The RDW-CV is a calculation based on the distribution curve's width and the mean cell size. It is calculated by dividing the standard deviation of the mean cell size by the MCV of the red cells and multiplying by 100 to convert it to a percentage. A normal range for the RDW-CV is approximately 11.0–15.0%. Because it is a calculation, the RDW-CV is dependent on the width of the distribution curve and the MCV of the red cell population and may not always reflect the actual variation in red cell size. Be aware that:
- A homogenous population of red cells with a narrow distribution curve and low MCV may have an elevated RDW-CV
- A heterogeneous population of red cells with a broad distribution curve and a high MCV may have a normal RDW-CV.
The RDW-SD is an actual measurement of the width of the red cell distribution curve in femtoliters (fL). The width of the distribution curve is measured at the point 20% above the baseline. Since the RDW-SD is an actual measurement, it is not influenced by the MCV and more accurately reflects the red cell size variance. The normal RDW-SD range for adults is 40.0–55.0 fL.
