Red Cell Distribution Width (RDW) Overview

Red Cell Distribution Width (RDW)

• RDW is a calculated index from RBC histograms used to identify anisocytosis (variation in red blood cell size).
• Recent hematology analyzers use two methods to calculate RDW:
  • RDW-CV (coefficient of variation): considers both the width of the red blood cell (RBC) distribution curve and the mean RBC size. Reference range for adults is 11.5% to 14.5%.
  • RDW-SD (standard deviation): measures the actual width of the RBC distribution curve in femtoliters (fL). Reference range is 39 to 46 fL.
• The earliest methods for measuring RDW were dependent on the width of the distribution curve and the mean corpuscular volume (MCV).
• RDW-SD is less affected by MCV and is considered a better measure of red blood cell variability, especially in abnormal conditions.
• The width of the curve in RDW-SD is measured at the point 20% above the baseline, not influenced by MCV.

Reference Ranges

• MCV: 80 to 100 fL
• RDW(CV): 11.5% to 14.5%

Bimodal Curve

• A bimodal curve indicates two distinct populations of red blood cells.
• This can be associated with conditions like anisocytosis, recent blood transfusions and schistocytes.