The Red Cell Distribution Width (RDW) is a standard component of the Complete Blood Count (CBC). A result of 18.6% is higher than the typical reference range, signaling a finding that requires further attention. This measurement offers insight into the quality of your red blood cells, which transport oxygen throughout the body. Understanding this elevated percentage involves clarifying what the RDW measures and the underlying processes causing this variation.
Understanding Red Cell Distribution Width
The Red Cell Distribution Width (RDW) is a laboratory value that quantifies the variation in size and volume among your red blood cells, or erythrocytes. Healthy red blood cells are generally uniform in size. The RDW value, usually reported as a percentage, is mathematically derived from the width of the red blood cell volume curve.
A typical normal range for the RDW is generally between 11.5% and 15.0%, though the exact figures can vary slightly between laboratories. A result of 18.6% is noticeably higher than this range, signaling a significant lack of uniformity in the size of the cells. This index is important because it provides a qualitative view of the red blood cell population, complementing other CBC parameters that only report average values.
The Clinical Significance of an Elevated RDW
An elevated RDW, such as 18.6%, directly signifies that your red blood cells vary greatly in size, a condition medically termed anisocytosis. This means that the sample contains a mixture of cells—some may be much smaller than normal, while others may be significantly larger. The variation in cell size suggests a fundamental issue with the production or lifespan of the red blood cells.
This measurement is often an early indicator that the bone marrow is struggling to produce a consistent population of new cells. The presence of both older, existing red cells and newly produced, abnormally sized cells leads to the wide distribution reflected in the high RDW. Because the RDW can become elevated before other red cell indices change, it can provide an initial clue about a developing health issue.
Primary Causes of Increased Red Cell Variation
A high RDW result is most commonly related to deficiencies in the building blocks required for proper red blood cell development. These shortages, along with other underlying conditions, cause the bone marrow to produce inconsistent cell sizes.
The primary causes of increased red cell variation include:
- Iron deficiency: A shortage of iron causes the bone marrow to release smaller-than-normal red blood cells, leading to a mixture of small, new cells and normal-sized older cells.
- Vitamin B12 or folate deficiency: These deficiencies impair DNA synthesis, resulting in the production of abnormally large red blood cells that mix with normal cells.
- Chronic diseases: Conditions causing inflammation, such as kidney or liver disease, can impair red blood cell production or lifespan.
- Genetic conditions: Disorders like thalassemia and sickle cell trait cause cells to be inherently misshapen or fragmented, leading to diverse sizes.
- Recent blood loss: The body’s recovery attempt involves the bone marrow releasing large, premature cells called reticulocytes, temporarily increasing the RDW.
Integrating RDW with Comprehensive Diagnosis
The RDW is rarely interpreted in isolation; instead, it is analyzed in conjunction with the Mean Corpuscular Volume (MCV) for a definitive diagnosis. The MCV measures the average size of the red blood cells, while the RDW measures the variation in that size. The combination of these two values helps to create a diagnostic map.
For example, a high RDW (18.6%) combined with a low MCV typically points toward iron deficiency anemia. Conversely, a high RDW alongside a high MCV suggests a deficiency in Vitamin B12 or folate. A high RDW with a normal MCV can suggest an early-stage nutritional deficiency or a mixed-cause anemia, such as both iron and B12 deficiency occurring simultaneously.
Healthcare providers use these patterns to determine the next steps, often ordering specific follow-up blood tests, such as ferritin levels for iron stores or B12 and folate assays. The clinical context, including a patient’s symptoms and medical history, is then used to confirm the underlying cause. A high RDW is a useful flag, but subsequent tests and clinical evaluation establish the final diagnosis and guide treatment.