The Red Cell Distribution Width (RDW) is a measurement included in a standard Complete Blood Count (CBC) that provides information about your red blood cells. Specifically, the RDW measures the variation in the size and volume of these oxygen-carrying cells within your blood sample. When this value is reported as high, it signifies a non-uniform population of red blood cells, which is often an early sign that the body’s process for producing or maintaining these cells is experiencing a disruption. Understanding an elevated RDW involves looking at the underlying processes that lead to this variation, from nutritional shortages to more complex chronic conditions.
What a High RDW Measurement Means
A high RDW, often expressed as RDW-CV (Coefficient of Variation), indicates that the red blood cells circulating in your bloodstream are significantly different in size from one another. This condition is technically known as anisocytosis, where the cells range widely from small to large instead of maintaining a consistent size. The percentage value represents how much the cell sizes deviate from the average red cell volume. A normal RDW suggests a uniform population of red blood cells, which is optimal for efficient oxygen transport. An elevated RDW value, generally above 15%, suggests a problem with the body’s ability to consistently produce healthy, mature red blood cells or an issue with their survival.
Causes Linked to Nutrient Deficiencies
The most common reasons for an elevated RDW involve a shortage of essential nutrients required for proper red blood cell formation.
Iron Deficiency
Iron deficiency is a frequent cause, as iron is necessary for producing hemoglobin. When iron stores are low, the bone marrow releases red cells that are smaller than normal (microcytes). These small cells circulate alongside older, normal-sized cells, creating a mixed population that drives the RDW value upward.
B12 and Folate Deficiency
Deficiencies in Vitamin B12 or folate (Vitamin B9) also lead to a high RDW through a different mechanism. These vitamins are required for DNA synthesis during red blood cell maturation. A lack of these nutrients causes red blood cells to become unusually large (macrocytosis) before being released into the bloodstream. The resulting mix of these large cells with existing normal cells creates a significant size disparity, elevating the RDW.
The high RDW is often a sensitive indicator of these nutritional issues, sometimes rising before other markers of anemia are clearly abnormal. The body may also experience a “mixed deficiency,” such as a simultaneous shortage of both iron and Vitamin B12. This situation is complex, resulting in the simultaneous presence of both small and large cells, which leads to a very high RDW value.
Causes Linked to Cell Destruction and Chronic Disease
Beyond simple nutrient deficiencies, an elevated RDW can be triggered by processes that either prematurely destroy red blood cells or disrupt their production due to chronic illness. Hemolytic anemia, a condition where red blood cells are destroyed faster than the bone marrow can produce them, is one such cause. To rapidly replace the lost cells, the bone marrow releases immature red cells, called reticulocytes, which are typically larger than mature red cells. This sudden introduction of larger, immature cells alongside the remaining mature cells creates a significant variation in size, thereby increasing the RDW.
Chronic diseases that affect major organs can also influence red blood cell size and uniformity. For instance, advanced liver disease can lead to changes in red cell membrane components, resulting in abnormally shaped or oversized cells. Similarly, conditions affecting the bone marrow, such as Myelodysplastic Syndromes (MDS), involve faulty cell production. In MDS, the bone marrow produces blood cells that are highly variable in size, making a high RDW a characteristic feature.
Interpreting the Result with Other Blood Markers
A high RDW is rarely interpreted in isolation; it is a clue that directs the physician toward the underlying cause when combined with other values from the CBC. The Mean Corpuscular Volume (MCV), which measures the average size of red blood cells, is the most frequently used co-marker. Analyzing the RDW and MCV together creates specific patterns that help narrow the diagnostic possibilities.
For example, a high RDW paired with a low MCV typically suggests iron deficiency anemia, where the cells are small but highly varied in size. Conversely, a high RDW combined with a high MCV points toward a deficiency in Vitamin B12 or folate, where the cells are large and varied. If the RDW is high but the MCV is within the normal range, it may suggest an early stage of iron deficiency or a case of mixed anemia. A healthcare professional uses this information to determine the next diagnostic steps, such as ordering specific tests for iron, B12, or folate levels.