What is RDW?
RDW measures the variation in size among red blood cells. A high RDW means cells vary considerably in size; a normal RDW means they are fairly uniform.
RDW is particularly useful when combined with MCV. For example, iron deficiency often raises RDW before MCV changes, making it an early clue to nutritional deficiency.
Why might RDW be high?
A high RDW (anisocytosis) indicates a wide variation in red blood cell size. This is commonly seen in iron deficiency, B12 or folate deficiency, and mixed anemias.
This information is educational. A single out-of-range result is rarely the whole picture — a healthcare professional can interpret your numbers alongside your overall health and history.
Context questions that matter
- →Was this result slightly or clearly outside the reference range?
- →Were related markers also high or low?
- →Is this new compared with previous tests?
- →Could recent illness, exercise, fasting, alcohol, supplements, or medication affect it?
- →Do you have symptoms that should be discussed with a clinician?
Markers often reviewed together with RDW
Clinicians rarely look at one number in isolation. These markers usually accompany RDW on a complete blood count panel and help put the result in context.
Questions to ask your doctor
- →How does this high RDW fit with the full CBC — WBC differential, hemoglobin, MCV, RDW and platelets?
- →Could recent illness, inflammation, bleeding, hydration or stress affect this result?
- →Should this be rechecked, or compared with my previous CBC results?
Don't guess from one flagged result.
Start with the most relevant panel and see how your markers fit together in plain English.
Frequently asked questions
What does high RDW mean?
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A high RDW (anisocytosis) indicates a wide variation in red blood cell size. This is commonly seen in iron deficiency, B12 or folate deficiency, and mixed anemias.
What is RDW?
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RDW measures the variation in size among red blood cells. A high RDW means cells vary considerably in size; a normal RDW means they are fairly uniform.
Should one CBC marker be read alone?
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Usually no. WBC and the differential, RBC, hemoglobin, hematocrit, MCV, RDW and platelets often make sense as a pattern. For example, MCV and RDW together help classify the type of anemia far better than hemoglobin alone.
Can recent illness affect CBC results?
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Recent infection, inflammation, bleeding, dehydration, stress or recovery can all influence CBC markers. A single mildly abnormal CBC, especially after an acute event, is often rechecked once the situation has settled.