Red Blood Cell Distribution Width (RDW) is a standard metric reported as part of a complete blood count (CBC), a common laboratory test. This value measures the variation in the size and volume of a person’s red blood cells (RBCs). While red blood cells are generally uniform in size, the RDW quantifies any irregularity in their volumes. An elevated RDW suggests a greater degree of size variability among the red blood cells circulating in the bloodstream. This measurement offers insight into red blood cell production and turnover.
Understanding Red Blood Cell Size Variation
The biological concept measured by RDW is known as anisocytosis, meaning a condition of unequal cell sizes. Healthy red blood cells normally maintain a relatively uniform size, typically falling within a narrow diameter range of 6.2 to 8.2 micrometers. This uniformity ensures the cells can efficiently navigate the body’s smallest capillaries to deliver oxygen.
When the RDW is elevated, it indicates that a mixed population of red blood cells is present—some cells may be much larger, while others are much smaller than average. This variation often reflects an issue in the bone marrow’s ability to produce stable, uniform cells, or it can be a sign that older cells are being destroyed or removed at an uneven rate. The size variation can also include poikilocytosis, the presence of abnormally shaped cells, further contributing to a higher RDW value.
The RDW is always considered in conjunction with the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells. The distinction between these two values is important for accurate diagnosis. For instance, a person could have a normal MCV, suggesting the average cell size is fine, but a high RDW, indicating that the average is created by a mix of very small and very large cells.
The RDW acts as a measure of the spread of red cell volume, rather than the average volume itself. Since the MCV is a single average, it can obscure the presence of mixed red cell populations that are clearly revealed by an elevated RDW. RDW can detect these deviations in size distribution, sometimes even before the MCV shows any change.
The Process of Measuring RBC Distribution
The measurement of Red Blood Cell Distribution Width is performed by automated hematology analyzers used to analyze blood samples during a Complete Blood Count. These analyzers rapidly count and size millions of individual red blood cells passing through a channel. The data collected on the volume of each cell is then used to generate a distribution curve, often called a histogram.
The RDW value is a statistical calculation derived from this volume distribution histogram. It is commonly reported in one of two ways: as a coefficient of variation (RDW-CV) or as a standard deviation (RDW-SD). The RDW-CV is expressed as a percentage and is mathematically derived by dividing the standard deviation of the red cell volume by the MCV.
The general reference range for RDW-CV in a healthy adult typically falls between 11.5% and 14.5%, though this range can vary depending on the laboratory and equipment used. A lower percentage indicates that the red blood cells are highly uniform in size, while a higher percentage means there is a greater spread of cell sizes. The RDW-SD is an absolute measure of the width of the distribution curve, expressed in femtoliters (fL). Unlike RDW-CV, RDW-SD is not influenced by the MCV.
Interpreting Abnormal Results and Associated Conditions
An elevated RDW is a key clinical finding, suggesting a problem with red blood cell production or lifespan. This finding is highly useful in the differential diagnosis of various types of anemia, especially when considered alongside the MCV. A high RDW value often precedes changes in other red cell indices, making it an early clue for potential nutritional deficiencies.
The combination of a high RDW and a low MCV is frequently associated with iron deficiency anemia. In this condition, the body struggles to make hemoglobin, leading to the production of small, varied cells (microcytic), which drives the RDW up while the MCV decreases. An elevated RDW can also help distinguish iron deficiency anemia from other conditions that cause small red cells, such as heterozygous thalassemia, which typically presents with a normal RDW.
A high RDW paired with a high MCV commonly suggests deficiencies in Vitamin B12 or folate. These deficiencies impair DNA synthesis, causing the bone marrow to release abnormally large, ineffective red blood cells (macrocytic), which increases both the size variation and the average cell size. When both iron and B12/folate deficiencies occur together, known as mixed deficiency anemia, the RDW is typically high, but the MCV may be normal, as the small and large cells average out.
A high RDW with a normal MCV can be seen in the early stages of iron, B12, or folate deficiency, before the average cell size has significantly shifted. This pattern is also a finding in conditions like chronic liver disease, sickle cell disease, or recent acute blood loss. Beyond anemias, an elevated RDW has been linked to various other systemic issues, including certain cancers, kidney disease, and chronic inflammatory states.
Conversely, a low RDW value is rare and is generally not considered clinically significant on its own. A result within the normal or low range simply means that the red blood cells are uniform in size. Even with an underlying health issue, some conditions, such as anemia of chronic disease or aplastic anemia, may present with a normal RDW.