Red Cell Distribution Width (CV)

RDW, short for red cell distribution width, is the one number on a blood count that tells you whether your red blood cells are all about the same size or a mix of small and large. A high RDW means a wide mix, and its most useful job is sorting out a low MCV: when your red cells are running small, a high RDW points to iron deficiency and a normal RDW points to thalassemia trait.

What to do next: if your RDW is flagged high, read it next to your MCV and hemoglobin from the same blood count, and bring all three to your clinician. A high RDW with a low MCV usually prompts a clinician to order a ferritin test, a measure of your iron stores; a normal RDW with a low MCV points instead toward checking for thalassemia trait. RDW sorts the possibilities but does not diagnose, so the next test is a clinician's call. One exception overrides the lab entirely: if you have chest pain, breathlessness at rest, a racing or pounding heart, or you feel faint, seek urgent care now regardless of your RDW.

What is RDW in a blood test?

Your red blood cells should be close to the same size. RDW puts a number on how much they actually vary, a measure called anisocytosis (cells of unequal size). Labs report it two ways: RDW-CV, a coefficient of variation given as a percentage (the common one, and the one this page covers), and RDW-SD, a standard deviation given in femtoliters (fL, a tiny unit of volume). A higher RDW-CV means a wider spread of sizes. It says nothing about whether the cells are small or large on average; that is what MCV (mean corpuscular volume, the average red-cell size) tells you. ³¹

What does a high RDW mean?

A high RDW means your red cells are an uneven mix of small and large rather than a uniform batch. The classic reason is iron deficiency. As iron runs low, your bone marrow keeps making red cells, but the newest ones come out small and pale, so your blood ends up holding a mix of older normal-sized cells and newer iron-poor ones. That mix is exactly what a high RDW captures, and it often shows up early, sometimes before the average size (MCV) has dropped below the normal range. ¹

• Iron deficiency, the most common cause and often the earliest sign
• Recovery from any deficiency, as the marrow releases fresh cells of a new size
• B12 or folate deficiency, which makes some cells large
• A mixed deficiency, iron plus B12 or folate together
• After a blood transfusion, which adds donor cells of a different size
• Some liver disease and certain blood disorders

A high RDW with a still-normal MCV can be an early or mixed picture: early iron deficiency before the average size drops, or iron deficiency and B12 or folate deficiency together, where small iron-poor cells and large B12-poor cells average out to a normal-looking MCV. ¹⁶

How RDW and MCV together separate iron deficiency from thalassemia trait

On its own, a low MCV (small red cells) has two common explanations: iron deficiency and thalassemia trait, an inherited change that makes smaller red cells. RDW is what tells them apart. Iron deficiency usually raises the RDW, because the cells are a mix of sizes. Thalassemia trait usually leaves the RDW normal, because the cells are uniformly small from the start. ¹ Reading the two indices together sorts most cases of anemia into a short list of causes.

The low-MCV row is where RDW does its most useful work, so here are the two causes side by side. One more tool helps: the Mentzer index, your MCV divided by your red-cell count (in millions per microliter). A result under 13 leans toward thalassemia trait; over 13 leans toward iron deficiency. ⁵

For the size half of this read, the MCV page sorts red cells into small, normal, and large:

What does a low RDW mean?

A high RDW when the rest of the count looks normal

A raised RDW sometimes turns up when hemoglobin, MCV, and everything else read normal. Most of the time it points to a mild or early deficiency worth checking. There is also a population-level pattern: across large groups of adults, a higher RDW tracks with a higher risk of dying from any cause, even when other measures look fine. In one large study of US adults aged 45 and older, each 1 percentage point rise in RDW came with roughly a 22% higher risk of death from any cause (hazard ratio 1.22). ⁴

Red blood cell distribution width is a widely available test that is a strong predictor of mortality in the general population of adults 45 years or older.Patel et al, Archives of Internal Medicine, 2009

That is a statistical association across populations. It does not diagnose anything or predict one person's future. A single high RDW does not mean something is wrong with your heart or your lifespan; it is a nudge to make sure a treatable cause, iron, B12, or folate, is not being missed. ⁴

How RDW is tested

RDW comes free with every complete blood count (CBC), the standard blood panel that also reports your hemoglobin, MCV, and white cells. The lab analyzer calculates it automatically from the same sample, so there is no separate order, no extra needle, and no added cost beyond the CBC itself. You do not need to fast for it. Because RDW rides along with every CBC, no one screens for it on its own; it shows up whenever a CBC is run. If a low MCV with a normal RDW raises the question of thalassemia trait, which is inherited, that is the situation where a clinician may suggest testing family members. ²

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