An elevated Red Cell Distribution Width (RDW) is a finding from a standard Complete Blood Count (CBC) test. The CBC assesses the different components of your blood, and the RDW specifically measures the uniformity of your red blood cells, which transport oxygen throughout the body. Understanding what a high RDW signifies is the first step toward clarifying this finding. This measurement provides an initial clue to your health status that requires further context and investigation.
Defining Red Cell Distribution Width
The Red Cell Distribution Width (RDW) is a statistical measure that quantifies the variation in the size and volume of the red blood cells within a sample. It indicates how much red blood cells differ from one another in size, which should be relatively uniform in a healthy individual. The most common measurement reported is the RDW-Coefficient of Variation (RDW-CV), expressed as a percentage. This calculation uses the standard deviation of red blood cell volume divided by the average red blood cell volume, known as the Mean Corpuscular Volume (MCV).
A less common measurement is the RDW-Standard Deviation (RDW-SD), which represents the width of the red blood cell volume curve in femtoliters (fL). The RDW-CV typically falls within a range of approximately 11.5% to 14.5% in adults, though this reference range can vary between laboratories. A result above the upper limit is considered elevated and suggests greater variability in cell size.
What an Elevated RDW Indicates
An elevated RDW indicates greater-than-normal heterogeneity in red blood cell size, a condition medically termed anisocytosis. This means the blood sample contains a mix of red blood cells that are much smaller, much larger than average, or a combination of both. Cell size variation signals a potential issue with how the body is producing, maturing, or clearing red blood cells.
The presence of cells of unequal size often results from an irregularity in the bone marrow’s production of new cells or an issue with the survival of circulating cells. The RDW is particularly useful because an elevation can sometimes precede changes in other red blood cell indices, offering an early diagnostic clue. The RDW result is always evaluated alongside the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells, to narrow down diagnostic possibilities.
Underlying Conditions That Increase RDW
A high RDW suggests a process disrupting the uniform production of red blood cells, often pointing toward a nutritional deficiency or an underlying disease. The most frequent causes involve deficiencies in the essential nutrients required for proper red blood cell synthesis. For instance, iron deficiency anemia often leads to the production of smaller-than-average cells, and the RDW becomes elevated as the body attempts to make new, small cells while older cells still circulate.
Deficiencies in Vitamin B12 or folate also cause an elevated RDW because they lead to the production of abnormally large red blood cells. When these deficiencies are present, the peripheral blood may contain a mix of large, poorly matured cells and normal-sized cells, significantly increasing the measured variation. If a person has both iron and B12 or folate deficiencies simultaneously, the RDW is highly elevated due to the presence of both very small and very large cells.
Beyond nutritional issues, certain genetic blood disorders, known as hemoglobinopathies, can also cause an elevated RDW. Conditions like Thalassemia or Sickle Cell Trait may result in a high RDW, even if the MCV appears normal, because the RDW is more sensitive to cell size variation. Chronic diseases, such as Chronic Kidney Disease, certain liver diseases, or inflammatory conditions, can also impair red blood cell production or survival, thereby elevating the RDW. An elevated RDW has also been noted as a potential marker in some cancers and cardiovascular diseases, often reflecting chronic inflammation or oxidative stress.
Next Steps After a High RDW Result
A high RDW result prompts further investigation, as it is rarely diagnostic on its own. The next course of action involves follow-up tests to pinpoint the specific underlying cause of the red blood cell size variation. A physician will typically order iron studies, including a serum ferritin level to assess iron stores, and sometimes transferrin saturation.
Testing for Vitamin B12 and folate levels is also common to rule out nutritional deficiencies. A peripheral blood smear review, where a technician manually examines the blood cells, may be performed to visually confirm the variation in cell size and look for other abnormalities. Treatment is directed at correcting the underlying disorder, not lowering the RDW itself. For deficiencies, this involves targeted supplementation, while for chronic diseases, the focus is on managing the primary condition.