Lab results explained
RDW Blood Test Explained: What a High Red Cell Distribution Width Means
RDW is one of the less familiar numbers on a complete blood count (CBC). It measures how much your red blood cells vary in size. On its own it says little, but read with MCV and hemoglobin it helps your clinician narrow down the cause of anemia or a nutrient deficiency.
Part of Complete blood count (CBC)
Typical ranges at a glance
- RDW-CV, usual
- About 11.5 to 14.5% (some labs up to 15.4%)
- RDW-SD, usual
- About 39 to 46 fL
- High RDW
- Above your lab's upper limit
Labs report RDW as a percentage (RDW-CV), in femtolitres (RDW-SD), or both. Use the range printed on your report.
What does RDW measure?
Red blood cells are normally similar in size. RDW (red cell distribution width) shows how much they vary. A high RDW means your blood contains a wider mix of small and large cells (called anisocytosis), often because new cells are being made under different conditions than older ones, for example as iron runs low.
Reading RDW with MCV
MCV is the average size of your red cells. Together the two are more useful: a high RDW with a low MCV often points to iron deficiency; a high RDW with a high MCV can point to low vitamin B12 or folate, or liver disease; a normal RDW with a low MCV is typical of thalassemia trait. A high RDW with a normal MCV can mean an early deficiency or two deficiencies at once.
Common reasons for a high RDW
Iron deficiency (from low intake, heavy periods, pregnancy or blood loss), vitamin B12 or folate deficiency, recent bleeding, a recent blood transfusion, hemolysis (red cells breaking down early), liver disease, heavy alcohol use and some cancer treatments. A slightly high RDW with an otherwise normal blood count is common and often not significant.
What about a low RDW?
A low RDW means your red cells are very uniform in size. It is rarely a sign of a problem on its own and usually does not need follow-up.
RDW and long-term health
In large studies, a persistently high RDW is linked with higher rates of heart disease and other chronic conditions. This is an association, not a diagnosis, and RDW is not used on its own to judge your risk. It is one more reason to look at the trend in your blood counts over time.
What usually happens next
If RDW is high together with an abnormal hemoglobin or MCV, clinicians often check ferritin (iron stores), vitamin B12, folate and sometimes a reticulocyte count or blood smear. Treatment is aimed at the cause, for example iron or vitamin replacement, and RDW usually returns toward normal over a few months as new red cells replace old ones.
Frequently asked questions
Should I be worried about a high RDW?
A high RDW on its own is a clue, not a diagnosis. If the rest of your blood count is normal, your clinician may simply repeat it. If hemoglobin or MCV is also abnormal, the next step is usually tests for iron, vitamin B12 and folate.
Can low iron cause a high RDW?
Yes. Iron deficiency is one of the most common causes. RDW often rises early, sometimes before hemoglobin falls, because new red cells made with too little iron are smaller than older ones.
Can dehydration affect RDW?
Dehydration mainly affects concentration-based results such as hemoglobin and hematocrit. It does not usually change RDW in a meaningful way.
How long does it take RDW to return to normal?
Red blood cells live about 120 days, so after the cause is treated RDW typically improves over two to four months as new, uniform cells replace older ones.
See your blood count as a trend
Add your results to VHT to keep every complete blood count in one timeline, and ECHNA, your AI health assistant, explains how RDW, MCV and hemoglobin moved together.
