A result of “RDW-SD high” on a blood test report can be confusing. This value is a standard part of a Complete Blood Count (CBC), one of the most common lab tests performed. An elevated RDW-SD means there is more variation in the size of your red blood cells than is typically seen. It is not a diagnosis on its own but serves as a piece of information that, along with other data, helps form a complete picture of your health.
Understanding the RDW-SD Measurement
Red blood cell distribution width (RDW) measures the variability in the size and volume of your red blood cells. The RDW-SD reports this variation as a direct measurement in femtoliters (fL). Ideally, red blood cells are uniform in size, resulting in a low RDW. A high RDW-SD is like a basket with a mix of small and large fruit, indicating a wide range of cell sizes.
This variation in red blood cell size is medically termed anisocytosis, and a high degree suggests the body’s red blood cell production may be affected. Lab reports often include RDW-CV, a percentage calculation. RDW-SD, however, is an absolute measurement of the size distribution, providing a direct look at the size differences.
Healthcare providers analyze the RDW-SD value with other red blood cell indices, especially the Mean Corpuscular Volume (MCV), which indicates the average size of the red blood cells. Comparing a high RDW with the MCV helps narrow down potential underlying causes of the size variation, guiding further investigation.
Potential Causes of Elevated RDW-SD
The most frequent reasons for an elevated RDW-SD are nutritional deficiencies that affect red blood cell production. Iron deficiency is a common cause, leading to iron deficiency anemia. Without enough iron, the bone marrow produces smaller red blood cells, but as the deficiency progresses, the size variation increases, raising the RDW.
Deficiencies in vitamin B12 and folate also commonly lead to a high RDW. These vitamins are necessary for the DNA synthesis required for red blood cell division in the bone marrow. A lack of B12 or folate disrupts this process, resulting in the production of fewer, but unusually large, red blood cells, which increases the overall size variation.
Other types of anemia can be associated with a high RDW. For instance, some inherited blood disorders like sickle cell anemia can cause variations in cell size and shape. Conditions that lead to the premature destruction of red blood cells, known as hemolysis, can also trigger the bone marrow to release immature red cells of different sizes.
Certain chronic health conditions are another potential factor. Chronic liver disease or kidney disease can interfere with the hormones and nutrients needed for healthy red blood cell production. A recent blood transfusion can temporarily elevate the RDW because it introduces donor red blood cells that may have a different average size than the recipient’s own cells.
Associated Symptoms and Next Steps
An elevated RDW-SD value does not directly cause symptoms; instead, they are due to the underlying medical condition. When the cause is anemia, which is common, symptoms often include persistent fatigue, weakness, and shortness of breath with physical activity. Other signs can include pale skin, dizziness, and cold hands and feet.
If your lab results show a high RDW-SD, the first step is to review the entire CBC panel, paying close attention to other red blood cell indices like the MCV and hemoglobin levels. This broader context is fundamental to understanding the significance of the RDW result.
Based on the initial CBC results and a review of your medical history, your healthcare provider may order follow-up tests. These could include specific tests for iron levels, or to measure your vitamin B12 and folate levels. A physical examination and a discussion of your diet, lifestyle, and symptoms will also guide the diagnostic process and allow for an accurate interpretation of your results.