A routine blood test often includes a Complete Blood Count (CBC), which measures various components of your blood, including red blood cells, white blood cells, and platelets. Among the red blood cell measurements is the Red Cell Distribution Width (RDW). A high RDW value is not a diagnosis in itself, but it serves as an important clue, alerting a healthcare provider to a possible underlying issue affecting the quality of your red blood cells.
Understanding Red Cell Distribution Width
The RDW quantifies the variation in size and volume of your red blood cells (RBCs). Normally, RBCs maintain a relatively uniform size to travel efficiently and deliver oxygen. A normal RDW range typically falls between 12% and 15%, indicating a consistent cell size distribution. A high RDW result means the red blood cells circulating in your bloodstream show a greater-than-normal variation in size, a condition known as anisocytosis. This elevated value suggests a disruption in the normal process of red blood cell production or survival, often because the bone marrow is releasing a mix of very large and very small cells into circulation.
Common Medical Conditions Linked to High RDW
An elevated RDW often acts as an early indicator of developing nutritional deficiencies or certain blood disorders. The body requires specific vitamins and minerals to produce healthy, uniformly sized red blood cells. When these resources are lacking, the resulting cells are of inconsistent size, leading to an increased RDW.
Nutritional Deficiencies
Iron deficiency anemia is a common cause. Without enough iron, the body cannot properly synthesize hemoglobin, and the bone marrow releases smaller-than-average red blood cells (microcytes). The mix of newly produced small cells and older, normal-sized cells causes the RDW to rise dramatically. Deficiencies in Vitamin B12 or folate also cause a high RDW. These vitamins are essential for DNA synthesis, and when insufficient, red blood cell precursors grow abnormally large before dividing, resulting in large, varied cells (macrocytes).
Other Causes
Certain genetic disorders affecting hemoglobin production, such as thalassemia, can also lead to an increased RDW. Chronic health issues like liver disease or kidney disease impact red blood cell production and lifespan. These conditions interfere with hormonal signals for red cell production, increasing size variation. An elevated RDW can also be seen in cases of mixed anemias, where a person has a combination of two different deficiencies simultaneously.
Correlating RDW with Other Blood Markers for Diagnosis
The RDW is rarely interpreted in isolation and is examined alongside other metrics from the CBC. The Mean Corpuscular Volume (MCV) is the most important marker to pair with RDW, as it measures the average size of the red blood cells. By comparing the RDW (variation in size) and the MCV (average size), a healthcare provider can narrow down the potential cause. For example, a high RDW coupled with a low MCV suggests iron deficiency, where the average cell size is small but highly varied. Conversely, a high RDW combined with a high MCV points toward a condition linked to a Vitamin B12 or folate deficiency. In cases where the RDW is high but the MCV is within the normal range, it may suggest a mixed deficiency or an early-stage nutritional deficit.