About the Test
Purpose of the test
The purpose of a CBC is to give your health care provider details about the state of your health. It is an important medical tool because it uses one sample to analyze the complete spectrum of cells found in the blood as well as some of the characteristics of those cells.
Because it provides information about every type of cell in the blood, the CBC can provide information related to a wide variety of medical problems.
The primary uses for the CBC are diagnosis, monitoring, and screening:
- Diagnosis is determining the cause of a patient’s symptoms. The CBC can identify many different abnormalities in the blood that can be linked to distinct medical problems. For this reason, the CBC is frequently used as a diagnostic test. In many cases, it can confirm or rule out certain conditions and may be used alongside other tests to arrive at a definitive diagnosis.
- Monitoring is the process of following a patient’s condition over time. A CBC can be used to monitor patients who have previously been diagnosed with blood cell disorders. It can help see how a person’s condition has responded to treatment and may be used to watch for side effects of some medical treatments.
- Screening is testing to find health problems before there are any symptoms. In some cases, a doctor may prescribe a CBC as a screening test during routine check-ups.
What does the test measure?
A CBC involves multiple measurements that include the number of blood cells and some of their physical features. A standard CBC includes several elements related to RBCs, WBCs, and PLT that are described in the following sections.
Red blood cell measurements
RBC, also called erythrocytes, carry oxygen from your lungs to the tissues and organs in your body. A CBC test includes several basic measurements of RBCs:
- RBC count is the total number of RBCs in your blood sample.
- Hemoglobin measures the amount of this oxygen-carrying protein that is found inside RBCs.
- Hematocrit measures the proportion of your total blood volume that consists of RBCs.
A CBC also provides details about the physical features of RBC. These are known as RBC indices, of which there are several kinds:
- Mean corpuscular volume (MCV) is a measurement of the average size of RBC.
- Mean corpuscular hemoglobin (MCH) is the average amount of hemoglobin inside each RBC.
- Mean corpuscular hemoglobin concentration (MCHC) is a calculated measurement of how concentrated hemoglobin is within RBC.
- Red cell distribution width (RDW) is a measurement of the variation in the size of your RBC.
The CBC may include the reticulocyte count, the total number of newly released young RBCs in your blood sample. It may also be measured as a percentage.
White blood cell measurements
WBCs, also called leukocytes, are an important part of the body’s immune system.
A standard CBC includes measures of the WBC count, which is the total number of WBCs in a sample of blood.
A common variation of the CBC is the CBC with differential. The WBC differential is a breakdown of the amount of each of five different types of WBCs:
- Neutrophils: Neutrophils make up the greatest percentage of WBCs and are produced by the bone marrow to fight a diverse array of inflammatory and infectious diseases.
- Lymphocytes: Lymphocytes such as B-cells and T-cells are found primarily in the lymph system and fight bacteria and other pathogens in the blood.
- Monocytes: Monocytes work in conjunction with neutrophils to combat infections and other illnesses while removing damaged or dead cells.
- Eosinophils: Eosinophils are WBCs activated in response to allergies and some types of infections.
- Basophils: Basophils are involved in the early identification of infections as well as wound repair and allergic reactions.
Initial blood testing may include a CBC with differential, or this test may be done after an abnormal initial standard CBC. Because each WBC type has a different function, the CBC with differential can be used to identify abnormal levels of specific WBCs, which may offer clues about an underlying health concern.
Platelets (PLT), also called thrombocytes, are cell fragments that circulate in the blood and play an essential role in blood clotting. When there is an injury and bleeding begins, PLT help stop bleeding by sticking to the injury site and clumping together to form a temporary plug.
A standard component of the CBC is the PLT count, which is the number of PLT in your blood sample.
In some cases, your doctor may have the laboratory also measure the mean PLT volume (MPV), which determines the average size of PLT.
When should I get a complete blood count?
The CBC is a very common test. You may have a CBC performed when you have a routine health examination, are being diagnosed or assessed for a disease or condition, or when your provider is monitoring the effectiveness of a treatment.
Because blood counts can be affected by a diverse range of health conditions, the CBC may be recommended by your doctor to help identify the cause of many different kinds of symptoms. Your health care provider is in the best position to address whether a CBC is appropriate in your specific situation.
Finding a Complete Blood Count Test
How can I get a complete blood count test?
A CBC requires a needle blood draw and is most commonly conducted by a licensed professional in a health care setting like a hospital or doctor’s office.
Can I take the test at home?
There are few options available for at-home CBC testing. Kits for use at home involve a fingerstick; you prick your finger with a small needle to produce a drop of blood. That blood is applied to a special test paper and is then sent to a lab.
After the lab is done analyzing the blood, it provides results either online or through a smartphone app. Results usually take two to three days after the sample is received.
These at-home tests do not provide a diagnosis or confirm a specific illness or disease. Diagnosis and disease confirmation can only be provided by a doctor. For this reason, it is important to talk to a doctor before and after taking an at-home blood count test.
How much does the test cost?
The cost of a CBC test depends on several factors, including whether or not the patient is paying out of pocket or has health insurance. Because the CBC is common and has many uses, the cost is often covered by insurance. However, there may still be charges for copays and deductibles.
Talk to your doctor or insurance provider for specific details about expected costs for a CBC. If you do not have insurance, a hospital or laboratory administrator may be able to provide information about typical costs for uninsured patients.
Taking a CBC
The CBC is ordered and conducted by a licensed professional. The blood sample is drawn from a vein, usually on the inside of your elbow.
Before the test
Unless specified by your provider, there is no special preparation required before a CBC.
In some instances, a CBC is done along with other blood tests. When you are getting more than one blood test, your provider may ask you to not eat anything for a certain amount of time prior to your test.
If you have questions or concerns about any test preparation, contact your health care provider for specific instructions.
During the test
There are several steps that you can expect during a needle blood draw for a CBC:
- An antiseptic alcohol wipe is used to cleanse your arm in the area that the needle will be inserted. This is most often either inside the elbow or at the top of the hand.
- To make the vein in your arm more visible and easier to access with a needle, a band called a tourniquet is tied around your upper arm.
- A needle is placed in your vein, and a test tube attached to the needle is filled with blood. When the needle is inserted there may be a pinch or a little pain.
- After the test tube or vial is filled, the needle is removed, and the test is over.
After the test
After the needle is removed, a bandage will be placed over the puncture site.
Slight bruising is a common side effect after any blood draw while dizziness or lightheadedness are less common. Your provider may have you stay for a few minutes after the blood draw to monitor you until they are sure you are safe to walk and/or drive.
If you notice any other effects or signs of bleeding or infection after your blood draw, it is important to promptly contact your health care provider.
Complete Blood Count Test Results
Receiving test results
Depending on the laboratory equipment that is used, the results from a CBC can be available in a few minutes to a few days after the blood sample arrives at the laboratory. You may receive a copy of your results by mail or through an electronic health portal. Your doctor may also call you to discuss the results or to schedule an appointment to review them together.
Interpreting test results
The results from a CBC test will include separate levels listed for each component of the test including RBC, WBC, and PLT.
For each test component, the report will also show a reference range that defines what the laboratory that performed the test considers normal. These ranges, which can vary from lab to lab, are based on the results of a large sample of healthy people and help your doctor determine typical and atypical blood levels.
It is essential to talk with your doctor about the significance of your CBC test results. Your doctor will review your levels and how they compare to the laboratory’s reference ranges. They may also look at the relationships between your different blood levels and will consider your current symptoms and health history.
The following sections describe some potential causes of high or low levels of RBCs, WBCs, or PLT, but it is important to remember that an abnormal test result is not always a sign of a medical problem. Some healthy people may have blood counts that fall outside the standard reference range.
Red blood cell measurements
RBCs carry oxygen through the body, and the RBC count shows the total number of RBCs found in your blood. Hematocrit and hemoglobin are other related measures.
Anemia is a condition marked by low levels of RBCs. There are many potential causes of abnormally low levels of RBCs, hematocrit, and/or hemoglobin, including:
- Excessive acute or chronic bleeding
- Destruction of RBCs, such as in a condition called hemolytic anemia
- Disorders that affect the bone marrow, which produces new RBCs
- Nutritional deficiencies such as low iron, folate, or vitamin B12
- Some types of cancer and cancer treatment
- Chronic illnesses involving inflammation or disrupted organ function
High levels of RBCs, hematocrit, and/or hemoglobin can also have a number of possible causes. Examples include:
- Heart or lung disease that reduces oxygen levels
- Certain kinds of kidney diseases
- Cigarette smoking
- Polycythemia vera, a rare disease causing overproduction of RBCs
In many cases, the CBC will include a set of measurements known as RBC indices that include the MCV, MCH, and MCHC. These indices can help distinguish between the most likely causes of anemia.
RBC counts are influenced by many different systems of the body, and sometimes abnormal levels are related to more than one factor. Your health care provider is in the best position to explain what the RBC measurements on your CBC mean for your health.
White blood cell counts
WBCs are the main players of the immune system, and the WBC count is the sum total of five different kinds of WBCs; each plays a role in immune function.
A low level of WBCs is known as leukopenia. Some of the possible causes of leukopenia include:
- Liver damage, including from alcohol abuse
- Severe infections
- An enlarged or damaged spleen
- Autoimmune diseases
- Conditions that disrupt bone marrow function
- Certain medications, including many chemotherapies for cancer
Having too many WBCs is known as leukocytosis. Excess WBCs may be related to several potential causes:
- Tissue death from injury, burns, or other physical trauma
- Allergies and autoimmune conditions
- Some medications
- Leukemia and some other cancers
If you have a CBC with WBC differential, your test report will show the specific levels of each kind of WBC. These are known as neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Knowing the counts of each type of WBC can provide detailed information to help your doctor interpret the findings of your CBC.
PLT are responsible for clotting the blood. Abnormal PLT function can mean you bleed too easily because your blood doesn’t clot properly, or your blood is prone to excess clotting.
Thrombocytopenia, a low PLT count, puts you at risk of excessive bleeding or bruising. Possible causes of a low PLT count include:
- Blood disorders that cause PLT destruction
- Some medications, including many chemotherapies
- An enlarged spleen
- Damage to the bone marrow
Having too many PLT is called thrombocytosis, and it is associated with a higher risk of cardiovascular complications from blood clots. Abnormally high PLT counts may be caused by several conditions including:
- Iron deficiency
- Some cancers
- Bone marrow dysfunction
- Recent infection or other tissue trauma
The test may also measure the MPV (the size of your PLT). The MPV level can help your doctor understand how rapidly your body is producing new PLT.
Examples of questions that you can ask your doctor to learn more about your CBC test results include:
- What does my CBC indicate about my health?
- Were any results from the test abnormal? If so, which levels were abnormal?
- Are there any diagnoses to be made based on my CBC results?
- Will any follow-up tests be needed based on my CBC results?
- Given my CBC results, is there anything that you would suggest I do to improve my health?
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