RDW (Red Cell Distribution Width)

What is RDW (Red Cell Distribution Width)?

RDW measures how much your red blood cells vary in size — a wider spread can be an early clue to anemia or a nutrient deficiency.

Red blood cells are normally fairly uniform in size; RDW quantifies how much that size actually varies using a histogram generated by the lab's automated counter. A high RDW means your red cells are more mixed in size (anisocytosis) than expected, which can show up before other classic anemia markers change — for example, it often rises early in iron, B12, or folate deficiency. RDW is almost always interpreted together with MCV (average red cell size) and hemoglobin rather than on its own.

What to know about RDW (Red Cell Distribution Width)

Understanding RDW (Red Cell Distribution Width) means knowing what it reflects, what tends to move it, and the signs that suggest it’s worth a closer look — so you can bring the right questions to your next appointment.

How to read a value in context

A normal RDW means the red blood cells are relatively uniform in size, which is reassuring — but it doesn't rule out anemia or a mild nutrient deficiency on its own. It's most useful when read together with MCV, hemoglobin, and other CBC values rather than in isolation.

Lifestyle factors that influence this marker

• Eat a varied diet with iron-rich foods (leafy greens, legumes, lean meats) and vitamin B12/folate sources (eggs, dairy, fortified grains, leafy vegetables)
• Limit excessive alcohol intake, which can interfere with folate absorption and red blood cell production
• Address chronic gut or digestive issues that can impair nutrient absorption, in consultation with a doctor
• Talk to your doctor before making changes to your diet or lifestyle based on lab results

Tests often checked alongside this

• Mean Corpuscular Volume (MCV) — read together with RDW to narrow down the type of anemia
• Iron Studies (Ferritin, Serum Iron, TIBC) — checked when RDW is elevated to rule in or out iron deficiency
• Vitamin B12 and Folate — checked when RDW is elevated alongside a high or normal MCV
• Reticulocyte Count — helps distinguish reduced red cell production from increased destruction as the cause of an abnormal RDW

How this can show up in how you look and feel

• RDW itself typically causes no felt symptoms — it's usually an incidental finding on routine bloodwork
• If an elevated RDW reflects underlying anemia, associated symptoms can include fatigue, shortness of breath, or dizziness
• Pale skin or unusual tiredness may be associated with the deficiency driving an abnormal RDW, not the RDW number itself
• Most people with a mildly abnormal RDW and normal hemoglobin feel completely well

What can affect this marker

• Iron, vitamin B12, or folate deficiency — classic causes of elevated RDW, often appearing before anemia is fully established
• Recent blood transfusion — introduces donor red cells of a different size, transiently raising RDW
• Chronic conditions such as liver disease, kidney disease, or long-term inflammation
• Hemolytic anemia or conditions causing early release of immature red cells (reticulocytes) from the bone marrow

What to track over time

• Whether RDW is trending upward alongside a falling or borderline MCV (could point to developing iron deficiency)
• Whether RDW is trending upward alongside a rising MCV (could point to developing B12/folate deficiency)
• Persistence of an elevated RDW across repeat tests rather than a single isolated result
• Whether RDW improves after starting any deficiency treatment your doctor recommended

What to ask your doctor

• Is my RDW elevated because of a nutrient deficiency, and should we check iron, B12, or folate levels?
• Does my RDW result change how you interpret my hemoglobin or MCV?
• Is a reticulocyte count needed to understand what's driving this result?
• Should I retest RDW after any dietary or supplement changes?

This glossary entry is for informational purposes only and is not a substitute for professional medical advice. Read our Medical Disclaimer.