What does Red Cell Distribution Width (RDW) measure?

Red Cell Distribution Width (RDW) is a measure of the variation in size of red blood cells (RBCs) in a sample. A higher RDW indicates a greater variability in RBC sizes, while a lower RDW suggests that the red blood cells are more uniform in size. This measurement is often utilized in the evaluation of different types of anemia; for instance, a high RDW can be indicative of iron deficiency anemia or other conditions where there is a mix of small and large cells due to underlying pathologies.

In the context of the other options, measuring the speed of erythrocyte settlement relates to the Erythrocyte Sedimentation Rate (ESR), which assesses inflammation but does not reflect the size of red blood cells. The volume of plasma in blood is associated with hematocrit and not RDW. Lastly, the amount of white blood cells pertains to a complete blood count (CBC) measurement, which quantifies leukocyte levels, not the distribution of red blood cell sizes. Thus, the focus of RDW on the range of RBC sizes makes it the correct and most relevant choice.