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RDW: What Red Cell Size Variation Tells You
All NiaHealth tests are processed in Canadian-certified labs, which are highly trusted by the medical community for their accuracy and reliability. Blood and urine samples are collected by licensed medical professionals and processed using state-of-the-art equipment. NiaHealth is SOC2 Type 2 certified.
What is it?
RDW is a measure of how much your red blood cells vary in size. A higher RDW can show up when the body is under stress that affects red blood cell production, like low iron, low B12 or folate, inflammation, or recovery from blood loss. RDW is not a diagnosis on its own — it is a clue that is most useful when looked at with the rest of the CBC and, if needed, nutrient labs.
Why it matters
RDW can be useful for adding context when evaluating anemia. For this it is most helpful when interpreted with hemoglobin, MCV, and other red cell indices. It can sometimes be an early signal of a developing nutrient issue, especially when other values are borderline.
It is also a general health risk signal in research. Across many studies, a higher RDW tends to track with higher illness burden and higher risk of adverse outcomes. This does not mean RDW causes risk — it is more likely reflecting underlying processes like inflammation, impaired red blood cell production, or nutrient deficiency.
Reference ranges
NiaHealth groups RDW into four categories: Low (at-risk), Optimal, Normal, and High (at-risk).
Low (At-risk): Your red blood cells are very similar in size. On its own, this usually isn't a concern if the rest of your CBC is normal.
Optimal: Normal variation in red blood cell size. If your other CBC markers are normal, this is reassuring.
Normal (In range): A bit more variation in red blood cell size than average, based on data from Canadians and other groups. If your other CBC markers are normal, this is still reassuring.
High (At-risk): More variation in red blood cell size. This can happen with low iron, low B12 or folate, inflammation, or recovery from blood loss. RDW should be looked at along with hemoglobin, MCV, and related labs, not on its own.
There is no single "best" RDW number to aim for. In large population studies, people with a higher RDW have tended to have higher rates of illness and death over time. But this is a pattern seen across big groups, not a prediction about any one person. A higher RDW reflects things like inflammation, low nutrients, or changes in how red blood cells are made. For some people, it reflects genetics or a benign medical condition. The most useful step for a high result is to look at the whole CBC and find and treat any cause, such as a nutrient deficiency.
Fava, C., Cattazzo, F., Hu, Z.-D., Lippi, G., & Montagnana, M. (2019). The role of red blood cell distribution width (RDW) in cardiovascular risk assessment: useful or hype? Annals of Translational Medicine, 7(20), 581.
Xanthopoulos, A., Giamouzis, G., Dimos, A., Skoularigki, E., Starling, R. C., Skoularigis, J., & Triposkiadis, F. (2022). Red blood cell distribution width in heart failure: Pathophysiology, prognostic role, controversies and dilemmas. Journal of Clinical Medicine, 11(7), 1951.
This information is educational and does not replace medical advice, diagnosis, or treatment. Your results should be interpreted with a qualified healthcare professional in the context of your health history.
