A high Red Cell Distribution Width (RDW) provides specific information about your red blood cells. It is a standard component of a Complete Blood Count (CBC), a test that evaluates the cells in your blood. The RDW measurement assesses the variation in the size of your red blood cells, also known as erythrocytes. This value does not measure the actual size of the cells but rather how different they are from one another, indicating a notable variety in their sizes.
Understanding the RDW Value
The RDW value gives your healthcare provider insight into the uniformity of your red blood cells. These cells, produced in the bone marrow, are typically similar in size. A standard reference range for RDW is between 12% and 15%, though this can differ by laboratory. A result above this range signifies anisocytosis, the medical term for having red blood cells of unequal size.
It is important to distinguish RDW from the Mean Corpuscular Volume (MCV), another measurement on a CBC. The MCV reports the average size of your red blood cells. In contrast, the RDW measures the distribution or range of sizes. Think of it like a bag of apples; the MCV would tell you the average size of all the apples, while the RDW tells you if the bag contains a mix of both very small and very large apples (a high RDW) or apples that are all roughly the same size (a normal RDW).
A high RDW suggests the body is having difficulty producing red blood cells consistently, which can be an early signal of certain underlying health issues. The measurement is calculated from a graph that plots the range of red blood cell sizes. When cell sizes are similar, they cluster together on the graph, resulting in a low RDW. When sizes vary greatly, they are more spread out, leading to a high RDW.
Potential Causes of High RDW
An elevated RDW is often one of the earliest indicators of a nutrient deficiency, with the most common cause being iron deficiency anemia. In the initial stages of iron deficiency, the bone marrow produces smaller red blood cells. These new, smaller cells mix with the existing, normal-sized cells, creating a significant size variation and increasing the RDW value.
Deficiencies in other nutrients, like vitamin B12 or folate, can also lead to a high RDW. Unlike iron deficiency, a lack of these nutrients results in the production of unusually large red blood cells (macrocytic anemia). The presence of these large cells alongside normal-sized cells increases size variability, elevating the RDW. In about two-thirds of cases involving B12 or folate deficiency, the RDW will be high.
While nutrient deficiencies are frequent culprits, other conditions can cause a high RDW. Chronic liver disease or certain autoimmune conditions may be associated with an increased RDW. A recent blood transfusion can temporarily raise the RDW because it introduces donor cells that may have a different average size than the recipient’s own cells.
What to Expect After a High RDW Result
Receiving a high RDW result is not a diagnosis in itself. The first step is to discuss the findings with the healthcare provider who ordered the CBC. A doctor will not look at the RDW in isolation; they will interpret it within the context of the entire blood panel and your overall health. Particular attention will be paid to other red blood cell indices, like the MCV and hemoglobin levels.
Your doctor will likely perform a physical examination and ask questions about your diet, lifestyle, and any symptoms you may be experiencing. Based on this evaluation, further testing may be ordered to pinpoint the underlying cause of the high RDW. These follow-up tests often include specific iron studies or blood tests to measure your vitamin B12 and folate levels.
Treatment is always directed at the underlying condition, not at the RDW value itself. For example, if iron deficiency anemia is diagnosed, treatment will involve iron supplementation and dietary changes. As the underlying issue is resolved and the bone marrow begins to produce uniform red blood cells again, the RDW value will naturally decrease and return to a normal range.