CBC Updated Apr 17, 2026

Red Cell Distribution Width (RDW)

Red Cell Distribution Width (RDW) is a CBC value that shows how much red blood cell size varies from cell to cell. On a Red Cell Distribution Width blood test, the result helps describe whether the red cell population is fairly uniform or more mixed in size. RDW is often read together with MCV, RBC, Hct, and hemoglobin for a fuller view of red cell measurements.

What Is Red Cell Distribution Width (RDW)?

Red Cell Distribution Width (RDW) is a CBC measurement that shows how much red blood cell size varies from cell to cell. It is reported on a Red Cell Distribution Width on a lab report or a Red Cell Distribution Width on a blood test as a percentage. RDW reflects how uniform or mixed the red cell population is, rather than how many red cells are present. RDW is one of the standard values used to describe red cell composition in a routine CBC.

Why Is Red Cell Distribution Width (RDW) Tested?

RDW is included in the complete blood count (CBC), which is one of the most common blood panels. A Red Cell Distribution Width test is often ordered or reported with other CBC values such as hemoglobin, hematocrit (Hct), red blood cell count (RBC), and mean corpuscular volume (MCV). The RDW test helps add detail to the overall pattern of red cell measurements on a lab report. RDW is not part of a CMP, lipid panel, or thyroid panel, but it is commonly reviewed alongside results from those panels when a broader blood profile is being read.

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Red Cell Distribution Width (RDW) Normal Range

Group Range Unit
Adult Male 11.5–14.5 %
Adult Female 11.5–14.5 %

Reference ranges may vary by laboratory and individual factors.

What Does High RDW Mean?

A high RDW means the red blood cells vary more in size than usual. On a Red Cell Distribution Width on a lab report, high RDW is commonly described when the value is above the local reference interval, often around 14.5% to 15.0% in adult reference systems. A high RDW on a blood test can reflect a mix of older and newer red cells, or a shift in red cell production that makes cell sizes less uniform. In practice, high RDW is interpreted with MCV, RBC, and hemoglobin, not by itself.

Associated factors

Recent blood loss — the marrow may release newer, larger cells while older cells remain in circulation, increasing size spread.
Low iron intake — reduced building material for red cells can lead to a wider mix of cell sizes.
Low folate intake — limited nutrient supply can alter red cell production and size consistency.
Low vitamin B12 intake — changes in red cell maturation can increase variation in cell size.
Recovery after blood loss or donation — a younger red cell mix can temporarily raise RDW.
Living at higher altitude — increased red cell production can create a broader size mix.
Smoking — may shift red cell turnover and the distribution of cell sizes.
Pregnancy — changing blood volume and red cell turnover can affect RDW.
Recent intense exercise — temporary fluid shifts and red cell turnover can influence a Red Cell Distribution Width test result.
Some medications — drugs that affect red cell production or turnover can change RDW values.

What Does Low RDW Mean?

A low RDW means the red blood cells are more uniform in size than usual. Low RDW is less often emphasized than high RDW, but it can appear when the red cell population is very even. A low RDW on a Red Cell Distribution Width test result is usually read together with MCV and RBC to understand the pattern. Many Red Cell Distribution Width normal range references include a narrow spread, so low RDW often means values below about 11.5% to 12.0% depending on the lab.

Associated factors

Very uniform red cell production — cells are released with similar size, narrowing the spread.
Stable nutrient intake — consistent iron, folate, and B12 supply can keep size variation small.
Low red cell turnover — fewer newly released cells may reduce variation in the mix.
Long-term consistent hydration — less fluctuation in plasma volume can make RDW more even.
Some inherited red cell traits — naturally uniform cell sizing can produce low RDW.
Measurement method differences — analyzer settings and lab technique can slightly shift RDW.
Recent fluid loading — increased plasma volume can make the measured spread appear smaller.
Lower altitude residence — less stimulation of red cell production can keep sizes more uniform.

How Red Cell Distribution Width (RDW) Relates to Other Values

RDW is read with mean corpuscular volume (MCV), red blood cell count (RBC), hemoglobin, hematocrit (Hct), and mean corpuscular hemoglobin (MCH). MCV shows the average red cell size, while RDW shows how spread out the sizes are. A Red Cell Distribution Width test result with a normal MCV and high RDW suggests the cell sizes are mixed even if the average size looks typical. RDW on a blood test is also more useful when compared with RBC and Hct, because those values show how many cells are present and how concentrated the sample is.

What Factors Affect Red Cell Distribution Width (RDW) Levels?

RDW can vary by age, sex, hydration, altitude, and recent blood loss or donation. Time of day and recent exercise can also shift a Red Cell Distribution Width test result slightly because plasma volume changes the measured concentration. Diet and nutrient intake affect RDW over time by changing red cell production and turnover. Different lab analyzers may report slightly different Red Cell Distribution Width normal range values, so the reference interval on the report matters. RDW may also differ modestly across populations because red cell size patterns are not identical in every group.

How It Is Tested

RDW is measured from a blood sample drawn from a vein, usually as part of a CBC. The lab analyzer counts and sizes red blood cells and calculates the spread in cell size, then reports the result as a percentage (%). The same Red Cell Distribution Width test is often printed with RBC, Hgb, Hct, and MCV on the report.

How to Prepare

No fasting is usually required for a Red Cell Distribution Width test as part of a routine CBC. RDW is commonly measured from the same sample used for other CBC values.

Frequently Asked Questions

What is the normal range for Red Cell Distribution Width?
A common Red Cell Distribution Width normal range is about 11.5% to 14.5%, though the exact interval depends on the lab and analyzer. On a CBC, RDW is usually reported as a percentage (%). The reference range printed on the lab report is the best match for that specific Red Cell Distribution Width test result.
What does RDW stand for?
RDW stands for Red Cell Distribution Width. It is a CBC value that describes how much red blood cell size varies. In a blood test, RDW is usually shown as a percentage.
What does a high Red Cell Distribution Width mean on a lab report?
A high RDW means the red blood cells vary more in size than usual. On a Red Cell Distribution Width on a lab report, this often reflects a mixed population of older and newer red cells. High RDW is read together with MCV, RBC, Hct, and hemoglobin to describe the overall red cell pattern.
What does a low Red Cell Distribution Width mean on a lab report?
A low RDW means the red blood cells are relatively uniform in size. Low RDW is less common as a focus, but it can still be part of the Red Cell Distribution Width test result. It is usually interpreted with MCV and RBC rather than on its own.
Can hydration affect Red Cell Distribution Width?
Hydration can affect a Red Cell Distribution Width test result slightly because changes in plasma volume can shift concentration-based measurements. RDW itself reflects size variation, but fluid changes may still alter how the result appears on a CBC. Exercise and fluid intake near the time of testing can also have small effects.
What is the difference between Red Cell Distribution Width and MCV?
RDW shows how much red blood cell size varies, while MCV shows the average size of the cells. A Red Cell Distribution Width test result can be normal even when MCV is unusual, or RDW can be high when MCV is still in range. Together, RDW and MCV give a fuller picture of red cell size patterns.
What unit is Red Cell Distribution Width measured in?
RDW is usually measured in percent (%). Some labs may report the value with a slightly different format, but CBC reports most commonly use %. The Red Cell Distribution Width reference range on the lab report is usually shown in the same unit.
How much can Red Cell Distribution Width change between tests?
RDW usually changes only a little between close-together tests if the blood sample and lab method are similar. Small shifts can happen from hydration, recent blood loss or donation, and analyzer variation. A change outside the Red Cell Distribution Width normal range is more noticeable than a small day-to-day shift.
Is Red Cell Distribution Width different for men and women?
Many labs use the same Red Cell Distribution Width normal range for adult men and adult women. Some reference systems may vary slightly by method or population, but the difference between men and women is usually small. The lab report should list the correct range for the specific test.
Why is Red Cell Distribution Width tested in a CBC?
RDW is included in a CBC because it adds detail to the red cell profile. A Red Cell Distribution Width test helps show whether the red cells are spread out in size or relatively uniform. It is most useful when read with MCV, RBC, hemoglobin, and hematocrit (Hct).
What does RDW mean on a blood test?
RDW on a blood test means Red Cell Distribution Width. It describes the amount of variation in red blood cell size. The result is part of the CBC and is usually reported as a percentage.

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.

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