An RDW blood test measures how much your red blood cells vary in size. Healthy red blood cells are roughly uniform, so a normal RDW falls between 12.0% and 15.0%. When your result is higher than that, it means your red blood cells are a mix of larger and smaller sizes, a condition called anisocytosis, which can point to nutritional deficiencies, certain anemias, or other underlying health issues.
RDW stands for “red cell distribution width.” It’s one of several values reported as part of a complete blood count (CBC), the routine blood panel most people get during a checkup or before a medical procedure. You don’t need to fast or do any special preparation beforehand.
What RDW Actually Tells You
Think of RDW as a measure of consistency. Your bone marrow constantly produces new red blood cells, and in a healthy person, those cells are about the same size. The RDW calculation looks at the spread of cell sizes in your blood sample and expresses it as a percentage. A low percentage means your cells are uniform. A higher percentage means there’s more size variation.
Most lab reports show RDW-CV, which is the coefficient of variation (normal range: 12.0–15.0%). Some labs also report RDW-SD, measured in femtoliters (normal range: 38.1–51.9 fL). RDW-SD gives a direct measurement of the width of that size distribution rather than a ratio. Your doctor typically looks at RDW-CV unless there’s a specific reason to examine the SD value.
On its own, RDW doesn’t diagnose anything. Its power comes from being read alongside other CBC values, especially MCV (mean corpuscular volume), which tells you the average size of your red blood cells. The combination of RDW and MCV helps narrow down what type of anemia or blood disorder might be present.
What a High RDW Means
A high RDW (above 15%) means your red blood cells are more varied in size than expected. The most common reason is iron deficiency anemia. When your body runs low on iron, it produces smaller-than-normal red blood cells while older, normal-sized cells are still circulating. That mix of sizes pushes the RDW up.
Other conditions that elevate RDW include:
- Vitamin B12 or folate deficiency: These produce abnormally large red blood cells, creating a size mismatch with normal ones.
- Sickle cell anemia: Misshapen cells vary widely in size.
- Autoimmune hemolytic anemia: Your immune system destroys red blood cells prematurely, and the bone marrow releases younger, larger cells to compensate.
- Liver disease or kidney disease: Both can disrupt red blood cell production.
- Thyroid disease: Both overactive and underactive thyroid conditions affect blood cell size.
- Myelodysplastic syndrome or myelofibrosis: Bone marrow disorders that produce abnormal blood cells.
Pregnancy commonly raises RDW as well. The increased demand for iron and nutrients during pregnancy can shift red blood cell production, and mild anemia with elevated RDW is a frequent finding on routine prenatal bloodwork. Certain medications can also cause anisocytosis.
How Doctors Use RDW With MCV
The real diagnostic value of RDW shows up when it’s paired with MCV, which classifies red blood cells as smaller than normal (under 80 fL), normal-sized (80–100 fL), or larger than normal (over 100 fL). Different combinations point to different conditions.
When red blood cells are small and RDW is normal, thalassemia trait is a common explanation, because it produces uniformly small cells. But when red blood cells are small and RDW is high, iron deficiency is more likely, since the body is making smaller cells alongside older normal-sized ones. This distinction matters because the treatments are completely different.
When red blood cells are normal-sized and RDW is high, early iron or folate deficiency is possible. The average size hasn’t shifted yet, but the increasing variation signals that something is changing. It can also indicate sickle cell disease or conditions where the bone marrow produces a mix of cell types.
When red blood cells are large and RDW is high, B12 or folate deficiency is the classic cause. When they’re large but RDW is normal, chronic liver disease, hypothyroidism, or alcohol use are more typical explanations, since these conditions tend to produce uniformly large cells.
What a Low or Normal RDW Means
A low RDW is not a cause for concern. It simply means your red blood cells are very uniform in size, which is normal and healthy. There are no medical conditions specifically associated with a low RDW value.
A normal RDW doesn’t rule out anemia or other blood disorders, though. Some types of anemia, like anemia of chronic disease or thalassemia, involve red blood cells that are abnormal in size but consistently so. The cells are uniformly too small or too large, which keeps the RDW in the normal range. That’s why doctors look at the full CBC panel rather than any single number.
RDW and Heart Disease Risk
Over the past two decades, researchers have found that elevated RDW correlates with cardiovascular risk in ways that go beyond anemia. A large meta-analysis of patients with coronary artery disease found that those with higher RDW values had roughly double the risk of cardiovascular events like heart attack, stroke, and heart failure (a pooled risk ratio of 2.13). For non-fatal cardiovascular events specifically, the increased risk was 86%.
What makes this finding notable is that the association held even after accounting for nutritional status, existing anemia, inflammation, and other illnesses. The exact mechanism isn’t fully understood, but elevated RDW appears to reflect a combination of chronic inflammation, oxidative stress, and poor nutritional status that together contribute to cardiovascular damage. Some researchers have proposed that RDW could serve as a simple, inexpensive marker of overall health decline, since so many different physiological problems cause red blood cell sizes to become uneven.
This doesn’t mean a slightly elevated RDW on your lab report predicts heart disease. These findings come from studies of people who already had coronary artery disease, and the clinical usefulness for screening healthy people isn’t established. But if your RDW is persistently high without an obvious nutritional cause, it’s worth discussing the broader picture with your doctor.