The Red Cell Distribution Width (RDW) is a standard measure included in a Complete Blood Count (CBC). The RDW specifically measures the variation in size and volume among a person’s red blood cells (erythrocytes). Normally, red blood cells are all close to the same size, but when production is disrupted, the sizes begin to vary. A high RDW value indicates a greater mix of cell sizes in the bloodstream, a condition called anisocytosis.
Understanding RDW Measurement
The RDW is calculated by a laboratory machine as either a standard deviation (RDW-SD) or a coefficient of variation (RDW-CV) of the red blood cell volume. The RDW-CV is the more common percentage value, representing the width of the cell volume distribution curve relative to the average cell size. A normal RDW suggests that the red blood cells are relatively uniform in size, indicating consistent production by the bone marrow.
An elevated RDW means the distribution of cell sizes is wide, indicating the presence of both smaller and larger cells than the average. This variation occurs because the bone marrow is releasing a mix of newly formed cells that are different sizes from the mature cells already circulating. If the body is struggling to produce normal cells, it might release some that are too small and some that are too large. The resulting wide range of sizes leads directly to a higher RDW value.
Interpreting Results with MCV
The RDW is rarely interpreted in isolation. It is considered alongside other red blood cell indices, particularly the Mean Corpuscular Volume (MCV). The MCV measures the average size of the red blood cells. By combining the MCV (average size) with the RDW (variation in size), clinicians can better categorize the potential cause of a blood issue.
A common pattern is a high RDW paired with a low MCV, which suggests an iron deficiency, where the body produces small, varied red blood cells. Alternatively, a high RDW with a high MCV often points toward a deficiency in Vitamin B12 or folate, as these deficiencies cause the production of fewer but larger red blood cells.
When the RDW is high but the MCV remains within the normal range, it can be an early sign of a nutritional deficiency, such as iron or B12, before the average cell size has shifted significantly. This pattern can also indicate a mixed deficiency where the patient lacks both iron and a B vitamin, causing small and large cells that average out to a normal MCV. Unlike a high result, a low RDW is generally not considered to have clinical significance.
Health Conditions Linked to High RDW
An elevated RDW is frequently linked to nutritional deficiencies, as the lack of specific building blocks impairs the bone marrow’s ability to create properly sized red blood cells. Iron deficiency prevents the red blood cell from filling with enough hemoglobin, resulting in small cells. Deficiencies in Vitamin B12 and folate disrupt DNA synthesis, leading to large, prematurely released cells that contribute to the size variation.
Beyond nutritional issues, a high RDW can be a marker for various chronic diseases that affect red blood cell production or lifespan. Chronic inflammation or oxidative stress, often present in conditions like heart disease, kidney disease, or liver disease, can impair the bone marrow’s function. This impairment results in a less uniform population of red blood cells, driving the RDW upward.
The RDW helps differentiate between certain inherited blood disorders and common deficiencies. For example, in uncomplicated inherited conditions like a thalassemia trait, the red blood cells are uniformly small, often resulting in a normal RDW. Conversely, iron deficiency anemia typically shows an elevated RDW because of the wide mix of cell sizes. This difference in RDW can guide further diagnostic testing, even when both conditions show a low MCV.