated with heart disease. In a standard lipid panel, fats measured include: Total cholesterol. A high total cholesterol level is associated with heart disease. LDL cholesterol. Low-density lipoprotein or “bad” cholesterol that is associated with atherosclerosis. HDL cholesterol. High-density lipoprotein or “good” cholesterol” that protects against heart disease. Triglycerides. High levels are associated with heart disease. VLDL (very low density lipoprotein). Another carrier of fat in the blood, that is associated with atherosclerosis. Blood calcium test. By measuring the calcium levels of blood, a physician may be able to support the diagnosis of a number of cardiovascular problems. Calcium is important for proper blood clotting, and abnormal levels may signal kidney or thyroid disorders. BNP blood test.

This new test measures the level of hormone called B-type natriuretic peptide, which has been shown to rise in heart failure. C-reactive protein test (CRP test). CRP is an inflammatory marker – a substance that the body releases in response to inflammation. CRP levels can provide physicians with information about a patient’s risk of having a heart attack or stroke. Carbon dioxide content. CO2 levels are generally used as an investigative and diagnostic tool for patients with breathing problems. This analysis may be helpful in the diagnosis of conditions such as chronic obstructive pulmonary disease (COPD). Complete blood count (CBC). This very common test can provide important information about the types of blood cells present and their condition and number (percentage) in relation to other cells. An elevated white blood count (WBC) may indicate a heart attack, infection or inflammatory disease, such as rheumatoid arthritis.Elevated levels of red blood cells, hematocrit (the percentage of available space taken up by red blood cells) or hemoglobin (the protein molecule in red blood cells that carry oxygen) may indicate a lack of oxygen in the body. This may be caused by smoking, congenital heart disease (a cardiac abnormality, defect or malformation that is present from birth), dehydration or kidney disease. Low hemoglobin or hemocrit may suggest anemia.

Anemia may suggest bleeding complications in patients receiving anticoagulants. Electrolyte panel. Tests that measure the amount of potassium, sodium, chloride and carbon dioxide levels in the blood in order to assess how well the major organ systems (e.g., the heart and cardiovascular system) are functioning. Erythrocyte sedimentation rate (ESR). A test that measures the rate at which red blood cells separate from plasma (the liquid part of blood) and fall to the bottom of a test tube to form a sediment. High levels may occur during a heart attack, rheumatic fever, giant cell arteritis, severe anemia, cancer relapse or other conditions. Low levels may be associated with heart failure, sickle cell anemia or other conditions. Cardiac enzyme tests. This test measures a number of heart enzymes, including creatine phosphokinase (CK). CK is a valuable tool for determining whether a heart attack is the cause of chest pain. Levels of CK typically rise within four to eight hours of a heart attack and return to normal by 72 hours after the heart attack. Glucose test. This is primarily used to screen for, diagnose or monitor patients with diabetes, a metabolic condition in which blood glucose levels are too high because the body cannot produce enough insulin in the pancreas or is desensitized to the effects of insulin. Following a heart attack or other serious illness, fasting glucose levels may be temporarily high. INR/prothrombin time tests.

A type of coagulation test that measures how long it takes blood to clot. Prothrombin is a protein substance that needs to be converted to thrombin in order for clotting to occur. People taking anticoagulants (such as warfarin) may have this test done regularly. Serum myoglobin test. A test sometimes used to measure the damage done to the heart muscle after a heart attack. Total serum protein. Blood contains large amounts of protein, and measuring these levels can give physicians valuable information about a patient’s nutritional state and kidney and liver functions. Abnormal protein levels may indicate congestive heart failure, high blood pressure (hypertension), or kidney or liver disease. Waste products test. A group of blood tests that measure the levels of specific waste products in the blood, including blood urea nitrogen, creatinine and uric acid. Abnormal results could be a sign of heart failure, heart attack or kidney disease.

Before and during the blood test

Preparation for blood tests varies according to the requirements of each specific test. In most cases minimal preparation is necessary. The patient may need to reduce or stop certain medications at some point prior to the test. Additionally, food intake and exercise may be temporarily restricted or suspended. Alcohol and caffeine should be avoided prior to a blood test. On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. The medical professional will also obtain the patient’s medical history before the test to determine if the patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems.

Drawing blood for a blood test is an easy and typically painless process for most people. If the physician needs only a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of a baby) can provide enough blood for testing. The technician will use a sterile, sharp lancet to prick the skin. The technician will then gently squeeze the puncture area to produce drops of blood that are collected in tiny glass tubes. Light pressure and sterile gauze are then applied to the puncture site to stop the bleeding. A bandage is not usually necessary. If a substantial amount of blood is needed, it is usually drawn from a vein in a process called venipuncture. Blood samples may be more difficult to obtain from: Infants or young children Overweight people whose veins are difficult to find Elderly people whose veins tend to roll away Patients with scarred or collapsed veins due to multiple transfusions or drug use During a venipuncture, a needle is inserted into a vein – usually at the inside of the elbow or on the back of the hand. The area around the puncture site is cleaned with rubbing alcohol and a wide elastic band or piece of latex tubing may be placed around the upper arm to slightly increase the pressure in the vein.

One end of a sterile double-ended needle that has been attached to an open-ended syringe (which contains an empty test tube) is inserted into the vein. Because the test tube contains a partial vacuum, blood flows directly from the vein through the double-ended needle and into the test tube. The precise amount of blood to be drawn is determined by the type and number of tests to be done. It is usually around 7 milliliters. The technician may change test tubes once or twice during the venipuncture to either allow for more blood to be collected or to change the type of tube being used. Tubes are marked with different colored tops that indicate the way in which the collected blood will be preserved: Lavender tops indicate that the tube contains an anticoagulant, which prevents the collected blood from clotting. Red tops indicate that the tube contains no anticoagulants, allowing serum to separate and the blood to form a blood clot. Gray tops indicate that the tube contains a preservative, which keeps glucose (blood sugar) from breaking down in the tube. After the necessary amount of blood is drawn, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site.

After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The entire process takes less than 10 minutes. Despite the precautions taken to avoid bruising and soreness, it does sometimes occur. Typically, this is not a cause for great concern. To minimize soreness, patients may immediately apply a warm compress to the puncture site and repeat this application every three hours until the discoloration or pain subsides. Blood tests can be performed on different parts of the blood, including: Whole blood (to which an anticoagulant has been added in the test tube to prevent clotting) Blood serum, the term for plasma (the liquid part of blood) that has had the clotting agents removed Blood plasma, the whitish yellow liquid that remains in unclotted blood once the blood cells have settled out to the bottom of the test tube Blood cells, the individual red, white and platelet cells If blood must be taken from an artery instead of a vein, (e.g., during an arterial blood gas study), it is usually drawn from a small artery located on the inside or the top side of the wrist.

After the blood test

Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s orders. Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing. Results are usually returned within 24 hours, depending on which tests need to be performed. Based on the test results, additional blood tests may be ordered.Test results are always evaluated in relation to the “normal range” for that test.

The range of values considered to be normal is the range of test results from the blood of normal, active healthy people. When someone has a disease or health problem, his or her blood test results may be higher or lower than normal – or “outside of the normal range.” When a physician sees that a blood test is outside of the normal range, he or she may order a repeat test to verify the results or additional tests to determine the underlying causes behind the abnormality.Normal ranges for some tests may vary slightly from lab to lab, especially between labs that use machines to perform blood tests and those that perform the tests by hand. Frequently, results of a patient’s blood test are compared to another “known” blood sample taken from a healthy individual that is run at the same time and is designated as the “normal control.” When the “normal control” sample falls within the normal range, or reaches a specific “known” measurement, the laboratory results confirm that the blood test has been carried out accurately.

Types and differences of blood tests

Hundreds of blood tests are performed every day in today’s modern labs. In general, there are four main types of blood tests: Hematology tests Biochemistry tests Microbiology tests Serology tests Hematology tests examine the blood to identify: The types and numbers of blood cells that are present (e.g., red blood cells, white blood cells and platelets) The appearance of the cells, especially their maturity The ability for the blood to form a blood clot and the speed at which clotting occurs Biochemistry tests measure the levels of normally occurring chemicals and biochemicals in the blood, both individually and in relation to other chemicals.

These measurements are compared to normal ranges and are used to determine whether blood biochemicals are in a proper and healthy balance. Biochemicals and other substances that may be studied include: Sodium Cholesterol and other fats Vitamins and minerals Hormones Blood gases Prescription drugs Recreational drugs Alcohol Not only can biochemical tests precisely measure these substances, but they can also be used to indicate how well some organs and organ systems are functioning. For instance, the amount of blood sugar (glucose) in the bloodstream can help diagnose or monitor diabetes, and indirectly reflect how much insulin is being produced by the pancreas. Microbiology tests examine blood for the presence of infectious microscopic organisms such as: Bacteria Fungi Viruses (in most clinical labs a serology test is used) Parasites Microbiology tests include: Smears, in which a small amount of blood is placed on a glass slide for examination under a microscope. Sometimes the blood smear is stained with special dyes before examination. Blood cultures, in which a small amount of blood is placed in a nutrient broth, incubated for days or weeks and then examined for growth of disease-causing bacteria.

Serology tests (tests done on blood serum) can detect the presence of antibodies that are produced by white blood cells. They are frequently used to detect viral diseases. Most hospital laboratories do not have the equipment or specially trained personnel necessary to isolate the viruses themselves, so serology tests are done instead to identify the infecting organism by studying the antibodies produced against it.

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