Anemia Biomarker
Collection Type: Blood
Related System: Anemia
RDW-SD (red cell distribution width standard deviation) is a laboratory measure of the actual width of the red blood cell (RBC) size distribution curve, reported in femtolitres (fL). It quantifies anisocytosis variability in RBC sizes and is reported as part of a complete blood count/anemia profile. Elevated RDW-SD indicates a wide spread of cell volumes and can be seen with iron deficiency, vitamin B12 or folate deficiency, hemolysis, recent transfusion, or mixed anemias. Low values are uncommon and usually not clinically important. Newborns and older adults may show different baseline values (older adults more prone to elevated RDW from nutritional deficiencies); gender differences are minimal.
Run our symptom checker to see if this test is right for you
Symptom Checker39-46 fL
Q: What does it mean when your RDW SD is high?
A: A high RDW‑SD means greater variation in red blood cell size (anisocytosis). It often occurs with iron deficiency, vitamin B12 or folate deficiency, recent bleeding, hemolysis, mixed anemias, or bone marrow disorders. Alone it isn’t diagnostic; clinicians interpret RDW‑SD alongside hemoglobin, MCV and reticulocyte counts and may order iron studies, B12/folate levels or a blood smear to identify the cause.
Q: What cancers cause a high RDW?
A: Several cancers are commonly associated with an increased RDW. These include solid tumors such as colorectal, lung (especially non‑small cell), gastric, pancreatic, hepatocellular, breast, ovarian and esophageal cancers, and hematologic malignancies like leukemia, lymphoma, multiple myeloma and myelodysplastic syndromes. Elevated RDW often reflects inflammation, nutritional deficiencies or bone‑marrow involvement rather than being cancer‑specific.
Q: Is it better to have a high or low RDW?
A: A low or normal RDW is generally better. Low RDW means red blood cells are similar in size, reflecting healthy production. A high RDW shows greater size variability and can indicate iron, B12 or folate deficiency, mixed anemias, inflammation, or chronic disease, and is linked with higher health risks. Persistently high RDW should prompt medical evaluation and further testing.
Q: How to cure high RDW SD?
A: High RDW‑SD is treated by identifying and correcting the underlying cause. Common steps: test for and treat iron, B12 or folate deficiency with supplementation and dietary changes; manage chronic inflammation or liver disease; treat hemolysis or bleeding; stop causative drugs; and refer for bone marrow evaluation if indicated. Repeat blood counts to monitor response and follow your healthcare provider’s guidance.
Q: What are the dangers of high RDW?
A: High RDW indicates greater variation in red blood cell size and commonly reflects anemia, nutrient deficiencies (iron, B12, folate), bone marrow dysfunction, chronic inflammation, or liver/kidney disease. It is associated with higher risk of cardiovascular events, heart failure, stroke, sepsis and overall mortality, and may signal worse prognosis or treatment response. Elevated RDW warrants further evaluation to identify and treat underlying causes.
Q: What happens if RDW SD is low?
A: Low RDW‑SD means red blood cells are unusually uniform in size, indicating minimal anisocytosis. By itself it’s usually not clinically significant. Causes can include recent transfusion (homogeneous donor cells) or technical/measurement factors. It should be interpreted with hemoglobin, MCV and peripheral smear results and clinical context; discuss abnormal symptoms or other lab changes with your clinician.