RDW, or red cell distribution width, is a measure of how much your red blood cells vary in size. It’s part of a complete blood count (CBC), one of the most commonly ordered blood tests. A normal RDW typically falls between 11.5% and 14.5%, though the exact range can vary slightly between labs. When your red blood cells are roughly uniform in size, your RDW is low or normal. When they vary widely, your RDW is high, a condition called anisocytosis.
What RDW Actually Measures
Your bone marrow constantly produces new red blood cells, and in a healthy person, those cells are fairly consistent in size. RDW captures how spread out the sizes are. Think of it like a statistical snapshot: if you lined up all the red blood cells in a sample, RDW tells you whether they’re mostly the same diameter or scattered across a wide range of sizes.
A high RDW means some cells are much larger or smaller than others. This can happen when your body is struggling to produce red blood cells normally, when it’s compensating for blood loss by releasing immature cells, or when a nutritional deficiency is distorting cell development. RDW alone doesn’t diagnose anything specific, but paired with other CBC values, it helps narrow down what’s going on.
Normal, High, and Low Results
Most labs report normal RDW somewhere around 11.5% to 14.5%. A result within that range means your red blood cells are relatively uniform, which is a good sign. A low RDW, meaning your cells are even more uniform than average, is not a sign of anemia and generally isn’t something to worry about.
A high RDW is where things get clinically interesting. Values above 14.5% suggest your red blood cells are coming in noticeably different sizes, and that variation usually points to an underlying issue worth investigating. Your doctor will almost always look at RDW alongside other values on the CBC before drawing conclusions.
There are also meaningful differences across demographics. Men tend to have slightly higher RDW values than women, and Black adults have modestly higher average values than white adults. A study of healthy U.S. adults found mean RDW values of 13.4% in Black men compared to 13.3% in white men, and 13.1% in Black women compared to 12.9% in white women. These differences are small but statistically significant, which is why your lab’s reference range matters more than a single universal cutoff.
Conditions Linked to High RDW
Anemia is the most common reason for a high RDW. Different types of anemia produce different patterns of cell size variation:
- Iron-deficiency anemia is the most frequent cause. When your body doesn’t have enough iron to build red blood cells properly, it produces smaller-than-normal cells alongside the older, normal-sized ones, driving RDW up.
- Vitamin B12 or folate deficiency causes the opposite problem. Your body makes abnormally large red blood cells, creating a mix of oversized and normal cells.
- Sickle cell anemia distorts cell shape and size, often producing a persistently elevated RDW.
- Autoimmune hemolytic anemia occurs when your immune system destroys red blood cells faster than your bone marrow can replace them, leading to a mix of old and newly released cells of varying sizes.
Beyond anemia, anisocytosis is also associated with liver disease, kidney disease, thyroid disorders, heart disease, and certain cancers including colon cancer. In many of these conditions, the elevated RDW reflects chronic inflammation or disrupted red blood cell production rather than a simple nutritional deficiency.
How RDW and MCV Work Together
Doctors rarely interpret RDW in isolation. The most useful pairing is RDW with MCV, or mean corpuscular volume, which measures the average size of your red blood cells. Together, these two numbers create a diagnostic shortcut.
If your MCV is low (under 80 femtoliters), your red blood cells are smaller than normal. Combine that with a high RDW, and iron-deficiency anemia is a strong possibility. If MCV is low but RDW is normal, the pattern points more toward thalassemia, a genetic condition that produces uniformly small cells.
If your MCV is high (over 100 fL) with a high RDW, B12 or folate deficiency is a likely culprit. A high MCV with normal RDW can suggest other causes, like certain medications or alcohol use.
One particularly tricky scenario: when someone has both a deficiency that shrinks cells (like iron) and one that enlarges them (like B12), the MCV can land in the normal range because the average balances out. In that case, a high RDW may be the only clue that something is off. It flags that the “normal” average is hiding two competing problems.
RDW as a Risk Marker Beyond Anemia
Over the past decade, researchers have found that elevated RDW is linked to cardiovascular disease and overall mortality, even in people without anemia. The connection is strong enough that some researchers have proposed using RDW as a screening marker, though it hasn’t become standard practice.
In a study of more than 240,000 healthy UK adults aged 40 to 70, the incidence of coronary artery disease and death from any cause began climbing once RDW exceeded 13% and was nearly three times higher in people with values above 15%. A separate Scandinavian study found that people in the highest quarter of RDW values had a 34% greater risk of heart attack compared to those in the lowest quarter, even after adjusting for traditional risk factors like blood pressure and cholesterol. Another large study of over 26,000 participants tracked for 14 years found that those with the highest RDW values had a 1.8 times greater risk of fatal coronary events.
These associations held up even after researchers accounted for anemia, vitamin deficiencies, and standard cardiovascular risk factors. The leading theory is that elevated RDW reflects chronic low-grade inflammation and oxidative stress, both of which accelerate blood vessel damage over time. This doesn’t mean a high RDW causes heart disease. It means your body may be under a type of systemic stress that raises your risk.
What Happens After an Abnormal Result
If your RDW comes back high, your doctor will typically look at the rest of your CBC first, especially your hemoglobin, hematocrit, and MCV. These values together paint a much clearer picture than RDW alone. From there, common follow-up tests include checking your iron levels, vitamin B12, and folate. These are simple blood draws that can confirm or rule out the most common nutritional deficiencies behind elevated RDW.
If nutritional causes are ruled out, further testing depends on the clinical picture. Your doctor might evaluate thyroid function, kidney function, or liver enzymes. In less common cases, a blood smear (where a technician examines your red blood cells under a microscope) can reveal specific cell shapes that point toward conditions like sickle cell disease or hereditary spherocytosis.
RDW is part of a standard CBC, which requires only a routine blood draw from a vein in your arm. No special preparation or fasting is needed specifically for the CBC, though if your blood draw includes other tests like a metabolic panel or lipid panel, your provider may ask you to fast beforehand.
Why a Single Number Isn’t a Diagnosis
RDW is a screening value, not a standalone diagnosis. A high result tells your doctor that your red blood cells are more variable in size than expected, which narrows the list of possibilities but doesn’t pinpoint a cause. A normal result is reassuring but doesn’t rule out all blood disorders. Some conditions, like thalassemia trait, produce uniformly small cells with a perfectly normal RDW.
The real value of RDW is in context. Combined with other CBC markers, your symptoms, and your medical history, it becomes one piece of a larger puzzle. If your result is flagged on a lab report, it’s worth a conversation with your doctor about whether follow-up testing makes sense, but an isolated high RDW in an otherwise normal CBC is rarely cause for alarm on its own.