Overview
A complete blood count (CBC) is a blood test used to evaluate your overall health and detect a wide range of disorders, including anemia, infection and leukemia.

A complete blood count test measures several components and features of your blood, including:

Red blood cells, which carry oxygen
White blood cells, which fight infection
Hemoglobin, the oxygen-carrying protein in red blood cells
Hematocrit, the proportion of red blood cells to the fluid component, or plasma, in your blood
Platelets, which help with blood clotting
Abnormal increases or decreases in cell counts as revealed in a complete blood count may indicate that you have an underlying medical condition that calls for further evaluation.

Why it's done
A complete blood count is a common blood test that's done for a variety of reasons:

To review your overall health. Your doctor may recommend a complete blood count as part of a routine medical examination to monitor your general health and to screen for a variety of disorders, such as anemia or leukemia.
To diagnose a medical condition. Your doctor may suggest a complete blood count if you're experiencing weakness, fatigue, fever, inflammation, bruising or bleeding. A complete blood count may help diagnose the cause of these signs and symptoms. If your doctor suspects you have an infection, the test can also help confirm that diagnosis.
To monitor a medical condition. If you've been diagnosed with a blood disorder that affects blood cell counts, your doctor may use complete blood counts to monitor your condition.
To monitor medical treatment. A complete blood count may be used to monitor your health if you're taking medications that may affect blood cell counts.
How you prepare
If your blood sample is being tested only for a complete blood count, you can eat and drink normally before the test. If your blood sample will be used for additional tests, you may need to fast for a certain amount of time before the test. Your doctor will give you specific instructions.

What you can expect
For a complete blood count, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm, usually at the bend in your elbow. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.

Results
The following are normal complete blood count results for adults:

Red blood cell count Male: 4.35-5.65 trillion cells/L*
(4.32-5.72 million cells/mcL**)
Female: 3.92-5.13 trillion cells/L
(3.90-5.03 million cells/mcL)

Hemoglobin
Male: 13.2-16.6 grams/dL***
(132-166 grams/L)
Female: 11.6-15 grams/dL
(116-150 grams/L)

Hematocrit
Male: 38.3-48.6 percent
Female: 35.5-44.9 percent

White blood cell count

3.4-9.6 billion cells/L
(3,400 to 9,600 cells/mcL)

Platelet count

Male: 135-317 billion/L
(135,000 to 317,000/mcL)
Female: 157-371 billion/L
(157,000-371,000/mcL)

* L = liter
** mcL = microliter
*** dL = deciliter
Not a definitive test
A complete blood count is typically not a definitive diagnostic test. Depending on the reason your doctor recommended this test, results outside the normal range may or may not require follow-up. Your doctor may need to look at the results of a CBC along with results of other blood tests, or additional tests may be necessary.

For example, if you're otherwise healthy and have no signs or symptoms of illness, results slightly outside the normal range on a complete blood count may not be a cause for concern, and follow-up may not be needed. Of if you're undergoing cancer treatment, the results of a complete blood count outside the normal range may indicate a need to alter your treatment plan.

In some cases, if your results are significantly above or below the normal ranges, your doctor may refer you to a doctor who specializes in blood disorders (hematologist).

What the results may indicate
Results in the following areas above or below the normal ranges on a complete blood count may indicate a problem.

Red blood cell count, hemoglobin and hematocrit. The results of your red blood cell count, hemoglobin and hematocrit are related because they each measure aspects of your red blood cells.

If the measures in these three areas are lower than normal, you have anemia. Anemia causes fatigue and weakness. Anemia has many causes, including low levels of certain vitamins or iron, blood loss, or an underlying condition.

A red blood cell count that's higher than normal (erythrocytosis), or high hemoglobin or hematocrit levels, could point to an underlying medical condition, such as polycythemia vera or heart disease.

White blood cell count. A low white blood cell count (leukopenia) may be caused by a medical condition, such as an autoimmune disorder that destroys white blood cells, bone marrow problems or cancer. Certain medications also can cause white blood cell counts to drop.

If your white blood cell count is higher than normal, you may have an infection or inflammation. Or, it could indicate that you have an immune system disorder or a bone marrow disease. A high white blood cell count can also be a reaction to medication.

Platelet count. A platelet count that's lower than normal (thrombocytopenia) or higher than normal (thrombocytosis) is often a sign of an underlying medical condition, or it may be a side effect from medication. If your platelet count is outside the normal range, you'll likely need additional tests to diagnose the cause.
For specifics about what your complete blood count results mean if they fall outside the normal ranges, talk to your doctor.

What is a Complete Blood Count?
A complete blood count or CBC is a blood test that measures many different parts and features of your blood, including:

Red blood cells, which carry oxygen from your lungs to the rest of your body
White blood cells, which fight infection. There are five major types of white blood cells. A CBC test measures the total number of white cells in your blood. A test called a CBC with differential also measures the number of each type of these white blood cells
Platelets, which help your blood to clot and stop bleeding
Haemoglobin, a protein in red blood cells that carries oxygen from your lungs and to the rest of your body
Hematocrit, a measurement of how much of your blood is made up of red blood
A complete blood count may also include measurements of chemicals and other substances in your blood. These results can give your health care provider important information about your overall health and risk for certain diseases.

Other names for a complete blood count: CBC, full blood count, blood cell count

What is it used for?
A complete blood count is a commonly performed blood test that is often included as part of a routine check-up. Complete blood counts can be used to help detect a variety of disorders including infections, anaemia, diseases of the immune system, and blood cancers/>.

Why do I need a complete blood count?
Your health care provider may have ordered a complete blood count as part of your check-up or to monitor your overall health. In addition, the test may be used to:

Diagnose a blood disease, infection, immune system and disorder, or other medical conditions
Keep track of an existing blood disorder
What happens during a complete blood count?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?
You don't need any special preparations for a complete blood count. If your health care provider has also ordered other blood tests, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?
A CBC counts the cells and measures the levels of different substances in your blood. There are many reasons your levels may fall outside the normal range. For instance:

Abnormal red blood cell, haemoglobin, or hematocrit levels may indicate anaemia, iron deficiency, or heart disease
Low white cell count may indicate an autoimmune disorder, bone marrow disorder, or cancer
High white cell count may indicate an infection or reaction to medication
If any of your levels are abnormal, it does not necessarily indicate a medical problem needing treatment. Diet, activity level, medications, a women's menstrual cycle, and other considerations can affect the results. Talk to your health care provider to learn what your results mean.

Is there anything else I need to know about a complete blood count?
A complete blood count is only one tool your health care provider uses to learn about your health. Your medical history, symptoms, and other factors will be considered before a diagnosis. Additional testing and follow-up care may also be recommended.