A complete blood count (CBC) is a common laboratory test that provides a detailed look at the cells circulating in your bloodstream. Among the metrics reported is the Red Cell Distribution Width (RDW), which focuses specifically on red blood cells. The RDW indicates how uniformly sized your oxygen-carrying cells are. Understanding this metric offers insight into the production health of your blood system.
What Does Red Cell Distribution Width Measure?
The RDW quantifies the variation in the size and volume of your red blood cells. This variation in cell size is medically known as anisocytosis. Ideally, all red blood cells should be of a similar size to efficiently transport oxygen throughout the body. A high RDW percentage indicates a significant difference in cell sizes, meaning a mixture of very small and very large red blood cells are circulating together. An elevated RDW often signals an issue with the bone marrow’s ability to produce consistently sized cells, which can occur due to problems with cell production or destruction.
Interpreting the RDW Result of 14.5%
To understand what an RDW of 14.5% signifies, it must be compared to the established reference range. The typical normal range for RDW is approximately 11.5% to 14.5% or 12% to 15%, though this varies slightly by laboratory. A result of 14.5% is considered to be at the upper limit of the normal reference interval or borderline elevated. This upper boundary suggests that the variation in red blood cell size is minimal, but at the threshold of being considered increased. If other metrics on the complete blood count, such as hemoglobin and mean corpuscular volume, are within normal limits, a 14.5% RDW may not be clinically significant on its own.
A result at this level can sometimes be an early warning sign of a developing condition. A borderline RDW may also indicate a mild, transient factor influencing cell production, such as a recent infection, strenuous physical activity, or the effects of certain medications. Because the RDW is a sensitive marker, a value on the high side of normal warrants attention and may lead a healthcare provider to repeat the test later.
Primary Reasons for RDW Variation
When the RDW is elevated above the normal range, it indicates that the bone marrow is releasing red blood cells with a wide range of sizes. This occurs when new cells are produced differently than the older cells already in circulation. The most frequent causes of an increased RDW are deficiencies in the nutrients required for proper red blood cell maturation.
A lack of iron prevents the synthesis of hemoglobin, causing the body to produce red blood cells that are smaller than average, increasing size variation. Deficiencies in Vitamin B12 or folate disrupt the DNA synthesis needed for cell division, leading to the creation of abnormally large red blood cells. These nutritional shortfalls cause specific types of anemia, such as iron deficiency anemia (small cells) or megaloblastic anemia (large cells).
The RDW can also become elevated in conditions involving chronic inflammation or increased red blood cell destruction. In these scenarios, the rapid destruction of existing cells forces the bone marrow to release immature, often larger red cells (reticulocytes) into the circulation. This mix of newly released large cells and older cells increases the overall size variability, pushing the RDW value higher.
Combining RDW with Mean Corpuscular Volume
The RDW is rarely interpreted in isolation and is most informative when considered alongside the Mean Corpuscular Volume (MCV). The MCV provides the average volume of the red blood cells, while the RDW measures the variation around that average. Using both values creates a diagnostic matrix that helps pinpoint the specific type of blood disorder.
For instance, a high RDW combined with a low MCV (small average cell size) is a classic pattern seen in iron deficiency anemia. Conversely, a high RDW paired with a high MCV (large average cell size) suggests deficiencies in Vitamin B12 or folate. If the RDW is high but the MCV is within the normal range, it can indicate an early-stage nutritional deficiency or the presence of two distinct populations of red cells. This dual analysis allows clinicians to distinguish between different causes of anemia.