The Red Cell Distribution Width (RDW) is a measurement included in a standard Complete Blood Count (CBC). RDW-SD, or Red Cell Distribution Width-Standard Deviation, measures the difference in size between the smallest and largest red blood cells in a sample. A high RDW-SD result suggests the red blood cells are not uniform in size, a condition known as anisocytosis. This metric serves as a screening tool, flagging potential issues with red blood cell production or survival that require further investigation.
Understanding Red Blood Cell Variation
The primary function of red blood cells is to transport oxygen efficiently. For this process to work optimally, these cells are typically uniform in size and shape. An elevated RDW-SD indicates that the red blood cell population is highly heterogeneous, meaning there is a mix of smaller and larger cells in circulation.
RDW-SD is measured in femtoliters (fL) and provides an absolute value of the size range of red blood cells. This measurement is a direct indicator of size variability because it is not influenced by the Mean Corpuscular Volume (MCV), which measures the average size of all red blood cells. A high degree of size variation can impair the ability of red blood cells to navigate small capillaries, potentially compromising oxygen delivery.
The presence of cells of greatly different sizes, or anisocytosis, warrants attention. In a healthy individual, the RDW-SD falls within a narrow reference range, reflecting a highly controlled red blood cell production process. When this process is disrupted, the bone marrow releases cells that have matured improperly, leading to the wide variation recorded as a high RDW-SD.
Physiological Reasons for High RDW-SD
An elevated RDW-SD reflects a disturbance in erythropoiesis, or red blood cell formation, within the bone marrow. One common mechanism is ineffective erythropoiesis, where the bone marrow attempts to produce new cells but lacks necessary building blocks, such as vitamins or minerals. This deficiency causes cells to develop with size abnormalities, resulting in a combination of normal-sized, small, and sometimes large cells being released simultaneously.
Another mechanism involves the premature destruction or fragmentation of existing red blood cells, a process called hemolysis. When older, damaged, or abnormally shaped cells are rapidly cleared, the bone marrow compensates by releasing newer, often larger, cells called reticulocytes. The presence of these newly released reticulocytes alongside the remaining older cells creates a mixed population with a wide size range, registering as a high RDW-SD.
A third reason for a high RDW-SD is the presence of two distinct populations of red blood cells. This occurs in patients who have recently received a blood transfusion, as donor cells may have a different average size than the recipient’s native cells. It can also happen when a patient begins treatment for a deficiency, where newly produced, healthy cells mix with existing, abnormal cells, temporarily skewing the size distribution.
Conditions Indicated by Elevated RDW-SD
The information from an elevated RDW-SD is most informative when combined with the Mean Corpuscular Volume (MCV) result, which helps categorize the underlying problem. A high RDW-SD paired with a low MCV (varied in size but small on average) often points toward iron deficiency anemia. Insufficient iron limits the body’s ability to produce adequate hemoglobin, causing the resulting red blood cells to be microcytic, or smaller than normal.
Conversely, a high RDW-SD combined with a high MCV (indicating large average cell size) is associated with deficiencies in Vitamin B12 or folate. These nutrients are required for DNA synthesis. A lack of them causes red blood cell precursors in the bone marrow to grow too large before division, leading to the release of macrocytic, or oversized, cells. Because the deficiency develops gradually, the blood contains a mix of older, normal cells and newer, oversized cells, resulting in wide size variation.
An elevated RDW-SD can also be an early indicator of a developing nutritional deficiency, sometimes appearing before the MCV or hemoglobin levels change. Beyond nutritional causes, a high RDW-SD is seen in chronic conditions that affect bone marrow health or cause systemic inflammation. These include chronic liver disease, which impairs the production and regulation of red blood cell components, and early-stage blood disorders like myelodysplastic syndromes, where the bone marrow’s ability to create healthy, uniform cells is compromised.
What to Do After a High Result
Receiving an elevated RDW-SD result means the initial screening has flagged a need for further investigation, not that a definitive diagnosis has been made. The first step is to consult with a healthcare provider who can interpret the result within the context of the complete CBC and individual medical history. They will consider all other blood parameters, such as hemoglobin and MCV levels, to narrow down potential causes.
A physician will order additional, specific blood work to pinpoint the exact issue. Follow-up tests often include serum ferritin, which measures the body’s stored iron, and direct measurements of Vitamin B12 and folate levels. If a hemolytic process or a blood disorder is suspected, tests like a reticulocyte count (measuring new red blood cells) or a peripheral blood smear for manual cell examination may be necessary.
In cases where the high RDW-SD stems from a chronic condition, such as liver or kidney disease, further testing may focus on organ function. The treatment plan will be directed at correcting the underlying cause, whether through dietary changes, supplementation for nutrient deficiencies, or management of a chronic illness. The RDW-SD result acts as a guide, prompting the medical team to look deeper for the root of the cell size variation.