The most important components of a Full Blood Count report are, of course, the Haemoglobin, the White Cell Count and Differential and the Platelet Count. However, modern haematology machines that produce the FBC results are able to calculate several other derived parameters that provide more information. These are the red cell and platelet indices - MCV, MCH, MCHC, RDW and the MPV and PDW. They can help a lot in diagnosis, but I suspect they are under utilised because many doctors are less familiar with them. Some have only recently been routinely added to FBC reports and their significance may not be well known. The RDW is one example.
Interpretation of the RDW
Microscopic examination of a blood film gives an impression of the variation of red cell size, which is reported as "anisocytosis". This is, however, quite subjective and can only be quantified very roughly. Modern automated electronic haematology instruments are all able to assess the volume of red cells very easily and accurately, allowing a precise measure of this variability of red cell volume. This measure is called the "Red Cell Volume Distribution Width", or RDW for short. This is expressed as a Coefficient of Variation (CV) and is defined as follows:
CV = Standard Deviation of red cell size MCV
MCV LOW
MCV NORMAL
MCV HIGH
RDW HIGH
Iron Deficiency
HbH Disease
S/Beta Thalassaemia HbAC MAHA Severe anaemia of chronic disorders
RDW is a numerical measure of anisocytosis. It may be useful in distinguishing certain causes of anaemia, in particular, in distinguishing Iron Deficiency (RDW raised) from Thalassaemia (RDW usually normal) and in distinguishing Megaloblastic Anaemia (RDW usually raised) and other causes of macrocytosis (RDW more likely to be normal).
Note that only raised RDW are of significance and subnormal values do not occur. The RDW is a more sensitive measure of abnormality in microcytic, rather than macrocytic disorders, and is only of help if the haemoglobin is low.
These blood films illustrate the variation in red cell size in normal blood (left picture) and moderate iron deficiency (right picture). While the normal film shows little variation in red cell size, the iron deficient cells shows variations in size (anisocytosis) and shape (poikilocytosis), as well as microcytosis (low average cell size) and hypochromasia (increased central pallor). The anisocytosis in this film is increased as it is post-transfusion and shows some dimorphism.
