complete blood countOne of the most common tests a doctor would ask you to get is a CBC (Complete Blood Count) test. This is advised to measure the counts and concentration of various components of your blood – the red blood cells, white blood cells, platelets, haemoglobin and more. 

Your doctor might order a CBC test if:

  • If you experience fatigue, weight loss, fever, bruising etc.
  • If you have abnormal bleeding, to check for its cause and possible effects on the blood cell counts.
  • If you have infections
  • There is a possible diagnosis of certain blood cancers like leukemia
  • Diagnose certain conditions like asthma and allergies
  • You are scheduled for a surgery
  • You are undergoing radiation and chemotherapy for cancer, to check their effects on your blood cells

Read: Blood tests: Understanding the basics

Red blood cell (RBC) count:  Normal values are 4.5 to 5.5 million cells/cubic mm. Since RBCs are essential to carry oxygen to different parts of the body, reduced levels mean decreased oxygen supply to the body leading to fatigue and weakness (anaemia). Low levels of RBCs are seen in iron deficiency anemia, thalassemia, stomach ulcers, sickle cell anemia and certain cancers.  If the levels of RBCs are increased, they stick to each other, form clumps and block 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 white blood cells are a group of 5 cells that help to protect your body from infection. The five cells are neutrophils, lymphocytes, basophils, eosinophils and monocytes. The normal WBC count is 4000 – 11000 cells/ cu mm. A value above 11000 is termed as leucocytosis and a value less than 4000 is termed as leucopenia.

When your WBC counts are high, the first thing to rule out is infection.  Sometimes very high count could indicate leukemia, a type of blood cancer. Slightly elevated counts can also be seen in pregnancy or in children below the age of 2 years, which is absolutely normal.

 Normal % of each type of WBC:

Neutrophil  40-70%

Lymphocytes  20-40%

Eosinophil   2-6%

Basophil   0-1%

Monocyte  2-8%

If the percentage of neutrophils shows an increase, look out for bacterial infections. Elevated levels of lymphocytes could indicate a viral infection. Increased levels of eosinophils could be an indicator of allergies or parasitic infection.

Low levels of WBC are found in patients who have taken steroids (or medicines containing them) or are undergoing treatment for cancers.  Very often, it could be an indicator of a bone marrow failure.

Platelet count:

Platelets  are the tiny cells in your blood that help in forming blood clots. Normal platelet count is around 1.5 to 4 lakhs/cu mm. A platelet count is a very important investigation especially in malaria and dengue cases. Dengue causes fall in platelets and this can be very dangerous as platelet levels below a certain level could cause bleeding in the brain.


Haemoglobin is the substance in the red blood cells that carries oxygen. A normal haemoglobin count in a male is 13-15 gms/ dl. In a healthy female it is 12 – 14 gms/ dl. A value less than 12 gms/ dl is termed as anemia. This could be either due to deficiency of iron, vitamin B12 or Folic acid. Deficiency of these could be due to a diet deficient in these nutrients, blood loss in your body or your body is unable to absorb these even though you are are taking it in adequate quantities. At times low haemoglobin could be an indicator of some chronic disease.  Low haemoglobin would result in the person complaining of tiredness, swelling of legs, strange desire to eat chalk or cement ( pica)   especially if you have iron deficiency anemia and sometimes even tingling and numbness in legs in case of Vitamin B12 or Folic acid deficiency which is called as megaloblastic anemia.

Peripheral smear:

Sometimes, your doctor could also ask for a peripheral smear. In this a drop of blood is taken on a glass slide, spread and stained to see the RBCs, WBCs and platelets. It also helps to look for any parasite in cases of malaria or filaria. In case of anemic patients, the smear also gives information about the type of anemia one is suffering from.

ESR (erythrocyte sedimentation rate):

Normal values are 2-6 mm at the end of one hour.  High values may be seen in infections. Very high values if reported might indicate tuberculosis or multiple myeloma (a form of cancer of the bone marrow).

Preparing for a CBC test:

A technician ties a band around your arm so that the veins on that hand are more prominent. The site of withdrawal of blood is cleaned with an alcohol swab. A new disposable syringe is used for each patient. The vein is entered with the syringe and the blood withdrawn.

After the procedure, you might get a bruise or two. Just maintain some pressure with your other hand on the site for a while. 

If you suffer from bleeding or clotting disorders or are taking medicines like aspirin, warfarin or other blood-thinning agents, the bleeding might take a while to stop. Do tell the doctor and the people at the lab before the test about the same.

 Also read: Blood tests: Understanding the different kinds 

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