A high RDW on a blood test means your red blood cells vary more in size than normal. This is a measure of anisocytosis, which is the medical term for unequal cell sizes. It is not a diagnosis on its own. Instead, it is a clue that helps your doctor narrow down what is happening, often pointing to conditions like iron deficiency, vitamin B12 deficiency, or early stages of anemia.
What Does RDW Actually Measure?
RDW stands for Red Cell Distribution Width. It tells your doctor how much your red blood cells differ in size from one another.
Normal red blood cells are fairly uniform in size. When your RDW is high, it means there is a wide range of sizes circulating in your blood. Some cells are small, some are large, and some are in between.
The test is part of a complete blood count, or CBC. It is almost always reported alongside other red blood cell measurements like MCV, which is the average size of your red cells.
RDW is reported as a percentage. A normal range is typically around 11.5% to 14.5%, though exact ranges vary slightly by laboratory. When your value goes above that upper limit, it is flagged as high.
Why Would Your RDW Be High On A Blood Test? The Main Causes
A high RDW happens when your bone marrow is producing red blood cells of different sizes. This typically occurs for one of three reasons: a nutritional deficiency, a condition that destroys red cells, or a recent response to treatment.
Iron deficiency anemia is the most common cause. When your body lacks iron, it cannot make hemoglobin properly. The red cells it produces are smaller than normal. At the same time, older and newer cells mix together, creating a wide size range. This combination produces a high RDW.
Vitamin B12 or folate deficiency also raises RDW. These vitamins are needed for DNA synthesis in red blood cell production. Without enough of them, the bone marrow produces large, immature cells called megaloblasts. These large cells mix with normal-sized cells, pushing RDW up.
Hemolytic anemia is another cause. In this condition, red blood cells are destroyed faster than they can be replaced. The bone marrow responds by releasing younger, larger cells called reticulocytes. The mix of young large cells and older normal cells raises RDW.
Recent blood loss or transfusion can also cause a temporary rise. Your body responds to blood loss by ramping up production. New cells entering circulation are larger than the older cells already present. This size mismatch shows up as a high RDW.
How RDW Works With MCV To Find The Problem
RDW is rarely interpreted alone. Your doctor will look at it together with MCV, the mean corpuscular volume. MCV tells you the average red blood cell size. RDW tells you how much variation exists around that average.
Together, these two numbers help separate different types of anemia.
High RDW with low MCV suggests iron deficiency anemia or thalassemia. Iron deficiency tends to produce a very wide variation in cell size. Thalassemia, a genetic condition affecting hemoglobin production, typically produces more uniform small cells, so RDW is often normal or only slightly elevated.
High RDW with normal MCV can indicate early iron deficiency before anemia develops. It can also appear in early B12 deficiency or in mixed deficiencies where small and large cells cancel out the average.
High RDW with high MCV points toward B12 or folate deficiency. It can also appear in liver disease or in people recovering from chemotherapy.
This pattern-based approach is how doctors use RDW in practice. The number alone does not tell them the answer. The combination of RDW and MCV narrows the list of possibilities significantly.
What A High RDW Does NOT Mean
A high RDW is easy to misunderstand. It is important to know what it does not tell you.
RDW does not tell you whether you have cancer. No study has shown that RDW can diagnose any malignancy. Some research has noted that chronically elevated RDW appears more often in people with certain chronic diseases, including some cancers. But this is a statistical association, not a diagnostic tool. Your doctor will not use RDW to screen for cancer.
RDW does not tell you how severe your anemia is. Severity is determined by hemoglobin levels, not RDW. A very high RDW can occur with mild anemia, and a normal RDW can occur with severe anemia.
RDW does not tell you your iron levels directly. It is an indirect marker. If your RDW is high, your doctor will order ferritin, iron, and total iron-binding capacity tests to confirm whether iron deficiency is the cause.
RDW is also not a test for heart disease. Some large population studies have found that people with high RDW have higher rates of cardiovascular events. Researchers do not fully understand why this link exists. It may reflect underlying inflammation or poor nutritional status rather than a direct effect on the heart. RDW has no role in routine cardiac risk assessment.
When A High RDW Is Not A Concern
Not every high RDW requires treatment. In some situations, it is a normal part of recovery.
After a significant bleed or after starting iron therapy, your bone marrow produces many new red cells at once. These new cells are larger than the older population. For a few weeks, RDW will be elevated. This is a sign your body is responding appropriately, not a sign of a new problem.
Pregnancy can also raise RDW. Blood volume expands and iron demands increase. Many pregnant women develop mild iron deficiency, which raises RDW. This is common and typically resolves with proper prenatal nutrition or supplementation.
Some people simply run slightly above the reference range without any underlying disease. A single borderline high value with otherwise normal blood counts is rarely a cause for concern. Your doctor will usually repeat the test or look at trends over time.
What Happens After A High RDW Result
If your RDW comes back high, your doctor will not stop at that single number. The next step is a systematic evaluation.
Your doctor will review your complete blood count, looking at hemoglobin, MCV, and platelet counts. They will ask about symptoms like fatigue, weakness, pale skin, shortness of breath, or unusual bleeding. They will ask about your diet, medications, and family history.
Based on this information, they may order additional blood tests. Ferritin and iron studies check for iron deficiency. Vitamin B12 and folate levels check for nutritional deficiency. A reticulocyte count tells them whether your bone marrow is actively producing new cells.
In most cases, the cause is straightforward. Iron deficiency is by far the most common reason for a high RDW in otherwise healthy adults. Treatment involves addressing the underlying cause, which may mean iron supplements, dietary changes, or investigating a source of blood loss.
If the cause is not clear from initial testing, your doctor may refer you to a hematologist, a specialist in blood disorders. Further testing might include hemoglobin electrophoresis to check for thalassemia or other hemoglobin variants.
Can You Lower A High RDW Naturally?
RDW is not a condition you treat directly. It is a marker of an underlying issue. When the underlying issue resolves, RDW returns to normal on its own.
If iron deficiency is the cause, correcting the deficiency will lower RDW. This means treating the reason for the deficiency, not just taking supplements. In premenopausal women, heavy menstrual bleeding is a common cause. In older adults, gastrointestinal bleeding must be ruled out.
Diet can help maintain healthy red blood cell production. Iron-rich foods include red meat, poultry, fish, lentils, beans, and fortified cereals. Vitamin B12 comes from animal products like meat, eggs, and dairy. Folate is found in leafy green vegetables, citrus fruits, and beans.
The evidence for supplements outside of diagnosed deficiency is weak. Taking iron, B12, or folate without a known deficiency will not lower a normal RDW and can cause harm. Excess iron, in particular, can damage organs. Do not self-treat a high RDW with over-the-counter supplements.
No food or supplement has been shown to directly lower RDW. The only reliable way to normalize it is to diagnose and treat the underlying condition.
When To Take A High RDW Seriously
Most high RDW results are not emergencies. But some situations warrant prompt attention.
If your high RDW comes with a low hemoglobin level, you have anemia. Anemia can cause fatigue, weakness, shortness of breath, and chest pain. Severe anemia requires treatment, which may include supplements, injections, or in rare cases, transfusion.
If you are experiencing bleeding, such as blood in your stool, black tarry stools, or unusually heavy menstrual bleeding, seek medical care. Bleeding can cause iron deficiency and anemia over time.
If you have unintentional weight loss, persistent fever, or night sweats along with abnormal blood counts, tell your doctor. These symptoms warrant a more thorough evaluation.
Otherwise, a high RDW found on routine blood work is a starting point for discussion, not a reason for panic. Ask your doctor what the value means in your specific situation and what testing, if any, is recommended next.
Frequently Asked Questions
Is a high RDW dangerous?
A high RDW itself is not dangerous. It is a marker that signals your doctor to look for an underlying cause, which may or may not be serious.
Can dehydration cause a high RDW?
Dehydration can artificially concentrate your blood and affect many CBC values, but it is not a recognized cause of a genuinely high RDW. If your RDW is elevated, your doctor will look for nutritional deficiencies or other blood disorders rather than dehydration.
What is a normal RDW level?
Most laboratories define a normal RDW as roughly 11.5% to 14.5%. Exact ranges vary slightly by lab, so your report will list the reference range used for your test.
Does a high RDW always mean anemia?
No. A high RDW can appear before anemia develops, such as in early iron deficiency. It can also appear in conditions that do not involve anemia at all.
