The complete blood count (CBC) is a standard laboratory test that offers a snapshot of the cells circulating in the blood, including red blood cells, white blood cells, and platelets. Within the red blood cell panel, one specific measurement is the Red Cell Distribution Width-Standard Deviation (RDW-SD). This marker provides a measure of the uniformity of red blood cell size, moving beyond simply counting cells or measuring their average size. Understanding RDW-SD offers insight into the body’s red cell production process and the potential presence of underlying conditions affecting it.
What RDW-SD Measures
RDW-SD is a precise, absolute measurement of the variation in the volume of red blood cells within a blood sample. The term “Standard Deviation” signifies that this value is a direct, calculated measure of how much the cell sizes deviate from one another. This variation in size, where red blood cells are not uniform, is a condition known as anisocytosis.
The measurement is taken from the red blood cell volume distribution curve. RDW-SD is reported in femtoliters (fL), and the typical reference range falls between approximately 39 and 46 fL. A high RDW-SD value indicates a significant difference between the smallest and largest red cells in the sample, suggesting a problem with cell maturation or survival.
This marker is distinct from the more common RDW-CV (Coefficient of Variation), which is mathematically influenced by the Mean Corpuscular Volume (MCV), or the average cell size. Because RDW-SD is an absolute measurement not dependent on the average cell size, it is often considered a more sensitive indicator of early size variation.
Interpreting RDW-SD Alongside Other Blood Markers
RDW-SD gains its clinical utility when interpreted in conjunction with other red blood cell indices. The most informative partnership is with the Mean Corpuscular Volume (MCV), which measures the average volume of the red blood cells. By combining the assessment of average size (MCV) and size variation (RDW-SD), hematologists can narrow down the potential categories of blood disorders.
A high RDW-SD coupled with a low MCV indicates a microcytic pattern with high cell variation. This suggests a mixed population of red blood cells, where some are unusually small and others are of a more normal size. Conversely, a high RDW-SD combined with a high MCV points toward a macrocytic pattern with significant variation, reflecting inconsistencies in the production of larger cells.
If the RDW-SD is elevated while the MCV remains within the normal range, it often suggests a condition is in its very early stages or that a mixed disorder is present. This occurs because the average cell size (MCV) is maintained by a balance of small and large cells, but the wide variation (RDW-SD) alerts clinicians to an underlying issue. A normal RDW-SD alongside a low MCV suggests a microcytic condition where all the cells are uniformly small.
Clinical Conditions Associated with Abnormal RDW-SD
An elevated RDW-SD is the most common and clinically relevant abnormality, directly reflecting the body’s struggle to produce red blood cells of a consistent size.
Iron Deficiency and Nutritional Deficiencies
One of the most frequent causes is Iron Deficiency Anemia, which typically presents with a high RDW-SD and a low MCV. Insufficient iron stores lead to the production of a wide range of small, hypochromic (pale) cells, creating a high degree of size variation. Nutritional deficiencies involving Vitamin B12 and folate also result in a high RDW-SD, but often with a high MCV, reflecting a macrocytic pattern. These vitamins are necessary for proper DNA synthesis, and a lack of them causes the bone marrow to release fewer, larger, and irregularly sized red blood cells into circulation. This results in a population that includes both the large, immature cells and older, normal-sized cells, significantly increasing the RDW-SD.
Other Causes
Certain hemoglobin disorders, such as Beta Thalassemia, can present with a low MCV but a normal RDW-SD in their minor forms. A high RDW-SD can still occur in more severe forms or when the condition is complicated by another factor, like iron deficiency. Conditions affecting bone marrow function, such as Myelodysplastic Syndromes, disrupt the normal cell maturation process, leading to the release of highly heterogeneous cells and a high RDW-SD. Chronic diseases like advanced liver disease, kidney failure, and certain inflammatory states can also be associated with an elevated RDW-SD. Conversely, a low RDW-SD is rare and typically not considered a significant clinical finding, generally indicating a very uniform population of red blood cells.