Red Cell Distribution Width (RDW) is a standard measurement included in a Complete Blood Count (CBC), one of the most common blood tests. This value provides specific information about your red blood cells, which are responsible for carrying oxygen throughout your body. The RDW test measures the degree of variation in the size of these cells. In a healthy state, red blood cells are uniform in size.
Understanding the RDW Value
The RDW value on a lab report quantifies the variability in the size of your red blood cells and is presented as a percentage. While reference ranges can vary by laboratory, a normal RDW is between 12% and 15%. A result of 18.6% is therefore considered high.
This elevated number signifies that there is a greater-than-normal variation in the size of your red blood cells circulating in your bloodstream. Some cells may be smaller than average while others might be larger, creating a diverse population of cells. This measurement provides a specific clue about the processes happening within your bone marrow, where these cells are produced.
Potential Causes of a High RDW
An elevated RDW is frequently associated with anemia, and iron-deficiency anemia is a common cause. When the body lacks sufficient iron, it struggles to produce hemoglobin, the protein in red blood cells that carries oxygen. This can lead to the production of new red blood cells that are smaller than usual (microcytic), which mix with older, normal-sized cells and increase the RDW.
Deficiencies in vitamin B12 and folate are another primary cause of a high RDW. A lack of these vitamins leads to the production of unusually large red blood cells (macrocytic). This process, known as megaloblastic anemia, also creates a significant size discrepancy among red blood cells, resulting in an elevated RDW. The presence of these large, immature cells is a direct reason for the increased size distribution width.
Less common reasons for a high RDW can include chronic liver disease or a blood transfusion, which introduces a population of donor red blood cells that can vary in size from your own. In some instances, a high RDW can also be linked to chronic inflammation or autoimmune disorders. The underlying mechanism is the disruption of normal, uniform red blood cell production.
The Role of Other Blood Test Results
An RDW result of 18.6 is rarely interpreted in isolation by a healthcare provider. Its diagnostic utility increases when analyzed alongside other values from the Complete Blood Count, particularly the Mean Corpuscular Volume (MCV). MCV measures the average size of your red blood cells, while RDW measures the variation around that average. Together, they provide a more detailed picture of red blood cell health.
This combination helps narrow down potential causes. For instance, a high RDW paired with a low MCV (smaller than average cells) suggests iron-deficiency anemia. Conversely, a high RDW combined with a high MCV (larger than average cells) points toward a deficiency in vitamin B12 or folate.
There are also scenarios where the RDW is high but the MCV is normal. This pattern can indicate early-stage iron or vitamin deficiencies, where the average cell size has not yet shifted significantly but the variation has increased. It might also suggest a mixed anemia, such as a concurrent iron and folate deficiency.
Next Steps and Follow-Up
An RDW of 18.6 is a starting point, not a final diagnosis. The most important action is to discuss the findings with the healthcare provider who ordered the test. They will interpret this value within the context of your overall health, symptoms, medical history, and the entire blood panel.
Your doctor will determine the appropriate next steps. If a nutritional deficiency is suspected, they may order more specific tests to confirm it. These could include an iron panel or tests to check your vitamin B12 and folate levels. These follow-up tests are designed to pinpoint the exact cause of the high RDW.