RDW (Red Cell Distribution Width) is calculated to measure the variation in the size of your red blood cells (RBCs). It is typically expressed as a percentage.
RDW Calculation Explained
RDW is usually calculated by dividing the standard deviation (SD) of the mean corpuscular volume (MCV) by the MCV and multiplying by 100 to yield a percentage value to be on behalf of the RBC size heterogeneity [6]. In simpler terms:
RDW = (Standard Deviation of MCV / Mean Corpuscular Volume) x 100
Here's a breakdown of the components:
- Standard Deviation of MCV (SD): This measures the spread of the red blood cell volumes. A higher SD indicates a greater variation in size.
- Mean Corpuscular Volume (MCV): This represents the average volume of your red blood cells.
Formula Breakdown in Detail
| Component | Description |
|---|---|
| Standard Deviation of MCV | How much the volume of individual red blood cells varies from the average. |
| Mean Corpuscular Volume (MCV) | The average size of your red blood cells. |
| Multiplication by 100 | Converts the result into a percentage, making it easier to interpret. This percentage reflects the degree of variation in red blood cell size. A higher percentage suggests greater variation. |
Practical Example
Let's say:
- Standard Deviation of MCV = 15 fL (fL = femtoliters, a unit of volume)
- Mean Corpuscular Volume (MCV) = 85 fL
Then:
RDW = (15 / 85) x 100 = 17.65%
This result indicates that there's a 17.65% variation in the size of the red blood cells.
Why is RDW Important?
RDW is used, along with other blood cell indices, to diagnose different types of anemias and other blood disorders.
