Complete Blood Count (CBC) Test : Purpose, Preparation, Procedure & Normal Range of Results

CBC or Complete Blood Count, is a common blood test done to diagnose any kind of infection, anemia, or anything in your body that is causing your symptoms.

Updated: March 3, 2019

CBC or Complete Blood Count, is a common blood test done to diagnose any kind of infection, anemia, or anything in your body that is causing your symptoms.

Why a CBC test is done?

There are many reasons for prescribing a CBC test. These include:

  • If you experience fatigue, weight loss, bruising or fever.
  • If any kind of infection is suspected in your body.
  • In case certain blood related cancers like leukemia is suspected.
  • If you suffer from abnormal amounts of bleeding, such as if you are bleeding much more than normal during your periods.
  • To diagnose certain conditions like asthma and allergies.
  • If you are undergoing radiation and chemotherapy as a part of any kind of cancer treatment, this test is done to check if and how the therapy is affecting on your blood cells.
  • If you are scheduled for a surgery.

How a CBC test is done?

A band will be tied by a technician around your arm so that the veins of the hand are more prominent to prick the injection. The vein is punctured with a fresh, disposable syringe and the blood is withdrawn after the site is cleaned with an alcohol swab.
A cotton swab will be given to you after drawing the blood to place on the place where the injection was pricked. You will be asked to apply pressure on the cotton in order to stop the bleeding. You might notice a bruise in the place where the injection was pricked on your skin. This is normal and should fade away in a few days.
Before you undergo this test, you should inform the the lab technician if you suffer from a bleeding or clotting disorder or are taking medicines like aspirin, warfarin or other blood-thinning agents, as in such cases the bleeding might take a while to stop.

What are the parameters and what do they mean?

The parameters in CBC blood test are:

  • Red blood cell (RBC) count
  • White blood cell (WBC) count
  • Platelet count
  • Haemoglobin
  • Peripheral smear
  • ESR (erythrocyte sedimentation rate)


Red blood cell (RBC) count:

The normal range of RBC is 4.5 to 5.5 million cells/cubic mm. RBCs are essential to carry oxygen to different parts of the body. Low levels of this blood cell indicate decreased oxygen supply to the body leading to fatigue and weakness or anaemia. Low levels of RBC is an indication of

  • Iron deficiency anemia
  • Thalassemia
  • Stomach ulcers
  • Sickle cell anemia
  • Certain cancers

If the levels of RBCs are higher than normal, the cells stick to each other forming clumps which blocks the blood vessels. High levels are seen in case of chronic smokers, alcoholics, people with long-term lung, kidney, heart or liver disease. It may also be increased in cases of dehydration, burns, diarrhoea and vomiting.

White blood cell (WBC) count:

The normal range of WBC is 4000 to 11000 cells/ cu mm. A value above 11000 is termed as leucocytosis where as a value less than 4000 is termed as leucopenia. White blood cells are consist of 5 cells that help to protect your body from infection. The normal range of each of those components are listed below:

Neutrophil        40 to 70%
Lymphocytes    20 to 40%
Eosinophil        2 to 6%
Basophil           0 to 1%
Monocyte         2 to 8%

The aim is to rule out any infection as the cause when your WBC counts are high. A slightly elevated count can be seen in pregnancy or in children below the age of 2 years, which is absolutely normal where as very high count could indicate leukemia. Elevated levels of each of the components in WBC indicates different type of infection.

  • Elevated levels of neutrophils could be an indication of a bacterial infections.
  • Elevated levels of lymphocytes could indicate a viral infection.
  • Increased levels of eosinophils could be an indicator of allergies or parasitic infection.

On the other hand low levels of WBCs are commonly seen in people who are either being treated with medications that contain steroids, are undergoing treatment for cancer or could indicate a malfunctioning of the bone marrow.

Platelet count:

The normal range is 1.5 to 4 lakhs/cu mm. Platelets are the tiny cells in your blood that help in the formation of blood clots. This count is especially important to determine malaria or dengue. An extremely low level of platelets in the body could lead to hemorrhaging and bleeding into the brain. Therefore this count is especially important in the case of dengue. Platelet count should be closely monitored if you are suffering from dengue.

Haemoglobin:

The normal range of hemoglobin count in case of a male is 13 to 15 gms/ dl where as in a healthy female it is 12 to 14 gms/ dl. Hemoglobin is the substance in your red blood cells that carries oxygen. A value less than 12 gms/ dl is an indication of anemia, which could be either be due to deficiency of iron, vitamin B12 or folic acid. The deficiency of these vitamins could be either due to a diet deficient in these nutrients or even though you are are taking it in adequate quantities your body is unable to absorb these. Blood loss in your body could be another reason for deficiency of these vitamins. A low hemoglobin count could also be an indicator of a chronic disease. A person complaining of tiredness, swelling of legs, strange desire to eat chalk or cement could have low hemoglobin count . These could be due to iron deficiency anemia. In case of Vitamin B12 or folic acid deficiency which is called as megaloblastic anemia, there will be complaint of tingling and numbness in legs.

Peripheral smear:

A drop of blood is taken on a glass slide, spread and stained to see the RBCs, WBCs and platelets in this test. This test helps to look for the presence of parasite in cases of malaria or filaria. Also the smear will also gives information about the type of anemia if you suffer from anemia.

ESR (Erythrocyte Sedimentation Rate):

The normal range is 2 to 6 mm at the end of one hour. High values are indication of infections. If the values are very high, it might be an indication of tuberculosis or multiple myeloma which is a form of cancer of the bone marrow.

CBC Test Result:

Here are the normal results for adults, but different labs may show little variations:

Blood componentNormal levels
red blood cellFor men: 4.32-5.72 million cells/mcL
For women: 3.90-5.03 million cells/mcL
hemoglobinFor men: 135-175 grams/L
For women: 120-155 grams/L
hematocritFor men: 38.8-50.0 percent
For women: 34.9-44.5 percent
white blood cell count3,500 to 10,500 cells/mcL
platelet count150,000 to 450,000/mcL


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