Do You Have To Fast For A CBC?

Do You Have To Fast For A CBC (Complete Blood Count Test)?

CBC is an abbreviation of the complete blood count which is a blood test carried out to an individual to check the body health and detect a variety of disorder among them leukemia, anemia, and infection among others. Before the test is done you should enquire from your doctor whether you have to fast for a complete blood count test is done.

What is A Complete Blood Count?

A complete blood count (CBC) test measures if the cell is as per expectation, decrease or increase in cell count is a sign of an ailment to the individual which may call for additional test. These are some of the body components of blood which are tested

  • White Blood Cells– Are responsible for fighting infection in our body.
  • Red Blood Cells– Are responsible for distribution of oxygen in our body.
  • Platelets– They help in blood clot individual with few platelets blood for quite long time hence venerable to anemia.
  • Hemoglobin– Is the protein contained in the red blood cells which aid in the transportation of the oxygen across our body.
  • Hematocrit – This the ratio of red blood cells to the plasma or the fluid components in individual blood.

Why is A CBC Test Done?

A different individual has a different reason as to why they like to have CBC done to them, some of the reason include
To Detect Any Medical Condition –  Your physician may recommend complete blood count if you are experiencing unexplained general body weakness, abnormal bleeding, fatigue, inflammation or even fever as these are symptoms more serious ailment in the body so CBC may aid in diagnoses of the ailment.

To Evaluate Your General Body Health – Your personal physician may suggest this test as part of a normal medical examination. This enables him to screen for a variety of condition like leukemia which is a blood cancer.
To Help In Medical Treatment – CBC may be used by medical practitioners to monitor individual health especially when taking medication that may have effects on the blood cells.
Help To Monitor Certain Condition– A complete blood count may be used by doctors to monitor individual diagnosed with a blood disorder, for example, anemia.

Do You Have to Fast for A Complete Blood Count Test?

It is test require fasting, therefore, an individual can drink and eat just normal before the test. Some the test which allows eating before the test is carried out include fecal occult blood test though individuals are discouraged from taking some food such as broccoli and red meat.
There are some other cases whereby the doctor will advise you to avoid taking food only water is allowed. Some of the blood tests that need fasting include:

  • Renal function panel.
  • Low-density lipoprotein level test.
  • Iron blood test.
  • Cholesterol test.
  • Lipoprotein panel.
  • Blood glucose test.
  • High-density lipoprotein level test.
  • Triglyceride level test

How Long Should you Fast before A CBC Test?

The different CBC blood test has a different time frame but the least time of all the blood test is 8 hours with only consuming water before the test while others may require an individual to fast for more than 12 hours.

CBC Test Results

The following are results of normal healthy adults
Red blood Cells Count
Male: 4.32-5.72million cells/ microliter
Female: 3.90-5.03million cells/ microliter
If the results show that the level of red blood cell is lower than normal then it is an indication that the individual as anemia. This condition is characterized by general body weakness and fatigue. Some of the causes of this condition include having a low level of iron. While a higher level of red blood cell may suggest the presence of heart disease or polycythemia Vera.

Male: 135-175 grams/Liter
Female: 120-155 grams/Liter
The results of hemoglobin are related with those of red blood cells so the condition is the same.

White Blood Cells Count
Male: 3,500 to 10,500 cells/ microliter
Female: 3,500 to 10,500 cells/ microliter
If the individual results show a low level of white blood cells it may suggest that the individual as an autoimmune disorder, cancer or bone marrow problem. An autoimmune disorder is responsible for the destruction of white blood cells which led to low white blood cells level. Higher level than normal may suggest the presence of bone marrow disease or an immune system disorder.

Platelets Count
Male: 150,000 to 450,000/ microliter
Female: 150,000 to 450,000/ microliter
Lower than normal platelets count a condition known as thrombocytopenia is a sign of underlying condition which may require medical attention just like higher than normal platelets count a condition known as thrombocytosis.

Male: 38.8-50.0 percent
Female: 34.9-44.5 percent
This is related with the red blood cell; therefore, the conditions are the same.

Purpose of Fasting for A Complete Blood Count (CBC)

Doctor may recommend fasting before CBC is carried out this is because when we consume food and drink like coffee and alcohol they are broken down in our stomach and eventual nutrients are absorbed into our body, these include fats, sugar, minerals, enzymes such as Gamma-glutamyl transferase (GGT), these substances may interfere with the results of testing of condition such as anemia, liver disease, high cholesterol or even diabetes.
If you don’t fast before undertaking these tests the results will eventually be inaccurate. If you happen to eat some snack before the results are carried out, it advisable to let your doctor know so she or he can advise you accordingly.

When Can I Eat After The Test?

Fasting is not an easy thing so it ideal to take some snacks with you while going for CBC test, after the blood has been taken from your body you can take your food immediately since no farther blood will be drawn.
The thing to avoid while going for a complete blood test
Apart from food these are other thing should not be ignored:

  • Smoking – Tobacco contains nicotine which when in the blood may interfere with accuracy of blood results.
  • Chewing gum – Both sugarless and sugar gum is discouraged as they speed up digestion which in turn releases substances in the blood which may interfere with the results.
  • Coffee – Just like other drink it should be avoided as it interferes with digestion.
  • Exercise – Doing exercise can speed up digestion which may release substances into blood.
  • Alcohol Alcohol interferes with blood sugar, therefore, should be avoided before the test is done.


Fasting is not easy but with a little bit of sacrifice in order to get accurate results is worth taking. It is important to ask your doctor whether you have to fast for a complete blood count test because not all blood tests requires fasting.

Medically Reviewed By
Dr. Kaushal M. Bhavsar (MBBS, MD)Assistant Professor in Pulmonary Medicine, GMERS Medical College, Ahmedabad