Traditionally performed in physician offices and laboratories, blood tests are useful tools for uncovering disorders in the human body. They detect abnormally high or low levels of substances that may indicate a variety of diseases, including cancer.
Blood tests may be ordered during diagnosis to rule out other diseases and disorders. Although they are used to help detect cancer, additional tests are usually necessary to verify the diagnosis. They may also be used to determine a person’s risk of developing cancer, or to screen for cancer before signs and symptoms develop.
In people with cancer, blood tests may be used to monitor the progression of the disease, and the effectiveness of treatment. They may also be helpful in detecting recurrent cancer (cancer that has returned after treatment). In many medical facilities, blood is drawn by a phlebotomist, who is a medical laboratory assistant specially trained in collecting and handling blood.
Depending on the amount of blood required for the test, blood can be drawn from a vein puncture (venipuncture), artery puncture (arterial puncture) or skin puncture. Obtaining a blood sample only takes a few minutes and most patients find them to be almost painless. Some blood tests may require fasting or other preparation. However, for most blood tests, there is very little preparation beforehand, and people can generally return to their usual daily activities following testing.
About blood tests
A blood test uses a sample of blood to detect and measure various factors in the blood. It is important to measure these factors because abnormally high or low levels may affect normal body function. The presence or absence of certain elements in the blood also may be an indication of a medical problem.
Blood tests serve a number of purposes related to the diagnosis and monitoring of cancer. These purposes include:
- To determine a person’s risk of developing cancer
- To screen for cancer before signs and symptoms develop
- To help diagnose cancer
- To rule out other diseases during diagnosis
- To monitor the progression of the disease
- To monitor the effectiveness of a patient’s cancer treatment
- To detect cancer-related complications
- To detect the spread of the disease to other areas of the body (metastasis)
- To help detect recurrence or remission
The blood tests ordered to detect and monitor cancer are used to uncover an abnormality in the body. The detected abnormality is usually nonspecific, meaning that it may indicate a variety of possible diseases, including cancer. Although blood tests may help detect cancer, additional tests are usually necessary to verify the diagnosis.
Samples of blood may be taken in one of three ways:
- Vein puncture (venipuncture). A needle is inserted directly into a vein to draw blood into one or more tubes. Blood is usually drawn from a vein in the inner arm, opposite the elbow.
- Skin puncture. Used when only a small amount of blood is needed to perform the desired blood test. The skin of the finger, earlobe or heel (in infants) is jabbed with a needle or lancet to produce a drop of blood that is then collected for testing.
- Artery puncture (arterial puncture). Used only when the blood test needs to be performed on a sample of oxygen-rich blood traveling from the heart to the body via the arteries. Blood is usually drawn from an artery in the side of a wrist.
Blood tests can be performed on different parts of the blood, including:
- Whole blood (blood that has not been separated into its many components)
- Blood plasma (the liquid part of the blood)
- Blood serum (plasma that has had the clotting agent removed)
- Blood cells (the individual red blood cells, white blood cells and platelets)
Types and differences of blood tests
Hundreds of blood tests are performed every day in hospitals, laboratories and in people’s homes. 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 number 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 of 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 and other blood electrolytes
- Cholesterol and other fats
- Vitamins and minerals
- Blood gases
- Prescription drugs
- Recreational drugs
- Sugar, calcium and many other elements
Biochemical tests precisely measure these substances and can also be used to indicate how well some organs and organ systems are functioning. For instance, when cancer spreads to the liver, it may affect the organ’s enzyme production. As a result, measuring the amount of certain enzymes in the bloodstream can indirectly reflect the progression of the disease and its effect on the liver. Kidney, bone and lung function, among others, can also be reflected in the blood.
Microbiology tests examine blood for the presence of infectious microscopic organisms such as bacteria, fungi, parasites and viruses, although most laboratories use serology tests for viruses.
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 performed on blood serum) can detect the presence of antibodies that are produced by plasma cells to attack microscopic organisms. 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 performed instead to identify the infecting organism by studying the antibodies produced against it.
Common blood tests for cancer
Blood tests commonly used to detect and monitor cancer include:
- Complete blood count (CBC). This test measures the number of red blood cells, white blood cells and platelets in a person’s blood as well as the amount of hemoglobin in the red blood cells and a number of other factors. A CBC is one of the most common blood tests ordered among the general population. In cancer patients, it may be used during treatment to determine if chemotherapy or radiation therapy is negatively affecting cell production in the bone marrow. In addition, a CBC can help detect diseases such as leukemia or multiple myeloma, and cancer-related complications including anemia. It may also be used to detect cancer that has metatasized (spread) to bone marrow.
- Tumor marker tests. These tests detect substances produced by cancer cells. They may be used to help diagnose certain cancers or determine the primary site of a suspected cancer. For example, the presence of a high level of the tumor marker CA-125 strongly suggests ovarian cancer, although it is not absolutely specific. Tumor marker tests are also used to plan cancer therapy and to monitor the effectiveness of treatment.
Following treatment, the tests may be used to detect recurrent cancer or to monitor the spread of the disease. Blood tests for levels of the tumor marker CA-15-3 can detect breast cancer recurrence before the patient displays any symptoms. In men, levels of prostate specific antigen (PSA) may be measured to detect prostate cancer in men without signs or symptoms of the disease.
- Blood chemistry tests. These tests measure the levels of specific chemicals in the blood. An abnormally high or low level of a substance may be a sign of disease in the organ or tissue that produces it. The presence or absence of certain elements also may indicate a medical condition. The tests are used to monitor changes in liver or kidney function caused by cancer or treatment with certain chemotherapy drugs. The tests may also be ordered to determine whether treatment is required to correct abnormally low or high levels of certain minerals.
- Enzyme tests. These tests measure levels of enzymes (proteins required for chemical reactions to take place in cells) in a blood and/or urine sample. Enzyme tests may be ordered to measure the amount of enzyme released by the liver. An abnormal amount of an enzyme may indicate that cancer has spread to the liver.
- Electrolyte panel. An electrolyte panel is a group of blood tests that measure electrolyte (minerals that help regulate a variety of body functions) levels to assess the general functioning of the patient’s organ systems. Electrolytes that are often measured include potassium, chloride and sodium, although there are many others. These tests may be ordered to monitor the effects of treatment.
- Genetic tests. These tests analyze human DNA, RNA, chromosomes, proteins and certain metabolites. They may be used to detect inherited genetic abnormalities and to help predict a person’s risk of developing cancer. For example, women who come from a family with a history of breast cancer may choose to have genetic testing for the BRCA1 and BRCA2 gene mutations. Tests for the p16 gene mutation help predict the risk of melanoma for patients with a family history of the disease. Many cancer research studies focus on identifying genes associated with specific cancers or that indicate which tumors will respond to specific types of therapy.
- Sedimentation rate (or sed rate). This test measures the rate at which red blood cells (erythrocytes) separate from the liquid part of blood (plasma) and fall to the bottom of a test tube, forming sediment. Also known as erythrocyte sedimentation rate, sed rates are generally used as a screening tool when assessing for disease, or to monitor the progress of chronic diseases such as Hodgkin’s lymphoma and other cancers.
- Alpha-fetoprotein test. This tumor marker test measures the levels of alpha-fetoprotein (AFP) in the blood. Normally present in high amounts in the blood of fetuses, AFP is a protein that usually disappears shortly after birth. When detected in the blood of adults, it may suggest liver cancer, testicular cancer or certain types of ovarian cancer. Following treatment for these cancers, AFP levels may be monitored to determine if the cancer is returning. AFP tests may also be used to detect early tumors in people at high risk for developing cancer.
- Coomb’s test. Also known as an antiglobulin test, this test examines a sample of blood under a microscope to determine if antibodies are on the surface of red blood cells. When antibodies bind to red blood cells, they can destroy them. The test may be ordered during the staging of certain cancers, including chronic lymphocytic leukemiaand to help in the classification of anemias (low red blood cell counts).
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 and vitamins 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 some blood tests.
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 ask questions about 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.
In many laboratories and medical facilities, blood is drawn by a phlebotomist who is a medical laboratory assistant specially trained in collecting and handling blood.
Drawing blood for a blood test is an easy and virtually painless process for most people. If the test only requires 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.
A sterile, sharp lancetis used 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 usually not necessary.
If a substantial amount of blood is needed, it is usually drawn from a vein in a process called venipuncture. During venipuncture, a needle is inserted into a vein – usually at the end of the elbow or on the back of the hand. The site may be changed if the professional is unable to draw blood from the chosen vein.
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 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 scheduled. It is usually around 7 cubic milliliters per tube. The technician may change test tubes once or twice during a venipuncture to either allow for more blood to be collected or to change the type of tube being used. Tubes are marked with different color tops that indicate the way in which the collected blood will be preserved:
- A lavender top indicates that the tube contains an anticoagulant, which prevents the collected blood from clotting.
- A red top indicates that the tube contains no anticoagulants, allowing plasma and the blood cells to form a blood clot.
- A gray top indicates that the tube contains a preservative, which keeps glucose 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, if necessary. The entire process takes less than 10 minutes.
Despite the precautions taken to avoid bruising and soreness, they may 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 the application every three hours until the discoloration or pain subsides.
If blood must be taken from an artery instead of a vein, it is usually drawn from a very small artery located on the inside or the top side of the wrist.
After the blood test
Following the withdrawal of blood in a physician’s office or laboratory, patients may resume medications and food intake according to their physician’s instructions. Immediately after the blood sample is taken, the test tubes are labeled with the date, the patient’s name and any other necessary information. In some places, such as in a medical oncology office, the blood test analysis may be completed on-site. Otherwise, the blood sample is sent to a laboratory for analysis. The length of time in which the results are returned depends upon the urgency and type of blood test. 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 cancer or another disease or condition, blood test results may be higher or lower than normal – or “outside of the normal range.” A physician who sees that a blood test is outside of normal range may order repeat tests to verify results or additional tests to determine the underlying causes behind the abnormality.
Normal ranges for some tests may vary slightly between laboratories, especially with facilities that use machines to perform blood tests and those who perform blood test analyses by hand. Frequently, results of a patient’s blood test analyses 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.
Although blood tests are very useful in the diagnosis and classification of cancer, additional tests are usually needed as well. In order to diagnose cancer, a physician must almost always perform a biopsy – a procedure in which a small sample of tissue is collected and examined under a microscope. In addition, urine tests and imaging tests (e.g., x-rays, CAT scan) may also be used to diagnose and classify the cancer.
Potential risks with a blood test
There are very few risks or side effects reported with blood tests. Occasionally, bruising is reported at the venipuncture site. This can be minimized by keeping direct pressure on the spot for several minutes after the needle has been removed. Greater care must be taken to prevent bleeding after an arterial blood sample because there is greater pressure in an artery. In rare situations, the vein used to obtain the sample may become inflamed. Applying warm compresses to the spot several times a day can help reduce the swelling.
People with bleeding disorders or who are taking anticoagulants (including aspirin) may have problems with continued bleeding. Those who experience these side effects should consult with their physician.
When drawing blood, only sterilized, single-use needles are used by the healthcare professional. In addition, they are trained in standard precautions to eliminate any risk of infection for themselves or their patients. In almost all instances, the technician wears disposable gloves when drawing or handling blood. Infections from nonsterile conditions are rare with blood tests.
Questions for your doctor about blood tests
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about blood tests:
- What type of blood test do I need?
- What do you hope to learn from my blood test?
- Can the blood test diagnose a specific condition?
- Does my blood test require any special preparation?
- What are the risks associated with this blood test?
- Where should I have the blood test done?
- How much blood will they need?
- When can I expect to receive the results and from whom?
- Will I need to repeat the blood test in the future?
- What tests might I need after the blood test?