The results of a complete blood count (CBC) include the Red Cell Distribution Width-Coefficient of Variation, or RDW-CV. This specific value measures the consistency in the size of your red blood cells, the oxygen carriers in your body. This article explains what a low RDW-CV result means for your health.
What RDW-CV Measures
The Red Cell Distribution Width (RDW) quantifies the variation in the volume of red blood cells. The “CV” stands for Coefficient of Variation, a statistical calculation used to standardize the measurement by factoring in the average cell size. The RDW-CV reflects the degree of size difference among red blood cells in the bloodstream.
A high RDW-CV indicates anisocytosis, where the red blood cells are highly varied in size. This size disparity often signals a production issue in the bone marrow, potentially due to a nutritional deficiency. Conversely, a low RDW-CV suggests the opposite: a highly uniform or homogeneous population of cells.
When the RDW-CV is low, the red blood cells circulating throughout the body are remarkably similar in volume. This consistency suggests the bone marrow is producing cells that are nearly identical in size. This indicates a stable and predictable cell production process. The normal reference range for RDW-CV is typically between 11.5% and 15.4%, though this can vary between laboratories.
The Significance of a Low RDW-CV Result
A low RDW-CV reading is less common than an elevated one and is generally not considered an independent cause for alarm. This result signifies that the red blood cells are exceptionally uniform, with little difference between the largest and smallest cell volumes. This finding is often interpreted as a positive sign of well-regulated red blood cell production.
A slightly low RDW-CV is often considered clinically insignificant, especially if all other CBC metrics are within the normal range. Unlike a high RDW, which often points toward dynamic issues like iron or vitamin B12 deficiencies, a low RDW rarely suggests a problem with nutritional status. The low value simply reflects a tight distribution of cell sizes.
The primary significance of a low RDW-CV arises when it is considered alongside other specific red blood cell indices. Medical professionals evaluate the RDW-CV in the context of the Mean Corpuscular Volume (MCV), which measures the average size of the red cells. This combined analysis helps pinpoint the underlying cause when a condition is present.
Conditions Associated with Low RDW-CV
To interpret a low RDW-CV accurately, it must be cross-referenced with the MCV result, which categorizes cells as small (low MCV), normal (normal MCV), or large (high MCV). The most common scenario where a low RDW-CV is diagnostically relevant is when the cells are also uniformly small, or microcytic.
A low RDW-CV combined with a low MCV is a classic pattern associated with certain microcytic anemias, specifically the carrier or minor form of thalassemia. In Thalassemia minor, a genetic defect causes the body to produce red blood cells that are consistently smaller than average. This results in an exceptionally uniform population of cells, which drives the RDW-CV down.
A low RDW-CV paired with a normal MCV is often the most benign finding, representing a healthy, stable red blood cell population. This combination rarely indicates a disease state and usually requires no further investigation. The combination of a low RDW-CV with a high MCV (uniformly large cells) is exceedingly rare. It is not associated with any established hematologic disorder and sometimes points to a technical or laboratory anomaly.
If a low RDW-CV is found, especially with a low MCV, the next step involves specialized testing. These tests, such as hemoglobin electrophoresis or iron studies, help confirm or rule out conditions like thalassemia. The RDW-CV is only one piece of the complete blood picture, and a physician must analyze all results together to determine a diagnosis and course of action.