---

title: "RDW Blood Test Results Explained — Red Cell Distribution Width" slug: "rdw-blood-test-results-explained" description: "Understand your RDW blood test results. Learn what Red Cell Distribution Width means, normal ranges, and what high or low RDW indicates about your health." date: "2026-03-14" category: "blood-tests" keywords: ["RDW blood test", "red cell distribution width", "RDW results", "RDW high", "RDW normal range"] reading_time: "7 min"

RDW Blood Test Results Explained — Red Cell Distribution Width

Your RDW (Red Cell Distribution Width) measures how much variation there is in the size of your red blood cells. It's a standard part of a Complete Blood Count (CBC) — one of the most commonly ordered blood tests — yet most people skip right past this number without understanding what it reveals.

A normal RDW means your red blood cells are fairly uniform in size. An elevated RDW means your red blood cells vary significantly, a condition called anisocytosis. This variation can point your doctor toward the cause of anemia or flag other health issues that might otherwise go unnoticed.

What Does RDW Measure?

Healthy red blood cells are roughly the same size — about 6 to 8 micrometers in diameter. Your bone marrow constantly produces new red blood cells to replace old ones that are broken down after roughly 120 days in circulation.

RDW quantifies the degree of size variation among your circulating red blood cells. Labs measure RDW by analyzing thousands of red blood cells using an automated hematology analyzer and calculating how much their volumes differ from the average.

There are two ways RDW is reported:

• RDW-CV (Coefficient of Variation) — the most commonly reported form. It expresses variation as a percentage of the mean cell volume.
• RDW-SD (Standard Deviation) — a direct measure of the width of the red blood cell size distribution curve, reported in femtoliters (fL).

Most lab reports show RDW-CV. When this guide refers to RDW without further specification, it means RDW-CV.

Normal Ranges

| Measurement | Normal Range | |---|---| | RDW-CV | 11.5–14.5% | | RDW-SD | 39–46 fL |

These ranges are generally the same for men and women and do not change significantly with age in adults. Newborns and infants have slightly higher RDW values, which normalize by early childhood.

Note that reference ranges may vary slightly between laboratories. Always compare your results against the ranges printed on your specific lab report.

What High RDW Means

A high RDW (above 14.5% for RDW-CV) means your red blood cells are more varied in size than normal. This condition, called anisocytosis, occurs when the bone marrow is producing red blood cells of abnormal sizes or when different populations of normal and abnormal cells are circulating at the same time.

Common causes of elevated RDW include:

Iron Deficiency Anemia

The most common cause of high RDW. When iron stores are depleted, the bone marrow produces smaller-than-normal red blood cells (microcytes) while older normal-sized cells are still in circulation. This mix of small and normal cells drives the RDW up. Iron deficiency RDW values can reach 18–20% or higher.

Vitamin B12 or Folate Deficiency

Deficiencies in B12 or folate cause the bone marrow to produce abnormally large red blood cells (macrocytes). As with iron deficiency, the mixture of large new cells and normal older cells increases the RDW. These deficiencies are common in older adults, vegans (B12), and people with malabsorption conditions.

Mixed Nutritional Deficiencies

Some patients have both iron deficiency and B12 or folate deficiency simultaneously. This produces both small and large red blood cells, resulting in a particularly high RDW. The MCV (Mean Corpuscular Volume) may appear normal because the small and large cells average out, but the RDW reveals the underlying problem.

Liver Disease

Chronic liver disease can raise RDW through several mechanisms: impaired folate metabolism, altered red blood cell membrane composition, and increased red blood cell destruction (hemolysis). Studies have found that elevated RDW in liver disease patients correlates with disease severity.

Hemolytic Anemias

Conditions that cause premature destruction of red blood cells — such as sickle cell disease, thalassemia, autoimmune hemolytic anemia, and hereditary spherocytosis — lead to increased production of young red blood cells (reticulocytes). These reticulocytes are larger than mature cells, increasing size variation and raising the RDW.

Recent Blood Transfusion

Receiving a blood transfusion introduces donor red blood cells that may differ in size from the recipient's own cells, temporarily raising the RDW.

Chronic Inflammatory Conditions

Chronic inflammation can impair iron metabolism (functional iron deficiency) and interfere with normal red blood cell production, leading to anisocytosis and elevated RDW.

Using RDW and MCV Together to Classify Anemia

One of the most clinically useful applications of RDW is interpreting it alongside the MCV (Mean Corpuscular Volume), which measures average red blood cell size. Together, these two values help narrow down the cause of anemia.

| MCV | RDW Normal | RDW High | |---|---|---| | Low MCV (< 80 fL) | Thalassemia trait, chronic disease | Iron deficiency anemia, sickle cell-beta thalassemia | | Normal MCV (80–100 fL) | Chronic disease, acute blood loss | Early iron deficiency, early B12/folate deficiency, mixed deficiency, sickle cell disease | | High MCV (> 100 fL) | Aplastic anemia, myelodysplastic syndrome | B12 deficiency, folate deficiency, liver disease, some medications |

This classification is a clinical starting point, not a definitive diagnosis. Your doctor will use it alongside your symptoms, physical exam, and additional lab tests (such as iron studies, B12 level, reticulocyte count, and peripheral blood smear) to determine the actual cause.

What Low RDW Means

A low RDW (below 11.5% for RDW-CV) means your red blood cells are very uniform in size. In most cases, this is completely normal and not clinically significant.

Rarely, a low RDW can be seen in:

• Macrocytic anemias with uniform cell populations — some cases of aplastic anemia or myelodysplastic syndrome produce uniformly large cells, resulting in a high MCV but normal or low RDW.
• Iron deficiency before anemia develops — in very early iron depletion, the body may produce uniformly small cells before a mixed population develops.

In practice, doctors rarely pursue a low RDW in isolation. It becomes relevant mainly when combined with other abnormal CBC values.

RDW and Cardiovascular Risk

Research published over the past decade has revealed a surprising connection between elevated RDW and cardiovascular disease. Multiple large studies have found that higher RDW — even within the upper range of normal — is an independent predictor of:

• Heart failure outcomes — elevated RDW is associated with higher mortality in patients with heart failure, independent of hemoglobin and other traditional risk factors.
• Coronary artery disease — higher RDW has been linked to worse outcomes after heart attacks and in stable coronary artery disease.
• Atrial fibrillation — some studies have found that elevated RDW predicts new-onset atrial fibrillation.
• Overall mortality — several population-based studies, including analyses of NHANES data, have found that higher RDW is associated with increased all-cause mortality in the general population.

The exact mechanisms are not fully understood. Leading theories suggest that elevated RDW reflects underlying chronic inflammation, oxidative stress, and impaired iron metabolism — all of which are risk factors for cardiovascular disease. RDW is not currently used as a standalone screening tool for heart disease, but it adds prognostic value when interpreted alongside other cardiovascular risk markers.

When to See a Doctor

A slightly elevated RDW on a single test is not usually an emergency. However, you should talk to your doctor if:

• Your RDW is above 14.5% and you have symptoms of anemia — fatigue, weakness, shortness of breath, pale skin, dizziness, or cold hands and feet.
• Your RDW is elevated along with a low hemoglobin or hematocrit, confirming anemia that needs further workup.
• Your RDW has been trending upward across multiple tests, which may indicate a developing nutritional deficiency or worsening chronic condition.
• You have known heart failure or cardiovascular disease and your RDW is elevated — this may carry prognostic significance worth discussing with your cardiologist.
• You have unexplained fatigue and your RDW is high while other CBC values appear normal — this can sometimes be the earliest sign of a developing B12 or iron deficiency before full anemia develops.

FAQ

What does a high RDW mean in a blood test?

A high RDW means your red blood cells vary more in size than normal, a condition called anisocytosis. The most common cause is iron deficiency anemia, but it can also result from B12 or folate deficiency, liver disease, hemolytic anemias, or chronic inflammatory conditions. Your doctor will interpret RDW alongside your MCV, hemoglobin, and other tests to determine the cause.

Can RDW be high without anemia?

Yes. RDW can be elevated before anemia fully develops — it may be one of the first markers to change in early iron, B12, or folate deficiency. RDW can also be elevated in conditions unrelated to anemia, including liver disease, cardiovascular disease, and chronic inflammation.

What is the difference between RDW-CV and RDW-SD?

RDW-CV (Coefficient of Variation) expresses red blood cell size variation as a percentage of the mean cell volume. Its normal range is 11.5–14.5%. RDW-SD (Standard Deviation) measures the actual width of the cell size distribution in femtoliters, with a normal range of 39–46 fL. Most labs report RDW-CV. RDW-SD can be more sensitive in detecting mixed cell populations because it is not affected by changes in MCV.

Does diet affect RDW?

Yes. Nutritional deficiencies are the most common cause of elevated RDW. A diet low in iron, vitamin B12, or folate can lead to abnormal red blood cell production and raise your RDW. Correcting the deficiency through diet or supplementation typically normalizes the RDW over several weeks to months as old cells are replaced by new, properly-sized ones.

How long does it take for RDW to normalize after treatment?

Because red blood cells live approximately 120 days, it typically takes 2 to 4 months for the RDW to normalize after the underlying cause is corrected. The reticulocyte count (a measure of new red blood cell production) usually improves within 1 to 2 weeks of starting treatment, providing earlier evidence that therapy is working.

Track Your RDW Trends with healthbook.my

A single RDW value tells you something, but tracking it over time tells you much more — whether a deficiency is developing, whether treatment is working, or whether your red blood cells are returning to normal. With healthbook.my, you can upload your CBC results and get instant AI-powered explanations of your RDW and every other blood cell marker — in plain language, with trends tracked automatically across multiple lab reports. See how your RDW has changed over time and understand what those changes mean for your health.