The Red Cell Distribution Width (RDW) is a measurement included in a standard Complete Blood Count (CBC) that provides information about your red blood cells. Red blood cells transport oxygen throughout the body, and their function relies partly on size uniformity. The RDW test specifically assesses the degree of variation in the size and volume of these cells within a blood sample. This variation is known as anisocytosis, which refers to the presence of red blood cells of unequal size. Understanding the RDW value helps healthcare providers investigate potential underlying health issues related to red blood cell production or survival.
What RDW Measures in Your Blood
The RDW test quantifies anisocytosis, which is the degree of size variation among red blood cells. Normally, red blood cells are uniform in size, allowing them to flow efficiently through narrow capillaries to deliver oxygen. When the bone marrow produces red blood cells of different sizes—some too small and others too large—the RDW value becomes elevated. This size heterogeneity signals a problem with the body’s ability to produce or maintain a healthy population of oxygen carriers. Unlike the Mean Corpuscular Volume (MCV), which reflects only the average size, the RDW is an independent measure that detects when mixed populations of cells are present.
Clarifying the RDW (SD) Measurement
RDW results are generally reported in two formats: Red Cell Distribution Width-Standard Deviation (RDW-SD) and Red Cell Distribution Width-Coefficient of Variation (RDW-CV).
RDW-SD
The RDW-SD is reported in femtoliters (fL) and measures the actual width of the red blood cell size distribution curve. This measurement is determined by calculating the width at the 20% height level of the cell distribution histogram. Because RDW-SD is a direct measurement, it is not influenced by the average cell size, known as the Mean Corpuscular Volume (MCV). This makes RDW-SD useful for identifying populations of larger cells (macrocytosis).
RDW-CV
In contrast, RDW-CV is reported as a percentage and is calculated by dividing the standard deviation by the MCV. Therefore, RDW-CV is a relative measure that is affected by changes in the average cell size.
Interpreting High and Low RDW Results
The RDW result is compared against a typical reference range, though these ranges can vary slightly between laboratories. For adults, RDW-SD typically ranges from 39 to 46 fL, and RDW-CV typically falls between 11.6% and 14.6%. Results outside these parameters warrant further investigation by a healthcare professional.
A high RDW, or elevated anisocytosis, is the most clinically significant finding and suggests a greater degree of variability in red blood cell size. This increase indicates the body is producing red cells that are significantly smaller, larger, or a combination of both. Such variation can occur due to a defect in the bone marrow’s production process or when red blood cells are prematurely destroyed in circulation. An elevated RDW often acts as an early indicator of a problem, sometimes appearing before changes in other red cell parameters become apparent.
Conversely, a low RDW result is generally not considered a sign of a medical problem on its own. A low RDW simply means the red blood cells are more uniform in size than average, which is usually a favorable finding. A normal RDW suggests uniform cells, but it does not rule out the presence of certain types of anemia.
Common Conditions Associated with Abnormal RDW
The RDW is primarily used in conjunction with the Mean Corpuscular Volume (MCV) to help distinguish between different causes of anemia. Iron deficiency anemia is a condition that frequently presents with a high RDW, often accompanied by a low MCV, as the body attempts to make small, poorly hemoglobinized cells.
Nutritional Deficiencies
Anemias caused by deficiencies in Vitamin B12 or folate often result in a high RDW alongside a high MCV. These nutritional deficiencies impair DNA synthesis, causing the bone marrow to produce abnormally large red blood cells (macrocytes). The high RDW in these cases reflects the mix of large, newly formed cells and the older, smaller cells still in circulation.
Inherited and Chronic Conditions
The RDW is particularly helpful in differentiating nutritional anemias from inherited disorders like thalassemia, which may show a low MCV but often have a normal RDW. This is because in thalassemia, the red blood cells are uniformly small, meaning there is low average size but low size variation. An elevated RDW can also be associated with chronic diseases, such as liver disease, kidney disease, and some inflammatory or malignant conditions, reflecting underlying systemic stress or chronic inflammation.