RDW blood test: what red cell distribution width measures, normal levels, and what a high RDW means

Healthy red blood cells are remarkably uniform in size. The automated analyzer that counts them also measures the volume of each one and builds a histogram of sizes; RDW is a statistical description of how wide that histogram is. Most US laboratories report RDW-CV, the coefficient of variation, as a percentage of the mean cell volume; some also report RDW-SD, the width in femtoliters at a set height of the curve, which is independent of MCV.

The clinical word for a wide distribution is anisocytosis, cells of unequal size. It happens when the marrow starts releasing cells that are built differently from the ones already in circulation: small iron-poor cells joining a normal population, large B12-deficient cells joining a normal population, young large cells pouring out after bleeding or hemolysis, or a transfusion of someone else's cells mixing with yours.

Because RDW responds to a change in production, it often moves before MCV does. In developing iron deficiency the average cell size is still normal while the newest cells are small, so RDW rises first; weeks later MCV follows. That timing is what makes RDW useful rather than redundant.

RDW has no symptoms of its own — it measures how varied red cells are in size — but a high RDW usually accompanies a developing anemia, and the symptoms are the anemia's:

Which anemia a high RDW points to depends on the MCV it is read with: a high RDW with a low MCV suggests iron deficiency; with a high MCV, B12 or folate deficiency; with a normal MCV, early or mixed deficiency, recent blood loss or a recent transfusion.

A high RDW with a normal hemoglobin and no symptoms is common and is usually followed by iron, B12 and folate testing rather than treatment; a low RDW has no clinical meaning.

RDW is part of every complete blood count and is never ordered alone. It is read whenever the hemoglobin is low, to help classify the anemia and to flag a mixed deficiency; whenever MCV is borderline, because RDW says whether the population is uniform or shifting; and in people at risk of iron deficiency before anemia has developed, such as those with heavy periods, frequent blood donors and pregnant women, in whom a rising RDW can be the first sign.

A large research literature associates a high RDW with worse outcomes in heart failure, cardiovascular disease, and many chronic conditions, probably because it reflects inflammation, poor nutrition and impaired red cell production together. Those associations are real at the population level but are not used to manage an individual; no guideline treats RDW as a risk score, and a high RDW in a person without anemia is investigated for the nutritional and marrow causes below, not for heart disease.

RDW arrives with the complete blood count, with or without differential, which can be ordered by your doctor or online through TestWell for a draw at a Quest or Labcorp patient service center, with a licensed physician reviewing the order and results in your portal within one to three business days. When the reason is fatigue or suspected anemia, adding ferritin and vitamin B12 with folate to the same draw answers the questions a high RDW raises without a second visit.

No fasting is required. A recent transfusion, recent iron or B12 treatment, and recovery from a bleed or an illness all widen the distribution for several weeks, so mention them. Results from different laboratories are comparable in direction but not in decimals; analyzers differ in how they compute RDW.

Most laboratories report a normal RDW-CV of about 11.5 to 14.5 percent, with some ranges extending to 15 or 16 percent; the range on your report applies. RDW-SD, when reported, typically runs about 39 to 46 femtoliters. RDW does not differ by sex and rises slightly with age.

RDW is interpreted in a grid with MCV. Low MCV with high RDW favors iron deficiency; low MCV with normal RDW favors thalassemia trait, in which cells are uniformly small. High MCV with high RDW favors vitamin B12 or folate deficiency, hemolysis or a marrow disorder; high MCV with normal RDW favors alcohol, liver disease or medication. Normal MCV with high RDW suggests early iron deficiency, a mixed deficiency, recent bleeding or a recent transfusion. Normal MCV with normal RDW in an anemic person points to acute blood loss, chronic disease or kidney disease.

A high RDW means red cells of noticeably different sizes are circulating together. The commonest reason is iron deficiency, in which each new generation of cells is smaller than the last, and RDW is typically the first index to leave the normal range, often before hemoglobin falls. Vitamin B12 and folate deficiency raise RDW the same way from the other direction, with new cells larger than old. When both an iron and a B12 or folate deficiency are present, as can happen after bariatric surgery, in celiac disease, in alcohol use disorder and in malnutrition, MCV may be normal while RDW is markedly high; that pattern is one of the few ways a mixed anemia is caught on a CBC.

Other causes are recovery from acute bleeding or hemolysis, when a wave of large young cells joins the old population; a recent transfusion; hemoglobin variants such as sickle cell disease; liver disease; and, in older adults, myelodysplastic syndromes and other marrow disorders. Chronic inflammation raises RDW modestly and is the likely reason for the population associations with heart disease and mortality.

The response to a high RDW is to look for the cause in that order: ferritin and iron studies, B12 with methylmalonic acid and folate, a reticulocyte count, and a review of recent bleeding, transfusion, alcohol and medications. If those are unrevealing and the RDW is persistently high with other abnormal counts, a blood smear and hematology evaluation follow.

A low RDW means your red cells are unusually uniform in size, and it has no established clinical significance. Some laboratories do not report a lower reference limit at all. A low RDW does not indicate a disease, does not need to be repeated, and does not change how an anemia is classified; when hemoglobin is low with a low or normal RDW, the anemia is read from the MCV and the clinical picture alone.

Ferritin with transferrin saturation for iron, vitamin B12 with methylmalonic acid and folate for the large-cell causes, and a reticulocyte count to see whether the marrow is actively releasing young cells. If the hemoglobin is also low in a man or a post-menopausal woman and iron is deficient, the source of blood loss, usually gastrointestinal, is the next question. RDW normalizes over about three months after a deficiency is corrected; during treatment it often rises briefly as new normal cells mix with the old abnormal ones, which is expected and not a setback.