High RDW Blood Test: What Does It Mean?

A high RDW (red cell distribution width) means your red blood cells vary in size more than usual. It is part of a complete blood count and is a clue rather than a diagnosis. Common reasons include low iron, vitamin B12 or folate, and recent bleeding, and it is read together with the MCV and hemoglobin.

This guide is for adults who have seen a raised RDW on a blood count report. It explains what the number measures, how it is reported, and the main patterns clinicians look at. It cannot tell you why your own RDW is high.

A high RDW causes no symptoms by itself. Symptoms usually come from anemia (a low red cell or hemoglobin level) or from the underlying cause. Healthdirect lists tiredness, a fast heartbeat, dizziness, shortness of breath, pale skin, cold hands or feet and chest pain among anemia symptoms, and notes that some people have none.

Use local emergency services for chest pain, severe breathlessness, fainting or heavy bleeding. Contact a clinician promptly if you notice black or bloody stools, very heavy periods, or symptoms that are getting worse while you wait for follow-up tests.

Cleveland Clinic gives a normal RDW of 12% to 15% and notes that the range varies between laboratories. A low RDW is not usually a concern. Always compare your result with the range printed on your own report.

MedlinePlus explains that RDW is usually shown as a percentage against a reference range, and that results are read alongside other blood tests rather than on their own.

Laboratories can report RDW in two ways. RDW-CV (coefficient of variation) is given as a percentage. RDW-SD (standard deviation) is given in femtoliters (fL), the same unit used for cell volume. Both describe how spread out red cell sizes are across the whole sample, not the size of any single cell.

A 2019 review in the Cleveland Clinic Journal of Medicine gives a lab-dependent reference range of about 11% to 16% for RDW-CV and about 39 to 46 fL for RDW-SD. Your report may show one or both. Because the units differ, compare each value only with its own reference range.

Many conditions can widen the spread of red cell sizes, so the RDW alone does not point to one cause. Sources describe these groups:

• Low iron, vitamin B12 or folate. The Association for Laboratory Medicine lists these among the commonest reasons for a high RDW.
• Recent blood loss or faster breakdown of red cells. The bone marrow responds by releasing young red cells (reticulocytes), which are larger than mature cells, so sizes become more varied.
• A recent blood transfusion, which can temporarily change blood count results. Tell the team if you have had one.
• Longer-term conditions. Cleveland Clinic and MedlinePlus mention liver disease, kidney disease, heart disease, chronic inflammatory illness and some cancers.
• Thalassemia and, less often, bone marrow disorders.

MCV (mean corpuscular volume) is the average size of your red cells, while RDW shows how much the sizes vary. Cleveland Clinic notes that clinicians often compare the two. Reading them together can narrow the possibilities, but it is still only a starting point.

An average can hide a mix. If some cells are small and others large, the MCV can land in the normal range while the RDW is high. A review in American Family Physician notes that a mixture of large and small cells may suggest mixed processes, such as low iron alongside low B12 or folate, and that an elevated RDW should prompt that thought.

If your MCV is also high, the SameCase guide on high MCV covers the causes of large red cells. If your MCV is low, the usual first questions are about iron stores and inherited hemoglobin traits.

A high RDW can appear before anemia develops. The 2019 review notes that in iron deficiency, the RDW often rises before the MCV falls, so it can be an early clue. If your ferritin has also come back low, the SameCase guide on low ferritin with normal hemoglobin covers that situation.

Large studies have linked higher RDW with higher death rates and worse outcomes in heart failure, but the same review describes using RDW to predict outcomes as still investigational. A single high RDW with an otherwise normal count is usually a reason to check the likely causes and repeat the test, not a reason for alarm.

Because many conditions can raise the RDW, the 2019 review recommends looking at a blood film, where cells are examined under a microscope. Healthdirect lists iron studies, vitamin B12 and folate levels and a blood film among the tests used to investigate anemia. A reticulocyte count can show whether the marrow is making extra young cells.

Mention any recent bleeding, heavy periods, transfusion, diet changes, alcohol use, and all medicines and supplements. Do not start iron or vitamin supplements before testing unless your clinician advises it.

Bring your report, any earlier blood counts and a list of medicines and supplements.

• Is my RDW reported as RDW-CV, RDW-SD or both, and how far above the range is it?
• What do my MCV, MCH and hemoglobin show alongside it?
• Could a mix of iron and B12 or folate problems be hiding behind a normal MCV?
• Could recent bleeding or a transfusion explain the result?
• Will you look at a blood film or check iron, B12 and folate?
• When should the count be repeated, and which symptoms should make me contact you sooner?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a high RDW described their full blood count, follow-up tests and timelines can help you decide what to bring and what to ask.

Someone else's result pattern may not match yours, and a similar case cannot diagnose you. We suggest using cases only to prepare questions for your own clinician.