The Complete Blood Count (CBC) blood test panel provides a snapshot of components circulating in the blood, including red blood cells, white blood cells, and platelets. Among the metrics reported is Red Cell Distribution Width-CV (RDW-CV). This value measures the variation in the size of red blood cells, offering important information regarding the body’s ability to produce uniform red blood cells.
Defining Red Cell Distribution Width-CV
RDW-CV stands for Red Cell Distribution Width—Coefficient of Variation. It is a calculated measure of the heterogeneity, or variation, of red blood cell volume. In healthy circulation, red blood cells are relatively consistent in size. However, certain conditions cause the bone marrow to release cells of widely differing sizes, a variation known as anisocytosis. The RDW-CV quantifies this size variation as a percentage.
The RDW-CV is mathematically derived by dividing the standard deviation of red blood cell volume by the Mean Corpuscular Volume (MCV), then multiplying by 100. Since the formula uses MCV (the average volume of a red blood cell), RDW-CV is a relative measure influenced by the average cell size. This contrasts with RDW-SD (Standard Deviation), which is an absolute measure of cell size variation. A high RDW-CV indicates a significant mix of small and large cells, reflecting a disruption in normal red blood cell production.
How to Interpret RDW-CV Results
The typical reference range for RDW-CV generally falls between 11.5% and 14.5%, though this may vary slightly by laboratory. A result within this range suggests that circulating red blood cells are mostly uniform in size. A normal RDW-CV does not guarantee perfect health, as some conditions cause uniform changes to cell size without increasing variation.
An elevated RDW-CV is the most clinically significant finding, indicating a high degree of anisocytosis (a large spread between the smallest and largest red cells). This increase often occurs when the body attempts to correct a problem, such as a nutritional deficiency, by releasing new cells that differ in size from older cells. Conversely, a low RDW-CV is infrequent and generally considered clinically insignificant, meaning the red cells are exceptionally uniform in size.
The RDW-CV is not interpreted in isolation and must be evaluated alongside the Mean Corpuscular Volume (MCV). The MCV provides the average size of the red blood cells, while the RDW-CV provides the variability around that average. Together, these two metrics create a framework that helps narrow down the potential causes of anemia or other red cell abnormalities.
Conditions Associated with Elevated RDW-CV
An elevated RDW-CV is most commonly associated with various types of anemia and nutritional deficiencies. It provides an early clue that the bone marrow’s ability to produce consistent red cells is impaired. The RDW-CV, particularly when combined with the MCV, is used to differentiate between specific disorders.
High RDW-CV and Low MCV (Microcytic Anemia)
This combination suggests a microcytic anemia, where red cells are smaller than normal but highly variable in size. This pattern is highly suggestive of Iron Deficiency Anemia. The body struggles to incorporate sufficient iron, resulting in a mixed population of small, iron-poor cells and older cells. Hereditary conditions like Thalassemia often present with a low MCV but a normal RDW-CV, which helps distinguish the causes of small red cells.
High RDW-CV and High MCV (Macrocytic Anemia)
This pairing indicates a macrocytic anemia, where the average cell size is larger than normal, and the size variation is high. This combination is characteristic of deficiencies in Vitamin B12 or Folate, which are necessary for DNA synthesis. The lack of these vitamins causes the bone marrow to produce abnormally large, immature cells alongside existing cells, resulting in significant size variation. Myelodysplastic Syndromes can also present with this pattern.
High RDW-CV and Normal MCV
This diagnostic possibility can occur in the very early stages of iron, Vitamin B12, or folate deficiencies. Here, the deficiency begins to affect new cell production, leading to size variability before the average cell size (MCV) changes significantly. This combination is also seen in conditions like Chronic Liver Disease or after a recent hemorrhage. Furthermore, an elevated RDW-CV has been associated with chronic inflammatory states and diseases such as kidney disease, heart disease, and diabetes.