What Does a High RDW (Red Cell Distribution Width) Result Mean?

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A high RDW result reflects red blood cells that vary more in size than expected, a pattern called anisocytosis. On its own, RDW does not diagnose a specific condition. It is a clue your doctor reads alongside the rest of your complete blood count, most often MCV, to sort between causes such as iron, B12, or folate deficiency, recent blood loss, liver disease, or another chronic condition. Many of these causes are common and treatable. This article walks through what can raise RDW, which symptoms may go with it, and when the underlying cause needs urgent care.

RDW is often reported as part of a complete blood count (CBC). The analyzer that runs the CBC records the size, or volume, of thousands of your red blood cells one at a time. Red cell distribution width (RDW) takes that data and reports how much variation exists across the whole population of cells, usually as a percentage called RDW-CV. MedlinePlus describes the test as a measure of how much the volume and size of your red blood cells varies. A low RDW describes red blood cells that are close to the same size as one another. A high RDW describes a wider mix, with some cells noticeably smaller and some noticeably larger than average. Doctors call this variation anisocytosis, and RDW is simply the number that quantifies it.

RDW does not tell you what is causing the variation on its own. It is a width, not a diagnosis. Two people can have the same high RDW number for very different reasons, which is why your doctor reads it next to the rest of your CBC rather than by itself.

Reference ranges differ from lab to lab. MedlinePlus notes that results are usually shown as a percentage and compared with the reference range of the lab that ran the test. RDW-CV commonly falls somewhere between about 11.5 and 15 percent in healthy adults, though the exact cutoff varies with the equipment used, so the range printed on your own report is the one that applies to you. A result above that upper limit is what gets flagged as high.

How far your RDW is outside the laboratory reference range can provide context, but the number alone does not tell you how serious the underlying problem is. Your doctor interprets it alongside hemoglobin, MCV, symptoms, and other test results. MedlinePlus also points out that even with a normal RDW, red blood cells can still be larger or smaller than usual, and anemia or another condition can still be present.

Iron deficiency is a common cause of a high RDW, particularly when it occurs with a low MCV, and NHLBI describes iron-deficiency anemia as the most common type of anemia. As your iron stores drop, your bone marrow can start producing smaller red blood cells while your older, larger cells are still circulating, so the sample contains both sizes at once and the width can widen before you become fully anemic. If your ferritin is low but your hemoglobin is still normal, our guide to low ferritin with normal hemoglobin explains what that early stage can look like. Vitamin B12 and folate deficiency can also widen the spread, this time because the marrow can produce unusually large cells while normal sized older cells remain in circulation. When someone has more than one deficiency at the same time, for example low iron alongside low B12, RDW can rise even higher because the sample now contains small cells and large cells together.

After you lose a meaningful amount of blood, whether from an injury, surgery, a gastrointestinal bleed, or heavy menstrual periods, your bone marrow releases young red blood cells called reticulocytes to replace what was lost. These new cells are larger than mature cells, so mixing them with your existing population can raise RDW. Bleeding also drains iron, and MedlinePlus lists heavy, long, or frequent menstrual periods, long-term use of aspirin, ibuprofen, or medicines that thin the blood, peptic ulcers, and cancers of the digestive tract among the causes of bleeding that can lead to iron deficiency anemia. A recent blood transfusion can also affect RDW because the blood sample may contain red blood cells from two different populations.

MedlinePlus lists chronic liver disease, heart disease, diabetes, kidney disease, and cancer, especially colorectal cancer, among the conditions a high RDW may be a sign of. A high RDW can also be seen with inflammatory conditions such as rheumatoid arthritis or inflammatory bowel disease, an underactive thyroid, pregnancy (when the body needs more iron), chemotherapy, heavy alcohol use, and hemolytic anemia, a condition where red blood cells break down earlier than they should and get replaced by cells of varying ages all at once. Bone marrow disorders, including myelodysplastic syndrome, can also raise RDW because the marrow may not produce cells of consistent size. Bone marrow disorders are much less common than the nutritional causes above, and a single high RDW result does not establish one. Many of the conditions on this list are manageable once identified, and a high RDW alone does not tell you which of them, if any, is involved.

RDW is easier to interpret once your doctor pairs it with mean corpuscular volume (MCV), the CBC value that reports the average size of your red blood cells. The combination can help narrow down the possible causes.

A high RDW with a low MCV can occur with iron deficiency and may help clinicians distinguish that pattern from thalassemia trait, an inherited condition that also produces small red blood cells. The two patterns can overlap, so additional testing, such as iron studies or special hemoglobin tests, is often needed, and the distinction matters because the two conditions are managed differently. For more on what a high MCV specifically indicates, see our guide to high MCV results.

A high RDW with a high MCV can occur with vitamin B12 deficiency, folate deficiency, liver disease, or heavy alcohol use. A high RDW with a normal MCV can appear early in a developing nutritional deficiency, before cell size has shifted enough to move the average, or with a chronic disease, or with a combined deficiency where small and large cells balance out the average even though the spread between them is wide.

If your report also flagged low hemoglobin, our guide to low hemoglobin causes and next steps covers what that combination can suggest. And if this result came back on a routine screening panel, our complete blood count overview walks through what the rest of a CBC report is telling you.

A high RDW found on a routine blood test can come with no symptoms at all. When symptoms are present, they usually trace back to the anemia or the underlying condition driving the result rather than to RDW itself.

General anemia symptoms include fatigue, weakness, pale skin, shortness of breath with activity, dizziness, cold hands and feet, a rapid or pounding heartbeat, brittle nails, and headaches. When anemia becomes more significant, the heart has to work harder to move oxygen around the body, which can cause chest pain, a racing heart, or shortness of breath even while resting, and some people feel lightheaded enough to faint, especially when standing up quickly.

Iron deficiency specifically can bring unusual cravings for ice or other non-food items, restless legs at night, and hair thinning. Vitamin B12 deficiency can bring numbness or tingling in the hands or feet, balance problems, a sore tongue, and, in longstanding cases, memory or concentration trouble. MedlinePlus notes that nerve damage from long-term B12 deficiency may be permanent if treatment is not started promptly. If the cause is ongoing gastrointestinal bleeding, watch for dark or tarry stools, visible blood in the stool or vomit, and unintended weight loss. Heavy or prolonged menstrual bleeding can cause iron deficiency and blood loss, which may contribute to an elevated RDW.

Some signs suggest that anemia or blood loss may be severe. These include vomiting blood or vomit that looks like coffee grounds, black tarry stools or a large amount of blood in the stool, chest pain, shortness of breath while resting, fainting, confusion, skin that turns pale, cold, and clammy, and very heavy or rapidly increasing vaginal bleeding, or heavy bleeding with dizziness, weakness, or a racing heartbeat. These need emergency care, as listed in the When to Seek Medical Care section below. Smaller amounts of blood in the stool, or periods that have become heavier or longer than usual, deserve a call to your clinician. MedlinePlus describes soaking through a pad or tampon every hour for 2 to 3 hours in a row as heavy menstrual bleeding, and bleeding like that is worth a same-day call.

If your doctor suspects a nutritional cause and is waiting on iron, B12, or folate testing, there are reasonable steps you can take in the meantime, though none of these replace the follow-up testing itself. Iron-rich foods include lean red meat, poultry, fish, beans, lentils, spinach, and iron-fortified cereals. Pairing these with a source of vitamin C, such as citrus fruit or bell peppers, can improve how much iron your body absorbs, while coffee, tea, and calcium-rich foods eaten at the same meal can reduce absorption. For B12 and folate, useful sources include meat, fish, eggs, dairy, fortified plant milks, leafy greens, beans, and citrus fruit.

It is best not to start an iron supplement on your own unless a clinician recommends it. MedlinePlus notes that taking more iron than your body needs can cause serious medical problems. Unnecessary iron can also cause side effects such as constipation, nausea, or stomach cramps, and it may delay finding the actual cause of an abnormal blood count. Iron pills can be dangerous for young children, so keep them out of reach and contact Poison Control at 1-800-222-1222 right away if a child swallows one. Folic acid deserves a similar conversation: MedlinePlus notes that, although the risk is low, higher levels of folic acid can mask vitamin B12 deficiency, and long-term B12 deficiency can damage nerves. Tell your clinician about any supplements you take or plan to take so they can decide what is appropriate. A simple symptom log, noting fatigue, dizziness, or anything unusual, gives your doctor useful information at your follow-up visit and helps track whether treatment is working once it starts.

Your doctor will usually start by looking at the rest of your CBC, especially hemoglobin, hematocrit, MCV, and platelet count, to see what pattern the high RDW fits into. Depending on that pattern, follow-up tests can include iron studies (ferritin, serum iron, and total iron binding capacity), vitamin B12 and folate levels, a reticulocyte count to see how actively your marrow is replacing cells, a peripheral blood smear where a technician examines your cells directly under a microscope, and kidney or liver panels. If gastrointestinal bleeding is suspected, a stool test for blood or a referral for endoscopy or colonoscopy may follow.

Treatment targets whatever is driving the result rather than the RDW number itself. Iron deficiency is usually treated with iron supplements and dietary changes, along with finding and addressing any source of blood loss. B12 or folate deficiency is treated with supplements or, for some B12 deficiencies, injections. Chronic conditions are managed on their own timeline with your specialist. Because RDW reflects a mix of old and new red blood cells, it does not return to normal the moment treatment starts. RDW may remain abnormal for weeks or longer after treatment begins because older red blood cells remain in circulation, and MedlinePlus notes that red blood cells circulate for about three to four months. The timing of normalization depends on the underlying cause and how quickly the red-cell population is replaced, so your doctor will choose when to recheck your CBC rather than expecting an immediate change.

When to Seek Urgent or Emergency Care

Emergency, call 911 or go to the emergency room immediately if:

• Vomiting blood, or vomit that looks like coffee grounds
• Black, tarry stools, or a large amount of blood in your stool, especially with weakness or dizziness
• Chest pain or pressure, or shortness of breath while at rest
• Fainting, or feeling like you are about to pass out
• Confusion, or skin that turns pale, cold, and clammy
• Very heavy or rapidly increasing vaginal bleeding, or heavy bleeding with dizziness, fainting, weakness, or a racing heartbeat

Frequently Asked Questions

Sources

• MedlinePlus (NIH) - RDW (Red Cell Distribution Width)
• MedlinePlus (NIH) - Complete Blood Count (CBC)
• National Heart, Lung, and Blood Institute (NIH) - Anemia - What Is Anemia?
• Mayo Clinic - Iron Deficiency Anemia - Symptoms and Causes
• MedlinePlus (NIH) - Iron deficiency anemia
• MedlinePlus (NIH) - Taking iron supplements
• MedlinePlus (NIH) - Vitamin B12 deficiency anemia
• MedlinePlus (NIH) - Folic acid in diet