Buried in your complete blood count results is a value most patients overlook: RDW. So what does RDW mean in a blood test? RDW stands for red cell distribution width, and it measures how much variation exists in the size of your red blood cells. A high RDW means your red blood cells vary significantly in size — some are much larger or smaller than average — while a normal RDW means they are relatively uniform. This seemingly obscure number can provide important clues about anemia, nutritional deficiencies, and even cardiovascular health.
What RDW Actually Measures
Your bone marrow constantly produces new red blood cells. Ideally, these cells are all roughly the same size — about 6 to 8 micrometers in diameter. RDW quantifies how much size variation exists within your red blood cell population. It is expressed as a percentage, with a normal range typically falling between 11.5 and 14.5 percent, according to MedlinePlus.
A higher percentage means more variation — a mix of large and small cells. The medical term for this variation is anisocytosis. Some size variation is normal, which is why the healthy range starts at 11.5 rather than zero. But when RDW climbs above 14.5 percent, it signals that something is disrupting normal red blood cell production.
RDW is calculated automatically by hematology analyzers as part of the standard complete blood count (CBC). Your doctor interprets it alongside other red blood cell values — particularly MCV (mean corpuscular volume, which measures average cell size) and MCH (mean corpuscular hemoglobin) — to narrow down potential diagnoses.
What Causes High RDW
An elevated RDW indicates that your red blood cells are being produced in inconsistent sizes. The most common causes include:
Iron deficiency anemia — the most frequent reason for elevated RDW. When iron stores become depleted, the body produces smaller-than-normal red blood cells. During the transition from normal to iron-deficient, both normal-sized and undersized cells coexist in the bloodstream, increasing the size variation. The World Health Organization identifies iron deficiency as the most prevalent nutritional disorder worldwide.
Vitamin B12 or folate deficiency — these deficiencies cause the body to produce abnormally large red blood cells (macrocytes). Mixed with remaining normal-sized cells, this creates high RDW. Chronic liver disease, which alters red blood cell metabolism and production. Recent blood transfusion, which introduces donor cells that may differ in size from the recipient’s cells. Hemolytic anemias, where red blood cells are destroyed prematurely and the bone marrow ramps up production of replacement cells in varied sizes.
What Does Low RDW Mean?
A low RDW — below 11.5 percent — is less common and generally not considered clinically significant by most hematologists. It simply means your red blood cells are very uniform in size. In most cases, a low RDW is a normal finding.
Rarely, a very low RDW can be associated with certain conditions where red blood cells are abnormally uniform, but this is not a primary diagnostic concern. If your RDW is on the low side and your other CBC values are normal, there is typically nothing to worry about.
RDW and Heart Disease: An Emerging Connection
One of the more surprising findings in recent research is the association between elevated RDW and cardiovascular disease. Multiple studies, including a large analysis published in the Archives of Internal Medicine, have found that higher RDW levels are independently associated with increased risk of heart attack, heart failure, and cardiovascular death — even in people without anemia.
The exact mechanism is not fully understood. Researchers hypothesize that elevated RDW may reflect underlying chronic inflammation, oxidative stress, or nutritional deficiencies that also contribute to cardiovascular disease. It is important to note that RDW is not used as a standalone predictor of heart disease — it is one piece of a much larger puzzle. However, persistently elevated RDW in someone without an obvious blood disorder may prompt a doctor to look more closely at cardiovascular risk factors.
How Doctors Use RDW to Classify Anemia
RDW becomes particularly powerful when interpreted alongside MCV. Together, these two values help doctors distinguish between different types of anemia:
High RDW with low MCV (small cells) strongly suggests iron deficiency anemia. High RDW with high MCV (large cells) points toward B12 or folate deficiency, or mixed nutritional deficiency. High RDW with normal MCV can indicate early iron deficiency (before cells become uniformly small), mixed deficiency anemia (simultaneous iron and B12 deficiency), or chronic disease. Normal RDW with low MCV may suggest thalassemia trait — a genetic condition where cells are consistently small but uniform in size. Normal RDW with high MCV can indicate aplastic anemia or certain bone marrow disorders.
This classification system, outlined in hematology references like StatPearls, helps doctors order targeted follow-up tests rather than guessing, which saves time and reduces unnecessary testing for patients.
When to Discuss Your RDW With Your Doctor
If your RDW is slightly above the normal range — say, 15 to 16 percent — and your other blood values are normal, it may not require immediate action. Your doctor may recommend rechecking in a few months. However, you should have a conversation with your healthcare provider if your RDW is significantly elevated (above 16 percent), if you have symptoms of anemia such as fatigue, weakness, or shortness of breath, if your RDW has been trending upward over multiple blood tests, or if other CBC values are also abnormal.
Never self-diagnose or start supplements based on RDW alone. Iron supplements, for example, can cause harm if you do not actually have iron deficiency — excess iron accumulates in organs and can lead to serious damage. Always work with your doctor to identify the root cause before starting treatment. For more on understanding your blood test results and other medical terminology, explore our wellness guide and our article on what STAT means.
Frequently Asked Questions
Can stress affect RDW levels?
Chronic stress alone does not directly raise RDW. However, stress can lead to behaviors and conditions — poor diet, alcohol consumption, sleep deprivation, chronic inflammation — that may indirectly affect red blood cell production and raise RDW over time. If your RDW is elevated and you are under significant stress, improving nutrition and overall health habits may help normalize it.
Is RDW included in a standard blood test?
Yes. RDW is automatically calculated and reported as part of a complete blood count (CBC), which is one of the most commonly ordered blood tests. If your doctor has ordered a CBC for any reason — annual physical, pre-surgical screening, symptom evaluation — your RDW is included in the results.
What does it mean if RDW is high but hemoglobin is normal?
This can indicate early-stage iron, B12, or folate deficiency — the body is beginning to produce red blood cells of varying sizes, but total hemoglobin has not dropped enough to meet the threshold for anemia. It can also reflect chronic inflammation or liver disease. Your doctor may order additional tests such as ferritin, iron studies, or vitamin levels to investigate further.
How quickly can RDW change?
Red blood cells live for about 120 days, so changes in RDW reflect shifts in red blood cell production over weeks to months, not days. If you start treatment for an iron or vitamin deficiency, you might see RDW initially rise (as new, properly sized cells mix with older, abnormal ones) before gradually normalizing over 2 to 4 months as the old cells are replaced.