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Complete Blood Count (CBC) Test

The complete blood count (CBC) test is one of the most commonly done blood tests. To understand this test, it is very important to know that blood consists of two major parts: primarily plasma and cellular elements.

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What is a complete blood count? 

The complete blood count (CBC) test is one of the most commonly done blood tests. To understand this test, it is very important to know that blood consists of two major parts: primarily plasma and cellular elements. This plasma is the part of the blood that is liquid, which allows the blood to flow easily. The other part of the blood consists of cells.

The three major cells in the blood are white blood cells (WBC), red blood cells (RBC), and platelets. And each of these types of cells carries out specific and important functions.

What are the other names for a CBC test?

CBC Test, Complete Blood Count Test, CBC, Blood Count test 


Why is a CBC done?

A Complete blood count is a common blood test that is done to check 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 screen for a variety of disorders, such as Anaemia or Leukaemia.

  • To diagnose a medical condition. Your doctor may order a complete blood count if you are experiencing weakness, fatigue, fever, inflammation, bruising, or bleeding. A complete blood count might help to diagnose the cause of these signs and symptoms. If the doctor suspects that you have an infection, the test can also help confirm that diagnosis.

  • To monitor a medical condition. If you have been diagnosed with a blood disorder that affects your 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 your blood cell counts.


What does a CBC detect?

A Complete Blood Count blood test can help your provider to diagnose a wide range of conditions, disorders, diseases, and infections, including:

  • Anaemia (when there are not enough red blood cells to carry oxygen throughout the body)

  • Bone marrow disorders, such as myelodysplastic syndromes. 

  • Disorders such as agranulocytosis, thalassemia, and sickle cell anaemia.

  • If infections or other problems that cause abnormally low white blood cell count or high white blood cell count

  • There are several types of cancer, including leukaemia and lymphoma.

  • The side effects of chemotherapy and some prescription medications

  • Vitamin and mineral deficiencies.


How Is a CBC Done?

It may be done in many different healthcare settings, including doctors' offices, clinics, urgent care facilities, emergency rooms, hospitals, and outpatient medical laboratories.


The complete blood count is  analysed by drawing a few milliliters (one to two teaspoons) of blood from a vein. Commonly, the sample is drawn from a vein that is visible from the skin, such as a vein on the back of the hand or the inner angle of the elbow (antecubital fossa).


The tourniquet is usually applied to the area proximal to the vein (closer to the centre of the body than the vein itself). This type of technique will make the vein more visible and plump by limiting the blood flow from the vein going back toward the heart. The tourniquet is only applied for a short period of time (a few minutes at the most) and it is removed as soon as the blood is drawn.


The skin overlying the vein is cleaned by using an alcohol pad, and then a needle is inserted through the area of cleansed skin into the vein below where the tourniquet is applied. The blood is then drawn from the vein via the needle by gently pulling the plunger on the syringe or by the connection of the needle to a special vacuum vial that collects the blood up to the mark.


This sample is done inversely for 8 to 10 times and then taken to the laboratory for analysis, where the complete blood count results may be available within hours of collection. Prompt delivery of the blood sample to the laboratory for analysis is very important. The sample that is not delivered in a timely manner may yield inaccurate results.

What Does a CBC Measure?

A Complete Blood Count involves multiple measurements that include the number of blood cells and some of the physical features. A standard CBC includes many elements related to the red blood cells, white blood cells, and platelets that have been described in the following sections.


Red blood cell measurements

They carry oxygen from your lungs to the tissues and organs in your body. A Complete Blood Count test includes several basic measurements of RBCs:

  • The RBC Count is the total number of red blood cells in your blood sample.

  • Haemoglobin analyses the amount of this oxygen-carrying protein that is found inside RBCs.

  • Hematocrit analyses the proportion of your total blood volume that consists of red blood cells. 


A CBC also provides information about the physical features of red blood cells. These are known as RBC indices. And there are several kinds of RBC indices:

  • The Mean corpuscular volume (MCV) is a measurement of the average size of red blood cells.

  • The Mean corpuscular haemoglobin (MCH) is the average amount of haemoglobin inside each red blood cell.

  • The Mean corpuscular haemoglobin concentration (MCHC) is a calculated measurement of how concentrated haemoglobin is within red blood cells.

  • The Red cell distribution width (RDW) is a measurement of the variation in the size of your red blood cells.

White blood cell measurements

White blood cells (WBCs) are also called leukocytes. They are a very important part of the body’s immune system.

A standard CBC analyses the WBC count, which is the total number of white blood cells in a sample of blood.


A variation of the CBC is the complete blood count with differential. The white blood cell differential is a breakdown of the amount of each of five different types of WBCs:

  • Neutrophils: The Neutrophils make up the greatest percentage of WBCs and are produced in 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 they fight the bacteria and other pathogens in the blood.

  • Monocytes: The Monocytes work in conjunction with neutrophils to combat infections and other illnesses while removing damaged or dead cells.

  • Eosinophils: Eosinophils are WBCs that are activated in the 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.

The initial blood testing may include a Complete Blood Count with differential, or this test may be done if an initial standard CBC was abnormal. Each white blood cell 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.

Platelet measurements

Platelets, 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, these platelets help to stop the bleeding by sticking to the injury site and clumping together to form a temporary plug.

Platelet count is a standard component of CBC, , which is the number of platelets in your blood sample.

In some cases, your doctor may also have the laboratory measure the mean platelet volume (MPV), which determines the average size of platelets.

How do you prepare for the test?

If the blood sample is being tested only for a complete blood count (CBC), you can eat and drink normally before the test. If your blood sample will be used for some additional tests, you may need to fast for a certain amount of time before the test. The doctor may give you specific instructions.

What happens during a complete blood count test?

During a CBC test, a lab technician will draw blood from a vein, typically from the inside of your elbow or the back of your hand. The test will take only 3 minutes. The technician:

  • Cleanse your skin with an antiseptic wipe.

  • Placing an elastic band, or tourniquet, around your upper arm helps the vein swell with blood.

  • He inserts a needle into you and collects a blood sample in one or more vials.

  • removing the elastic band.

  • He covers the area with a bandage to stop any bleeding.

  • Label the samples and send them to a lab for analysis.

A blood test can be slightly uncomfortable. When the needle is inserted into your skin, you might feel a prick or pinching sensation. Some people may also feel faint or light-headed when they see blood. Afterward, you might have minor bruising, but it will clear up within a few days.


Are there any risks to the test?

These blood tests are very common and don’t carry any significant risks. You may have a  slight pain like an ant bite when the needle gets inserted, and a small bruise may develop there.


CBC Results

CBC results are given in numbers, and often it depends on certain factors, including sex, age, and medical history. It should be compared to a normal reference range or to previous results to have meaning. Normal ranges for CBCs might vary slightly from lab to lab.


A Complete Blood Count (CBC) usually provides a piece of general information that can give doctors clues to possible health problems. This information from a CBC helps the doctor decide whether other tests or procedures are needed to make a diagnosis. The information might also help the doctor to develop or revise treatment plans.

The doctor who is familiar with your medical history and overall health is the best person to explain your CBC results and what they mean for you.


The common terms used to describe CBC results are:

  • Anaemia —  not enough healthy RBCs or hemoglobin

  • Leukopenia: a low number of WBCs

  • Neutropenia—a low number of neutrophils

  • Leukocytosis—an increased number of WBCs

  • Thrombocytopenia (a low number of platelets)

  • Thrombocytosis - an increased number of platelets

CBC results can be low or high for several reasons. Some of the examples of abnormal CBC results related to cancer are listed below.

Abnormal RBC counts

A low RBC count may be due to:

  • Anaemia is due to prolonged bleeding or blood loss (haemorrhage), a diet lacking iron or certain vitamins, certain types of chemotherapy, blood disorders, or chronic disease.

  • Hodgkin's lymphoma and other lymphomas

  • The cancers of the blood, such as leukaemia and multiple myeloma

  • Some myeloproliferative disorders may

A high RBC count may be due to:

  • Severe dehydration, such as from severe diarrhoea

  • Kidney tumours

  • Lung diseases

  • Polycythemia vera (a myeloproliferative disorder)


Abnormal WBC counts

A low WBC count may be due to:

  • Viral infection

  • A severe bacterial infection

  • Suppression of bone marrow is caused by treatments like chemotherapy or radiation therapy.

  • Bone marrow diseases, such as leukaemia or myelodysplastic syndrome (MDS)

  • Anaemia

A high WBC count may be due to:

  • infection

  • Leukaemia

  • Some myeloproliferative disorders

  • Some types of cancer, such as bronchogenic carcinoma

  • Certain drugs, such as colony-stimulating factors

  • stress, allergies, or tissue injury.


Abnormal platelet counts

A low platelet count may be due to:

  • Some of the cancer types, such as leukaemia or lymphoma,

  • Autoimmune diseases

  • bacterial infection

  • Viral infection

  • chemotherapy or radiation therapy.

  • Having many blood transfusions

  • Certain drugs like these, such as nonsteroidal anti-inflammatory drugs (NSAIDs), including acetylsalicylic acid (ASA, Aspirin, salicylate) and ibuprofen, (Motrin, Advil, Nuprin)

A high platelet count may be due to:

  • prolonged bleeding or blood loss (haemorrhage)

  • Anaemia from low iron levels

  • infection (inflammation)

  • Surgical removal of the spleen (splenectomy)

  • polycythemia vera

  • Some types of leukaemia

CBC test normal ranges

What is the cost of a CBC test?

What is the type of sample required?

This test requires a blood sample.

Who performs a CBC test?

A provider then sends the samples to a lab where a medical laboratory scientist prepares the samples and performs the tests on machines known as analysers.


What other tests might I have along with this test?

Complete Hemogram


FAQs on CBC Test

What is the full form of CBC?

The full form of CBC is Complete Blood Count.      


What are the 5 components of a complete blood count analysis?

The 5 components of a complete blood count include haemoglobin, hematocrit, red blood counts (RBC), or erythrocyte count, white blood cells (WBC), or leukocyte count, and WBC differential counts.


Why is a complete blood count important?

A complete blood count is a common blood test that is done for several reasons, to monitor your overall health-related issues like anaemia, blood cancers, diseases of the immune system, allergies, and thrombocytes. 


Can a CBC detect heart problems?

The CBC test is done to detect CHF (congestive heart failure) since it can diagnose anaemia which is known to induce CHF and create similar symptoms to CHF. CBC analyses the platelets, packed cell volume (hematocrit), and haemoglobin values along with WBC counts help to assuage risks related to coronary heart defects and chances of a heart attack in a patient. 


Does CBC require fasting?

Usually, no special preparation is required for a CBC. But if your healthcare provider ordered other tests on your blood sample, you might need to fast for several hours before the test.

How to book a CBC test at home?

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