FULL BLOOD COUNT: UNDERSTANDING FBC

Full blood count (FBC)

Called complete blood count (CBC) in the USA. Before the advent of modern haematology blood analysers the blood count consisted of a Hb concentration (estimated using a manual colorimetric technique), a white cell count and manual platelet count. Other parameters such as MCV had to be mathematically calculated (derived) using the measured variables Hb, RCC and PCV.


Modern analysers use a variety of methods to provide a huge range of FBC variables including electronic impedance, laser light scatter, light absorbance and staining characteristics. The resultant FBC provides measured variables such as Hb, PCV and RCC along with derived (mathematically) MCV, MCH and MCHC. These machines also provide automated platelet counts and a 5-part differential WBC.
Sample: peripheral blood EDTA; the sample should be analysed in the laboratory within 4h, if possible.

Main parameters measured


1. Hb concentration.
2. Red cell count (RCC).
3. MCV.
4. MCH.
5. MCHC.
6. Haematocrit (Hct) or PCV.
7. Red cell distribution width (RDW).
8. White cell count.
9. WBC differential.
10.Platelet count.

Some machines are even more sophisticated and will measure reticulocyte counts in addition to  determination of reticulocyte Hb and MCV.


Role of the FBC

Why ask for a FBC? How will this aid the diagnosis or management of the patient? The FBC assesses several different parameters and can provide a great deal of information. The red cell variables will determine whether or not the patient is anaemic. If anaemia is present the MCV is likely to provide clues as to the cause of the anaemia. The white cells are often raised in infection—neutrophilia in bacterial infections and lymphocytosis in viral (but not always so). Platelets (size or number) may be abnormal either as a direct effect of underlying blood disease or may simply reflect the presence of some other underlying pathology. Most of us take a somewhat cursory glance at the FBC when the report arrives on the ward or in clinic, but a more detailed look may reveal a great deal more!

FBC parameters


Haemoglobin concentration (Hb)
Units: g/dL or g/L (Europe uses SI units; the USA still uses g/dL or
grams%).