The test is done using about 2 ml of blood withdrawn from a vein most probably in the patient's arm. The blood will then undergo laboratory testing where most of the components in the blood gets analysed automatically using a device and some manually by expert medical laboratory technicians. The analysis will give rise to a set of values which will then be compared with the normal range of such values by a physician in arriving at a diagnosis or assessing the status of a disease process.

The following are some components of the CBC and brief explanation of what it can demonstrate with regard to the condition of the body.

Red blood cells: This will show the amount of Red Blood Cells that is present in a standard measure. The physicians are capable of identifying abnormal blood counts that may appear when the production is low or excessive cells are being produced. The follow up for this measure should be a blood picture that will further explain the abnormality.

Haemoglobin (Hb) - This indicated the oxygen carrying capacity of the blood and lower levels will suggest a diagnosis of anaemia and further investigations. The levels can drop in sudden blood loss as well as blood loss occurring in long term. Higher levels of haemoglobin present in the circulation is also disruptive for its function therefore removal of such blood is indicated.

Packed cell volume (PCV or Haematocrit): The value will give an idea about the level of hydration a patient is having. This would prompt a physician to start on a saline drip or to advice taking more fluids depending on the underlying problem.

MCV (Mean Cell Volume) - The size of the red blood cells matter in its proper functioning. In certain nutrient deficiencies and medical conditions including cancers these levels are disrupted and could be identified in the CBC.

Platelets: A vital component in the CBC which indicated the ability of the body to prevent continuous bleeding from small blood vessels. Low platelet levels means the body is deficient in its capacity to plug a blood leak and make a clot and the possibility of excessive bleeding is high. Platelet count can go down even in normal viral fevers and liver diseases. But, it is vital to monitor its level in Dengue Fever as it can fall to dangerous levels very quickly and cause death in such patients.

White Cell Count (WCC) : The white cells are the components that battle with infections and are maintained by the body according to the immune requirement. Excessively high levels can be seen in infections and excessively low levels can be seen in cancers of the blood as well as certain viral infections.

Neutrophils (Neu): This is just one component of the WCC and will demonstrate a bacterial infection if its proportion has gone up.

Lymphocytes (Lym.): These cells are mostly related with viral infections.

Other components of WCC would be Basophils, Monocytes and Eosinophils, which will increase its numbers in certain disease processes.

Apart from giving a value, modern day CBC machines would be able to give a variation of the cells with in the blood sample and could be made use of in clinical diagnosis. One important thing to keep in mind is, the CBC itself is not adequate enough to arrive at a definitive diagnosis and needs to relate the findings with patient condition, history, examination findings and other relevant investigations.