The medical profession has come a long long way from just mystical powers. Today we, as doctors rely on clinical skills and investigations alike to diagnose, predict and treat illnesses. Despite our already formidable arsenal of investigations, more and more investigations are discovered everyday. These new findings are based on new associations that are made between a disease and a particular phenomenon that is associated with that disease. Today, we are going to try to understand one such phenomenon. 

In the wards of todays hospitals, one cannot find a single file that does not have a normal hemogram in it. Fundamentally, the hemogram consists of an analysis of the cells of the blood. The cells in the blood are of three types: red, white and platelets. Each of these cells - their number, their size, they colour and even their shape - can tell us a lot about the disease that a patient has. 

In recent times, a new investigation called the Red Cell Distribution Width or RDW has come into light. Research over the past few years has showed that RDW is associated with numerous non communicable disease like cerebrovascular accidents, myocardial infarctions and the like. The aim of this article is to better understand this particular diagnostic and prognostic test. 

The red blood cells, normally, have a biconcave shape and are generally ovaloid. Their biconvex shape helps them navigate through tighter blood vessels in the body as it allows them to be more flexible. Now, where there is a systemic disease, the synthesis of these red blood cells gets impaired and that causes the shape of the cells to change. But obviously, in milder disease, the change in shape will not be so obvious as it is in a severe disease. So how would you measure the degree of change in shape?

This is where RDW comes into play. Let us first understand the formula used to calculate the RDW. 

Red Cell Distribution Width = (Standard Deviation of Red Cell Volume / Mean Cell Volume) x 100

The standard deviation of a particular set of numbers tells you how far from the average, the numbers in that set fall. For example in a class of students if the average score on a test is 70 and the standard deviation is high, then it means that many students had scores a lot higher than and a lot lower than 70. Similarly, if the standard deviation is low, then it means that the most of the scores in that class fall close to 70. 

The normal red cell distribution is approximately : 11-15 percent. 

So imagine if the standard deviation of the red cell volume. It indicates how much change in shape there is from the normal. 

Now then if we look at this formula again: 

Red Cell Distribution Width = (Standard Deviation of Red Cell Volume / Mean Cell Volume) x 100

We see that the RDW formula is actually a percentage. It is basically the percentage of change from the mean red blood cell volume. Again, remember that it not a measure of how much the volume has changed. It is a measurement of how much the volume has shifted from the middle. 

Lets try to understand that. 

Consider the example if megaloblastic anaemia which is caused by B12 or folic acid deficiency and iron deficiency anemia. In both these cases, the RDW is high. But in iron deficiency anemia, the actual cells are small where as in megaloblastic anemia, the actual cells are large. This is because the degree of change of size in both conditions is the same even though the actual change in size is different. 

Now, lets look at another example. In cases of anaemia of chronic disease, the mean cell volume is low and the RDW is normal. How does this work? Anemia of chronic disease has multiple causes and has multifactorial etiology and as a result, the cells that come are varied in size. But for the most part, due to its chronicity, all these cells are quite close to the mean cell volume. So the ratio is low and as a result the RDW is normal. 

So RDW is a useful indicator in helping us predict how much the size of a red blood cell has changed, in relative to the majority population of red blood cells. Now, how is this helpful?  This allows us to understand the duration of the disease and also to understand how good or bad the nutritional status of the patient is. It is also serving as a marker for inflammation, both acute and chronic, as they both cause different changes in the body that affect erythropoesis.