Rdw
Red cell distribution width — how much your red blood cells vary in size. A wide spread can be an early clue to certain anemias.
Typical reference range
| Range | Unit |
|---|---|
| 11.5–14.5 | % |
What Rdw measures
RDW, or red cell distribution width, measures how much your red blood cells vary in size. On a complete blood count (CBC) it sits alongside other red-cell indices, and while any single value tells you little on its own, RDW adds useful context to MCV (average red-cell size) and hemoglobin.
Because red cells are normally fairly uniform, a rise in RDW flags that the population has become more mixed in size. That makes it a handy pointer when a clinician is working out the cause of anemia and wants to know whether one process or several are at play. It is a supporting index, best read as part of the whole CBC rather than in isolation.
What a high Rdw can mean
A high RDW means your red blood cells vary more in size than usual, a state called anisocytosis. It is often one of the earliest clues on a CBC that something is affecting red-cell production.
Common contributors include iron deficiency and shortages of vitamin B12 or folate, and sometimes a mix of causes at once, where newer and older cell populations differ in size. The pattern with other indices matters: a high RDW paired with a low MCV often points toward iron deficiency, while other combinations suggest different paths.
A high RDW is a signal to look closer, not a diagnosis. It is best read together with the rest of the CBC and your history by a clinician, who can decide whether further testing is worthwhile.
What a low Rdw can mean
A low or normal RDW means your red blood cells are fairly uniform in size, with little of the size variation described as anisocytosis. A normal RDW is common and generally reassuring.
On its own a low value carries little standalone meaning, and it is not usually something to act on. RDW earns its value through context: it is most useful when interpreted together with MCV and hemoglobin rather than read in isolation.
If your other red-cell numbers are steady and within their expected range, a low or normal RDW simply adds to a consistent picture. If any related marker looks off, a clinician can weigh the indices together and decide whether anything needs a closer look or follow-up over time.
Track your Rdw over time in Quanome — it reads your lab reports (PDF or photo) on your device, charts each marker against its range, and an AI coach explains what your trends mean alongside your Apple Health and DNA.