An RDW blood test measures how much your red blood cells vary in size. RDW stands for Red Cell Distribution Width, and it’s a standard part of a complete blood count (CBC), one of the most commonly ordered blood panels. The normal range for most labs falls between 11% and 15%. A result outside that range doesn’t diagnose anything on its own, but it gives your doctor an important clue about what might be going on with your blood cells.
What RDW Actually Measures
Your red blood cells aren’t all identical. They naturally come in slightly different sizes. RDW captures the degree of that size variation. When your red blood cells are fairly uniform, your RDW is low or normal. When there’s a wide mix of large and small cells circulating together, your RDW goes up. The medical term for this size mismatch is anisocytosis.
The number you see on your lab report is typically the RDW-CV, a percentage that represents how spread out the cell sizes are relative to the average cell volume. Some labs also report RDW-SD, which is an absolute measurement in femtoliters. The key difference: RDW-CV can be slightly skewed if your average cell size is unusually high or low, while RDW-SD isn’t affected by that. In practice, most doctors work with RDW-CV, and that’s the number in the 11% to 15% reference range.
Why Your Doctor Looks at RDW
RDW is most useful when read alongside another value on your CBC called MCV, or mean corpuscular volume, which tells you the average size of your red blood cells. Together, RDW and MCV help narrow down what type of anemia you might have, or whether anemia is even the issue.
For example, if your red blood cells are small (low MCV) and your RDW is elevated, iron deficiency anemia is a likely suspect. But if your cells are small and your RDW is normal, that pattern points more toward a genetic condition like thalassemia. Large cells with a high RDW suggest a vitamin B12 or folate deficiency, while large cells with a normal RDW lean toward chronic liver disease or certain bone marrow disorders. This pairing narrows the diagnostic window significantly before any further testing.
A normal RDW doesn’t rule out a problem. Several conditions, including anemia caused by chronic disease, acute blood loss, and some bone marrow cancers, can show up with a completely normal RDW. That’s why your doctor interprets RDW as one piece of a larger puzzle rather than a standalone answer.
What a High RDW Means
A high RDW (above 15%) means your body is producing red blood cells that are more varied in size than expected. The most common reason is a nutritional deficiency. Iron deficiency is the leading cause, followed by deficiencies in vitamin B12 and folate. These nutrients are essential for building red blood cells of the right size, and when supply runs low, the body starts churning out cells of inconsistent dimensions.
One particularly useful feature of RDW is its sensitivity to early iron deficiency. In the progression from healthy iron stores to full-blown iron deficiency anemia, RDW is often the first blood value to shift. Hemoglobin and other red blood cell markers tend to change only in the later stages, which means an elevated RDW can flag a problem before you’d technically meet the criteria for anemia. A study of adolescent girls found that those with iron deficiency had a median RDW of 16.3%, compared to 14.9% in those with sufficient iron stores, even when hemoglobin levels hadn’t dropped dramatically.
Beyond nutritional deficiencies, a high RDW can reflect a range of other conditions: hemolytic anemias (where red blood cells are destroyed faster than normal), sickle cell disease, inflammatory bowel disease, kidney disease, recent blood transfusions, and pregnancy. Chronic inflammation also drives RDW up, because inflammatory signals interfere with the normal production of red blood cells in bone marrow.
What a Low RDW Means
A low RDW simply means your red blood cells are very uniform in size. This is not associated with any particular disease and is generally not a cause for concern. It’s essentially a sign that your red blood cell production is consistent, which is what you’d expect in a healthy person.
RDW as a Broader Health Marker
Over the past decade, researchers have found that RDW predicts outcomes well beyond anemia. Elevated RDW is now recognized as an independent marker of mortality risk, particularly in people with heart disease or diabetes. In a large study of patients with diabetes, those in the highest RDW group had roughly 2.4 times the risk of dying from any cause and about twice the risk of dying from cardiovascular disease compared to those with the lowest RDW, even after adjusting for 17 other health factors. The risk of death began climbing at an RDW of around 12%, and values above 15% were associated with approximately a tenfold increase in mortality.
RDW has also shown prognostic value in heart failure, coronary artery disease, stroke, and chronic kidney disease. The connection likely runs through inflammation and oxidative stress. RDW correlates with C-reactive protein (a well-known inflammation marker) and with markers of oxidative damage. When the body is under chronic stress from inflammation or organ dysfunction, bone marrow produces red blood cells of irregular size, and RDW captures that disruption. There’s also evidence that misshapen, variably sized red blood cells are stiffer and less deformable, which can worsen blood flow and contribute to heart rhythm problems like atrial fibrillation.
None of this means a slightly elevated RDW on its own should alarm you. It does mean that when RDW is persistently high and nutritional deficiencies have been ruled out, it may prompt your doctor to look at cardiovascular or metabolic health more closely.
How the Test Works
You don’t need to fast or do any special preparation for an RDW test. It’s part of a standard CBC, so if you’ve had routine bloodwork done, you’ve almost certainly had your RDW measured already. A technician draws a small blood sample from a vein, and an automated analyzer measures the volume of thousands of individual red blood cells, then calculates the variation.
Results are typically available within a day. The reference range of 11% to 15% is used by most labs, though slight variations exist depending on the equipment and methodology. Children have a similar normal range, with a median of around 13.2%. There is no universally standardized reference range across all laboratories, so it’s best to compare your results to the specific range printed on your lab report.
What Happens After an Abnormal Result
An elevated RDW will usually lead your doctor to look at the rest of your CBC in detail, particularly hemoglobin, MCV, and your red blood cell count. If the pattern suggests a nutritional deficiency, the next step is typically a blood test for iron levels, ferritin (your body’s iron storage protein), vitamin B12, and folate. If deficiency is confirmed, treatment is straightforward: supplementation and dietary changes, with follow-up bloodwork to confirm improvement.
If nutritional causes are ruled out, further investigation depends on what other values look abnormal. Your doctor might order a peripheral blood smear, where a technician examines your blood cells under a microscope to look for unusual shapes, or tests for kidney function, liver function, or inflammatory markers. The path forward depends entirely on the clinical picture. RDW alone doesn’t point to a single diagnosis, but it reliably signals that something about your red blood cell production deserves a closer look.