Red Blood Cell Distribution Width (RDW): Definition and Calculation
The red cell distribution width (RDW) is a measurement derived from the red blood cell distribution curves generated on automated hematology analyzers and is an indicator of variation in red blood cell (RBC) size within a blood sample. The RDW is used along with the indices (MCV, MCH, MCHC) to describe a population of RBCs. The RDW measures the deviation of the RBC width, not the actual width or size of individual cells.
The two RDW measurements currently in use are the red cell distribution width - coefficient of variation (RDW-CV) and the red cell distribution width - standard deviation (RDW-SD).
The RDW-CV is a calculation based on both the width of the distribution curve 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 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 not only on the width of the distribution curve but also 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 that is 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.