RDW measures how much your red blood cells vary in size (and volume). Healthy blood tends to have red cells of fairly uniform size; RDW puts a number on how mixed the sizes are — clinicians call a wide spread "anisocytosis." It is reported as a percentage (a higher percentage means more variation) and is technically derived from the coefficient of variation, or standard deviation, of red cell volume.
RDW is reported automatically as part of a complete blood count (CBC), one of the most common blood panels. Its main role is to help investigate anemia — flagging it and helping sort out the cause. It is most informative when read alongside the MCV (mean corpuscular volume, the average red cell size); the MCV-plus-RDW pair is a routine first step in classifying an anemia. A clinician may pay particular attention to it with symptoms that can accompany anemia (such as fatigue, weakness, shortness of breath, or dizziness) or risk factors like a low-iron diet, chronic illness, recent blood loss, or a family history of inherited blood disorders. It is always read together with other results, not alone.
Reported as a percentage. As rough orientation only, published references put the normal adult interval at roughly 11.5% to 15%, and another gives it as about 13% plus or minus 1.5% (i.e., about 11.5–14.5%). These are general orientation, NOT a diagnostic cutoff — normal values VARY between laboratories and can shift with age and sex, so the authoritative range for your result is the one printed on your own lab report. Do not use these numbers to self-diagnose.
A high RDW means your red cells vary in size more than usual (more anisocytosis). By itself it names no cause and is not a diagnosis; it is a clue that can help a clinician work up anemia and can be raised in a range of conditions. Its value is mainly in combination with MCV: for example, a small-cell (low-MCV) picture with a high RDW commonly steers a workup toward iron deficiency, whereas a large-cell (high-MCV) picture with a high RDW may direct attention toward vitamin B12 or folate deficiency; a high RDW can also suggest a "mixed" picture with more than one process. These are directions for further testing, not conclusions — only a clinician, using additional tests, can determine what is actually going on.
A normal or low RDW means your red cells are relatively uniform in size. On its own this is generally unremarkable and, by itself, is not a sign of anemia and usually not something to worry about. However, a normal RDW does not rule everything out — some conditions can occur with a normal RDW — so it is still interpreted in the context of the MCV, the rest of the CBC, and your symptoms and history.
Reference ranges vary by laboratory, age, sex, and method — the range on your own report is what applies to you. A single value out of range doesn’t confirm any condition; your clinician interprets it alongside your symptoms, history, and other results. This page is general information, not medical advice.
General reference, not medical advice, and not a substitute for your clinician. Lab reference ranges and interpretation depend on the laboratory and on your individual situation — discuss your results with a licensed healthcare professional.