Red Cell Distribution Width – Standard Deviation (RDW-SD) is a measurement frequently encountered in complete blood count (CBC) results. It provides information about red blood cells, which transport oxygen throughout the body. RDW-SD offers insights into certain characteristics of these cells.
What RDW-SD Measures
RDW-SD quantifies the variation in the size of red blood cells within a blood sample. This variability in cell size is known as anisocytosis. Healthy red blood cells typically maintain a relatively uniform size. RDW-SD measures this consistency directly in femtoliters (fL), reflecting the width of the red blood cell size distribution.
While Mean Corpuscular Volume (MCV) indicates the average size of red blood cells, RDW-SD specifically assesses the uniformity of their sizes. Even if the average cell size (MCV) is normal, a high RDW-SD can still indicate significant differences among individual red blood cells. This distinction provides a more nuanced view of red blood cell health beyond just their average dimension.
Understanding High RDW-SD Levels
An elevated RDW-SD signifies a greater-than-normal variation in red blood cell size. This increased variability often points to conditions affecting the production or lifespan of red blood cells. Several types of anemia commonly lead to a high RDW-SD.
Iron deficiency anemia, a common form of anemia, frequently presents with high RDW-SD. In this condition, the body produces smaller, newly formed red blood cells alongside existing normal-sized cells, creating a diverse population. Deficiencies in vitamin B12 or folate can also cause an elevated RDW-SD, leading to the production of abnormally large red blood cells that increase size variation when mixed with normal cells.
Other conditions can also result in a high RDW-SD. Hemolytic anemias, characterized by the premature destruction of red blood cells, prompt the bone marrow to release new, often immature and varied-sized cells into circulation. Chronic diseases, such as kidney or liver disease, can impact red blood cell production and turnover, contributing to size variability. Myelodysplastic syndromes, disorders affecting the bone marrow’s ability to produce healthy blood cells, can also manifest with increased RDW-SD due to abnormal cell production. Recent blood transfusions can temporarily elevate RDW-SD as the recipient’s cells mix with donor cells of potentially different sizes.
Understanding Low RDW-SD Levels
A low RDW-SD indicates that red blood cells are very uniform in size. This finding is less common than an elevated RDW-SD and is typically not a cause for concern. A low RDW-SD often suggests stable red blood cell production, where the body consistently produces cells of similar dimensions.
In most cases, a low RDW-SD value falls within or slightly below the normal reference range, generally between 39 and 46 fL. While some rare conditions, such as certain types of thalassemia minor, can result in uniformly small red blood cells and a low RDW-SD, it is generally considered a normal or favorable result. Healthcare providers typically focus on other blood parameters if there are concerns, as a low RDW-SD alone rarely points to a specific disease state.
Why RDW-SD Matters
RDW-SD provides valuable insights when interpreted alongside other blood parameters, particularly the Mean Corpuscular Volume (MCV). This combined analysis helps healthcare professionals differentiate between various types of anemia and other blood disorders. Examining RDW-SD and MCV together can narrow down potential causes of blood cell abnormalities.
For instance, a high RDW-SD accompanied by a low MCV can suggest iron deficiency anemia, where red blood cells are both varied in size and smaller than average. Conversely, a high RDW-SD with a high MCV might indicate a deficiency in vitamin B12 or folate, leading to larger, varied red blood cells. If RDW-SD is high but MCV is within the normal range, it could signal early stages of certain nutritional deficiencies or conditions like sickle cell disease. This combined approach helps guide further diagnostic tests and informs appropriate treatment strategies.