What a CBC measures
A complete blood count, or CBC, is a simple blood test that counts and describes the three main types of cells in your blood. Red blood cells carry oxygen using haemoglobin. White blood cells fight infection. Platelets help blood clot and stop bleeding. The report also gives measurements of the size and haemoglobin content of red cells, which help doctors understand the type of problem when something is abnormal.
Why doctors order it
A CBC is part of routine health check-ups and antenatal care, and is often ordered when someone has fever, tiredness, weakness, breathlessness, unexplained bruising or bleeding, or weight loss. During the monsoon, doctors commonly use it in patients with fever to look for patterns seen in dengue, malaria or typhoid, especially changes in platelets and white cells. It is also used to monitor people on certain medicines and those with long-term illnesses.
How to prepare
A CBC usually needs no fasting. A small amount of blood is taken from a vein in the arm, and results are often ready the same day. If other tests such as blood sugar or cholesterol are being done at the same time, you may be asked to fast; follow the instructions given by your doctor or the lab. Continue your regular medicines unless told otherwise, and tell the lab about any blood thinners you take.
Red cell values
Haemoglobin (Hb) shows how much oxygen-carrying protein is in your blood. Commonly quoted adult ranges are about 13 to 17 g/dL for men and 12 to 15 g/dL for women, though each lab sets its own range and values differ in children and pregnancy. A low value suggests anaemia, and a high value can occur with dehydration, smoking, lung disease or some blood disorders.
Other red cell values include the red cell count, haematocrit (PCV), MCV, which shows average red cell size, and MCH and MCHC, which reflect haemoglobin in each cell. Small red cells are often seen in iron deficiency or thalassaemia trait, while large ones may point to vitamin B12 or folate deficiency. RDW shows how much red cell sizes vary.
White cell values
The total white blood cell count (WBC or TLC) in adults is commonly about 4,000 to 11,000 cells per microlitre, but labs vary. A high count often occurs with bacterial infections, inflammation, stress or steroid medicines. A low count may be seen in some viral infections, including dengue, and with certain medicines or bone marrow problems.
The differential count breaks white cells into neutrophils, lymphocytes, monocytes, eosinophils and basophils. Neutrophils often rise in bacterial infections, lymphocytes in some viral infections, and eosinophils with allergies, asthma and worm infections. The pattern helps your doctor, but it must always be read along with your symptoms and examination.
Platelets
Platelets are commonly in the range of about 1.5 to 4.5 lakh per microlitre in adults, though labs differ. A low platelet count may be seen in dengue and other viral fevers, some medicines, and liver or blood disorders. Very low counts can increase the risk of bleeding. A high count can occur after infections, with iron deficiency or in some blood conditions. Occasionally, platelets clump in the sample and give a falsely low result, so doctors may repeat the test or check a blood smear.
Reading your report sensibly
Every report shows your value next to the lab's reference range, and values outside it are often marked high or low. Being slightly outside the range does not always mean illness. Results also vary with age, sex, pregnancy, recent illness and hydration. Do not start iron, vitamins or any other medicine based only on a CBC. Your doctor will interpret the whole picture and may order further tests such as ferritin, vitamin B12, a peripheral smear or tests for specific infections.
When to repeat the test
How often you need a CBC depends on your situation. Healthy adults may have it with a routine check-up. During fevers such as dengue, doctors may repeat it daily or every few days to watch platelets. People being treated for anaemia usually repeat it after some weeks to check improvement, and those on certain long-term medicines need regular monitoring. Consult your doctor to understand your results and decide when to test again.
See a doctor urgently if
- Bleeding gums, nosebleeds, blood in urine or stools, or red spots on the skin during a fever
- Severe tiredness, breathlessness at rest, chest pain or fainting
- Fever with severe abdominal pain, persistent vomiting or restlessness
- Frequent or severe infections, mouth ulcers or unexplained weight loss
Tests that may help
This article is general health information, not medical advice. Do not start, stop or combine medicines on your own — always follow your doctor's prescription and ask your pharmacist if unsure.