⚠ Educational only. LabPlain does not provide medical advice or diagnosis. Always discuss your specific results with your healthcare provider.
What this test measures
RDW stands for Red Cell Distribution Width. Despite the word "width," it doesn't measure the actual physical size of your red blood cells. Instead, it measures the variation in size among your red blood cells. It evaluates how uniform or mixed your cell sizes are.
Normally, your red blood cells should be roughly the same size (about 6 to 8 micrometers in diameter). If your RDW is low or normal, it means your cells are mostly uniform. If your RDW is high, it means you have a mixture of unusually small cells and unusually large cells mixed together—a condition medically known as anisocytosis.
Your doctor typically orders an RDW as part of a Complete Blood Count (CBC). It is exceptionally helpful for distinguishing between different types of anemia, especially when evaluated alongside the MCV (Mean Corpuscular Volume), which measures the average size of those cells.
Normal reference range
Adult Standard Range: 11.5% – 15.0%
The RDW is expressed as a percentage. This percentage represents the mathematical variation (the coefficient of variation) of red blood cell volumes. A value within this range implies your red blood cells are structurally uniform.
Reference ranges can vary slightly depending on the lab processing your blood, as well as factors like age and sex. Always check the exact reference range printed on your specific lab sheet.
⏰ Low RDW is not an issue
While lab reports highlight high values, they rarely flag low values. A low RDW (under 11.5%) simply means your red blood cells are incredibly uniform in size. This is entirely healthy and normal—it is never considered a medical problem.
What your result might indicate
↑ If High
A high RDW indicates significant variation in cell sizes. This is an early warning sign of nutritional deficiencies (like iron, B12, or folate deficiency), early bone marrow issues, or recent blood loss where the body is rushing new, mixed-sized cells into circulation.
↓ If Low / Normal
A normal or low RDW means your cells are uniform. However, you can still have anemia with a normal RDW if all your red blood cells are uniformly small (like in Thalassemia trait) or uniformly large. Normal RDW does not completely rule out blood issues.
Symptoms associated with abnormal RDW
↑ High RDW Symptoms
Chronic fatigue and low energy
Shortness of breath during exertion
Pale skin, gums, or nail beds
Dizziness, lightheadedness, or vertigo
Cold hands and feet
Brittle nails or unusual hair loss
Rapid or irregular heartbeat
↓ Low / Normal Symptoms
No distinct symptoms for low RDW
Healthy red cell distribution
Uniform cell size profiles
General anemia symptoms if MCV is abnormal
Common causes of abnormal RDW
What can raise RDW?
The most common cause of an elevated RDW is Iron Deficiency Anemia. In its early stages, the bone marrow begins producing smaller red blood cells due to lack of iron, creating a wide mix of old normal cells and new small cells, spiking the RDW percentage.
Other major causes include Vitamin B12 or Folate deficiencies, which cause the body to produce oversized, malformed red blood cells. Chronic conditions can also interfere with healthy red blood cell production and lead to elevated variation, including chronic kidney disease, liver disease, chronic inflammation, recent blood transfusions, or certain cardiovascular diseases.
What can lower RDW?
There are no medical conditions that cause a pathologically low RDW. If your RDW is lower than the reference range, it simply reflects that your red blood cells are highly identical in size. This is considered an optimal finding and requires no clinical treatment or follow-up.
Tests often ordered alongside RDW
RDW is virtually never evaluated by itself. To solve the diagnostic puzzle, your doctor will look at other markers in your Complete Blood Count (CBC) and iron panel:
MCV (Mean Corpuscular Volume) — Measures the average size of your red cells. High RDW + Low MCV points directly to iron deficiency. High RDW + High MCV points to B12/folate deficiency.
Hemoglobin & Hematocrit — Determines the severity of your overall anemia status.
Ferritin & Iron Panels — Evaluates the exact iron storage reserves inside your body.
Reticulocyte Count — Measures how fast your bone marrow is producing brand-new red blood cells.
What to do next
An elevated RDW is a highly sensitive clue that something is altering the production of your blood cells, but it requires context. Your doctor will cross-reference your RDW with your MCV and hemoglobin levels to find the exact root cause. Most causes, like basic nutritional deficiencies, are straightforward to treat with targeted dietary shifts or short-term supplementation. Avoid self-diagnosing or treating with iron supplements until your healthcare provider identifies the exact cause.
Questions to ask your doctor
01My RDW is elevated. What does my MCV (average cell size) look like alongside it?
02Could this specific RDW value indicate an early iron deficiency, or should we check B12 and folate levels?
03Do I need a full iron panel or a ferritin test to check my actual iron stores?
04Could any chronic conditions or recent illnesses be causing this size variation in my blood cells?
05If an iron or vitamin deficiency is confirmed, what specific dosages or dietary changes do you recommend?
06When should we run a follow-up Complete Blood Count (CBC) to see if my RDW has stabilized?
Frequently asked questions
Can you have anemia if your RDW is completely normal?
Yes. If all your red blood cells are uniformly small or uniformly large, your RDW percentage will look completely normal because there isn't much size variation. For example, conditions like Thalassemia trait or anemias caused by chronic disease often display a normal RDW alongside low hemoglobin.
Can dehydration or stress cause a high RDW?
Dehydration can artificially concentrate your blood count numbers, but it typically doesn't alter the size variation of your cells. High RDW is usually caused by an underlying production issue in the bone marrow or an ongoing nutritional deficiency rather than short-term stress or daily hydration fluctuations.
How long does it take for a high RDW to return to normal?
Red blood cells have a natural lifespan of roughly 120 days. Once you start treating the root cause (such as taking iron or vitamin supplements), your bone marrow will begin creating uniform cells immediately, but it can take 3 to 4 months for old, uneven cells to cycle out completely and the RDW percentage to normalize.
Why does a blood transfusion cause a high RDW?
When you receive a blood transfusion, you are mixing a donor's red blood cells with your own native red blood cells. Because these two populations of cells naturally differ slightly in their age distribution and size profiles, the automated lab analyzer registers a massive variation, which temporarily spikes your RDW score.