A high RDW means your red blood cells vary more in size than they should. The normal range is typically 12% to 15%, and anything above that signals your body is producing red blood cells of uneven sizes, a condition called anisocytosis. This isn’t a diagnosis on its own, but it’s a useful clue that points toward nutritional deficiencies, chronic disease, or other conditions worth investigating.
What RDW Actually Measures
RDW stands for red cell distribution width. It’s a standard part of a complete blood count (CBC), which means you’ve probably had it measured many times without paying attention to it. The number reflects how much your red blood cells differ from one another in size. In a healthy person, red blood cells are roughly uniform. When something disrupts their production, you end up with a mix of larger and smaller cells, and the RDW percentage climbs.
The calculation is straightforward: it’s the variation in red blood cell volume divided by the average cell volume, expressed as a percentage. A tighter cluster of similarly sized cells gives you a low number. A wider spread gives you a high one. Labs may report this as RDW-CV (the percentage form) or RDW-SD (measured in femtoliters), but the percentage version between 12% and 15% is what most people see on their results.
Common Causes of a High RDW
Nutritional Deficiencies
The most frequent reason for an elevated RDW is a deficiency in iron, vitamin B12, or folate. These nutrients are essential for building properly sized red blood cells. When your body runs low on any of them, it starts producing cells that are too large, too small, or a chaotic mix of both. One important detail: RDW often rises before other red blood cell markers change, making it one of the earliest signs that a nutritional deficiency is developing. You could have a high RDW and still have a normal hemoglobin level, which is why it’s worth paying attention to even when the rest of your bloodwork looks fine.
Iron deficiency anemia, in particular, produces a characteristic pattern. Your red blood cells tend to be smaller than normal (low MCV) while your RDW is elevated. This combination is so distinctive that doctors have used it for decades to distinguish iron deficiency from thalassemia trait, a genetic condition that also causes small red blood cells but typically keeps the RDW closer to normal. An RDW above 21% has been shown to reliably separate the two conditions, with about 90% sensitivity and 77% specificity.
Vitamin B12 deficiency can be trickier. It usually produces abnormally large red blood cells, but in some cases the average cell size stays normal while the RDW shoots up. This happens because the blood contains a mix of very large and very small cells that average out to a normal-looking number. Researchers at a university hospital documented multiple cases of severe, life-threatening B12 deficiency where the average cell size appeared completely normal, but the elevated RDW revealed the underlying problem.
Chronic Inflammation
Inflammatory conditions can raise your RDW through a different mechanism. When your body is fighting chronic inflammation, signaling molecules interfere with the normal production of red blood cells in your bone marrow. This disruption causes newer, larger cells to be released into your bloodstream before they’re fully mature, creating the size variation that drives up RDW. Autoimmune diseases, inflammatory bowel disease, and chronic infections can all trigger this pattern.
Heart and Vascular Disease
Elevated RDW has a surprisingly strong connection to cardiovascular problems. Studies have linked higher RDW values to worse outcomes in heart failure, heart attacks, stable coronary artery disease, stroke, and peripheral artery disease. One large study found that each 1% increase in RDW was associated with a 10% increased risk of death in patients with peripheral artery disease. This relationship holds even in people without known heart disease: elevated RDW values are associated with increased risk of death from all causes, including cardiovascular events. Researchers aren’t entirely sure why the connection is so strong, but chronic low-grade inflammation and oxidative stress, both of which are central to heart disease, likely play a role in disrupting red blood cell production.
Symptoms You Might Notice
A high RDW itself doesn’t produce symptoms directly. What you feel depends on the underlying cause. If the cause is anemia, which is often the case, you may experience fatigue, weakness, shortness of breath, dizziness, pale skin, headaches, or a fast or irregular heartbeat. Some people notice a whooshing sound in one ear, which happens when your heart works harder to compensate for fewer healthy red blood cells. Chest pain can occur in more severe cases.
If your RDW is only mildly elevated and your hemoglobin is normal, you may feel nothing at all. That doesn’t mean it should be ignored, especially if the number is trending upward over time.
What Your RDW Means Alongside Other Results
Your doctor won’t interpret RDW in isolation. The most useful pairing is RDW with MCV, which measures the average size of your red blood cells. Together, these two numbers narrow down the likely cause considerably.
- High RDW with low MCV: Points toward iron deficiency anemia. Your body is making red blood cells that are too small, and they vary a lot in size.
- High RDW with high MCV: Suggests B12 or folate deficiency. Your red blood cells are larger than normal on average, with significant size variation.
- High RDW with normal MCV: This is the trickiest pattern. It can indicate early iron deficiency (before cells shrink), B12 deficiency masked by coexisting iron deficiency, chronic inflammation, or liver disease. It requires more investigation.
Other parts of your CBC matter too. Your hemoglobin, red blood cell count, and reticulocyte count (which measures how many new red blood cells your bone marrow is producing) all provide context. A peripheral blood smear, where a lab technician examines your blood cells under a microscope, can reveal specific shapes and sizes that point toward a diagnosis.
What Happens After a High RDW Result
A high RDW typically triggers follow-up testing rather than immediate treatment. The goal is to figure out why your red blood cells are uneven. Depending on your other bloodwork and symptoms, your doctor may order iron studies (which measure iron levels and your body’s iron stores), B12 and folate levels, markers of inflammation, or a reticulocyte count to see how actively your bone marrow is producing new cells.
If the cause turns out to be a nutritional deficiency, the fix is usually straightforward: supplementation with the missing nutrient and dietary changes. Iron deficiency responds to iron supplements, though it can take several months for your RDW to normalize as your body gradually replaces the misshapen cells with properly sized ones. B12 deficiency may require high-dose oral supplements or injections, depending on whether the issue is dietary or related to absorption.
If the elevated RDW is linked to a chronic condition like heart failure or an autoimmune disease, treatment focuses on managing that condition. The RDW itself isn’t treated directly. Instead, it serves as a marker that reflects how well the underlying problem is controlled. In cardiovascular patients, a persistently high or rising RDW can signal worsening disease and a higher risk of complications, making it a useful number to track over time.
Why a Mildly High RDW Still Matters
It’s tempting to dismiss a result of 15.5% or 16% as barely outside the range, but even mildly elevated RDW values carry meaningful information. Because RDW rises early in the course of nutritional deficiencies, a borderline result could be the first sign of a problem that hasn’t fully developed yet. Catching iron or B12 deficiency at this stage, before anemia sets in, makes correction simpler and prevents symptoms from ever appearing. If your RDW has been creeping upward across multiple blood tests, that trend is worth flagging even if each individual result is only slightly above normal.