When your doctor orders a blood test, he or she chooses from a list of chemical studies to be performed in a laboratory on your blood sample. These lab tests can provide important clues about what's going on inside your body.
If your doctor suspects you may have cancer, he or she may test your blood to:
- measure the number of red cells, white cells and platelets
- detect biomarkers that may indicate cancer activity
- examine various chemicals that can indicate how other parts of your body are functioning, including your liver, kidney, heart and lungs
How Are Blood Tests Done?
Ask your doctor if you need to follow any special instructions. For example, your doctor may ask you to fast (not eat or drink) before having your blood taken. Blood tests are usually done in one of two ways:
A needle is inserted into a vein (usually in the fold of your arm) and a sample of blood is withdrawn. You may feel a slight pinprick. Your blood sample is placed in a test tube(s) and sent to a laboratory for analysis.
If only a small amount of blood is needed, your doctor can obtain blood by simply pricking your finger. Your blood sample is placed on a glass laboratory slide to be examined under a microscope or in a test tube for analysis.
Types of Blood Tests
Complete Blood Count
A complete blood count (CBC) is a common test that gives a general picture of your health. A CBC measures the number of red cells, white cells (neutrophils, eosinophils, basophils, monocytes and lymphocytes) and platelets and levels of hemoglobin and hematocrit in your blood. Many health conditions cause increases or decreases in blood counts. Your doctor may order a CBC regularly to monitor your condition or track your response to treatment.
How Is It Done? The blood is placed in a test tube containing an anticoagulant (to prevent clotting) and sent to a laboratory to be examined by a pathologist. Dyes are added to the blood sample so that different types of blood cells are noticeable. The slide with the sample of blood cells is examined under a microscope to count the number of cells and see whether they're normal or, if abnormal, the nature of the changes.
What Do the Results Mean? Normal ranges vary slightly among different labs, so ask your doctor to review the results with you. Results above or below normal ranges may signal health problems.
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Your doctor may order a blood smear (also called a peripheral blood smear or manual differential) if your CBC results are abnormal or unclear or if he or she thinks a disorder or disease may be disrupting normal blood cell production. This test helps determine whether red cells, white cells and platelets are normal in appearance and number. It's also used to determine the proportion of each type of white cell relative to the total white cell count. The results also help your doctor monitor cell production and cell maturity before and during blood cancer therapy.
How is it done? A single drop of blood is spread on a glass slide, dried and then stained with a special dye. The sample is then examined under the microscope to calculate the number of each type of blood cell. The doctor also compares the size, shape and general appearance of the sampled cells to "normal" cells.
What do the results mean? This test can show the presence of abnormal or immature cells, which may indicate an underlying condition, provide information about its severity and suggest the need for further testing.
Your blood chemistry is examined using a group of tests called "chemistry panels," which provide information about your general health. Depending on the type of panel, these tests can measure:
- electrolyte balance (such as sodium or potassium)
- blood glucose (sugar)
- chemical substances that indicate liver and kidney function
- antibodies, including those developed from vaccinations (such as poliovirus antibodies)
- hormones (such as thyroid hormone)
- minerals (such as iron, calcium or potassium)
- vitamins (such as B12 or folate)
How Is It Done? You'll be asked to fast before getting the blood test. Once your blood is drawn, it's placed in a tube(s) and usually left to clot. The fluid portion of the blood that remains after clotting, called the serum, is used for various chemical studies.
What Do the Results Mean? The results give your doctor information about your overall health and identify potential problems that may need treatment. Higher levels of certain blood proteins can be signs of disease severity (for example, tumor size and growth rate). High levels of uric acid can sometimes indicate disease as well.
White Cell Differential
A white cell differential (also called a CBC plus differential or a differential) measures the amount of the different kinds of white cells (leukocytes) in the blood. A white cell differential is often included as part of the CBC. This test helps determine your body's ability to react to and fight infection. It can also identify various types and stages of blood cancers, detect the existence and severity of infections and measure your response to chemotherapy. The absolute neutrophil count (ANC) is the number of neutrophils (a type of white cell) in the blood that will fight infection.
How Is It Done? After your blood is drawn, it's placed on a stained blood slide and examined. The pathologist determines the percentage of different types of white cells present.
What Do the Results Mean? Abnormal patterns of white cells may point to infections, leukemia, immune disorders, inflammation and other problems.
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Fluorescence in Situ Hybridization (FISH)
Fluorescence in situ hybridization (FISH) is a lab test performed on your blood or bone marrow cells to detect chromosome changes (cytogenetic analysis) in blood cancer cells. FISH helps identify genetic abnormalities that may not be evident with an examination of cells under a microscope. This helps ensure that the proper treatment is used. Once treatment begins, doctors use FISH - usually every three to six months - to determine whether therapy is working.
Flow cytometry analyzes your blood or bone marrow cells to find out whether a high white cell count is the result of blood cancer. The test involves identifying cells as they flow through an instrument called a flow cytometer. Flow cytometry measures the number and percentage of cells in a blood sample and cell characteristics such as size, shape and the presence of biomarkers on the cell surface. This method helps to subclassify cell types so your doctor can decide on the best treatment plan. Flow cytometry can also detect residual levels of disease after treatment. This helps your doctor identify disease relapse and restart treatment as needed.
Immunophenotyping identifies a specific type of cell in a sample of blood, bone marrow or lymph node cells. This procedure can be important in helping to choose the best treatment. For example, immunophenotyping can distinguish myeloid leukemic cells from lymphocytic leukemic cells, normal lymphocytes from leukemic lymphocytes, and B-cell lymphocytes from T-cell lymphocytes. Immunophenotyping also reveals whether your cells are monoclonal (derived from a single malignant cell).
A karyotype test identifies and evaluates changes to the expected chromosome arrangement, size, shape and number in a sample of blood or bone marrow cells. This test provides a map of the 46 human chromosomes of a cell. In some cases, a dye called Giemsa may be used as a stain to make the banding pattern of chromosome pairs easier to see. This is also referred to as "G-banding." The findings can help your doctor develop a more specific treatment plan.
Polymerase Chain Reaction
Polymerase chain reaction (PCR) is an extra-sensitive test that measures the presence of certain biomarkers in blood or bone marrow cells. It measures any remaining blood cancer cells not found by cytogenetic methods such as FISH. PCR is used to diagnose and check a patient's molecular response to treatment. PCR can detect a specific DNA abnormality or marker found in patients with certain blood cancers such as acute promyelocytic leukemia and chronic myeloid leukemia. PCR allows more sensitive follow-up of patients in remission and can help determine whether additional treatment is needed.