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Complete Blood Count Test: Why and How is it Done?

Medically reviewed by डॉ. स्नेहल सिंह · होम्योपैथी · Wellness Online Clinic Healing Arts

Written by Nikita · Updated 02/09/2021

Complete Blood Count Test: Why and How is it Done?

What is the Complete Blood Count Test?

A complete blood count test or CBC test shows the overall blood cell counts and detects health conditions if any.

A complete blood count test is important to understand the functioning and amount of blood cells in the body that help to analyse the overall health. The CBC test results show the increase or decrease in blood cells along with the approximate normal CBC levels according to age and gender.

However, if there are low blood cells, it is a sign of a health condition that needs to be treated. The doctor reads the test results and suggests proper medication, treatment, or any health recommendations.

A CBC test can help detect many health conditions ranging from anaemia to infection and even raise the suspicion of serious disorders like cancer.

The complete blood count (CBC) test measures the changes in blood cell levels while a doctor estimates the overall physical fitness and detects health issues. CBC measures three basic blood cell types such as Platelets, Red Blood Cells and White Blood Cells.


Experts find platelets help control bleeding and blood clot. Doctors explain when a person experiences a cut, the platelets help stop the bleeding. However, if there are any changes in platelet levels, it can risk excessive bleeding and may be a sign of severe medical issues.

Red Blood Cells

Red blood cells remove carbon dioxide and supply oxygen throughout the body. A CBC test measures two things about red blood cells like Hematocrit that finds the percentage of RBC in the blood and Haemoglobin, an oxygen-carrying protein. According to health experts, low hematocrit and haemoglobin levels are often considered as signs of anaemia, a condition that occurs when the body lacks iron.

White Blood Cells

White blood cells in the body help fight infections. A complete blood count test measures white blood cell types and numbers in the body. Any uncommon increase or decrease in white blood cell numbers and types could be a sign of body inflammation, infection, or cancer.

Why is the Complete Blood Count Test Done?

According to health experts, a complete blood count test can be done for numerous reasons like to:

  • Understand overall health: A health expert may suggest a complete blood count test as one of the parts of a person’s general medical examination. This medical examination looks at overall fitness and finds health disorders if any.
  • Monitor medical treatment: A CBC test helps to observe a person’s health if he or she is using a medicine that may affect RBC, WBC, and platelets.
  • Review a health condition: If a person is suffering from a blood disorder that affects the overall blood cell counts, a doctor may use a CBC test to monitor the condition frequently.
  • Diagnose a health condition: Experts may recommend taking a CBC test if a person is experiencing excess bleeding, weakness, bruising, fever, inflammation, and fatigue. A CBC can help detect the signs and causes of body inflammation.

Read More : FBS Test (Fasting Blood Sugar): Why and How is it Done?

Prerequisites for the Complete Blood Count Test

Normally, a person can eat or drink before undergoing a complete blood count test. However, a person may be required to fast or follow other procedures if he or she is undergoing other tests too.

Let your doctor know about the medicines or drugs you take daily to prevent any changes in the CBC test. Also, a person should inform if he or she underwent a blood transfusion or is used to smoking. It is because these things can affect the CBC results and the doctor may give important instructions regarding these before taking the test.

Read More : Urinalysis (Urine Test): Why and How is it Done?

Understanding Complete Blood Count Test Results

The results of the complete blood count test depend on the blood cell counts. Below is the list of normal levels of each blood cell that you should:

White Blood Cells

  • Normal levels: 3,500 to 10,500 cells/mcL


  • Normal levels: 1,50,000 – 4,50,000/mcL

Red Blood Cells

  • Normal levels in men: 4.32 – 5.72 million cells/mcL
  • Normal levels in women: 3.90 – 5.03 million cells/mcL


  • Normal levels in men: 135 – 175 grams/L
  • Normal levels in women: 120 – 155 grams/L


  • Normal levels in men: 38.8 – 50.0 per cent
  • Normal levels in women: 34.9 – 44.5 per cent

Experts believe a complete blood count test is not a definitive diagnostic test. The increase or decrease in blood cells could be an indication of any specific health condition. Health conditions that can cause CBC abnormality and may require additional examinations:

  • Bone marrow failure
  • Leukaemia
  • Autoimmune disorders
  • Anaemia
  • Dehydration
  • Long-term or short-term medicines
  • Vitamin and mineral deficiencies
  • Infections
  • Inflammation
  • Cancer
  • Abnormal bone marrow development
  • Antibiotics reactions
  • Chemotherapy reactions
  • Thalassaemia

If a CBC test suspects these conditions, the doctor may recommend other tests to confirm the suspected health disorder.

When should the Complete Blood Count Test be Repeated?

If CBC test is done to monitor a particular health condition or effectiveness of medical treatment, the test should be repeated according to the doctor’s advice. For specific health conditions, doctors may advise repeating CBC after the treatment course is completed to confirm recovery.

For routine medical health evaluation, CBC may be performed regularly with other routine health tests or as a part of annual health check-up. Some doctors advise going for medical evaluation every three or six months to keep an eye on the overall health and fitness, depending on your health.

So, always consult a doctor, before and after undergoing a blood test and get proper medicines and treatment. If there is any increase or decrease in blood cells, take proper medication, home remedy, and follow lifestyle changes to bring blood cells to a normal level.

Read More : Glucose Challenge Test: Why and How is it Done?

Procedure of the Complete Blood Count Test

A CBC test requires a lab technician who takes a small sample of blood from a vein in your arm. The test procedure takes a few minutes.

A technician:

  • Cleans the area with an antiseptic wipe
  • Ties an elastic band around the upper arm to swell vein with blood
  • Injects a needle and draws small amount of blood in one or more vials
  • Removes the elastic band from the upper arm
  • Applies a small bandage on the area
  • Labels the sample and sends it to the lab for analysis

Doctors believe a blood test may be a little uncomfortable. Some people may feel a pinch or pricking sensation while inserting the needle. Few individuals also feel lightheaded when they are about to get injected or see blood.

Most complete blood count results are available within a few hours.

For children and infants, a nurse normally sterilises the heel of the foot and with a small needle called a lancet pricks the area. Then, the nurse squeezes the heel gently to let blood flow outside and collects it in a vial.



Medically reviewed by

डॉ. स्नेहल सिंह

होम्योपैथी · Wellness Online Clinic Healing Arts

Written by Nikita · Updated 02/09/2021

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