- 01High RDW splits small-cell results: iron deficiency versus thalassemia trait
Two people can have the same low MCV. If RDW is high, iron deficiency is the likely cause and ferritin checks it; if RDW is normal, an inherited thalassemia trait is more likely. RDW often rises before MCV or hemoglobin fall, so it can be the earliest flag of low iron 12.
A high red cell distribution width (RDW) on a complete blood count means a second batch of red cells, smaller or larger than the rest, has joined your blood. With a normal hemoglobin (no anemia), a slightly high RDW is common and rarely urgent. The useful step is to read RDW beside the MCV (mean corpuscular volume, the average red cell size), because the pair turns one flag into a short list of causes and a clear next test 2.
Where an example RDW-CV of 16.2% falls%
What counts as a high RDW: RDW-CV and RDW-SD
Labs report RDW in one of two forms. RDW-CV is a percentage that compares the spread of red cell sizes with the average size. RDW-SD is a direct width of the size curve in femtoliters (fL) 1. Many labs use an adult RDW-CV range of roughly 11.5 to 14.5%, and RDW-SD limits differ by analyzer, so the range printed on your own report is the one to go by 15. Because RDW-CV divides by the MCV, the two forms can disagree when cells are very small or very large, and the RDW-SD page covers that split. A low RDW is uncommon and rarely matters on its own; this page covers the high result, and the RDW-CV marker page covers the test itself.
| Your RDW-CV | Where it sits | How it is usually read |
|---|---|---|
| About 14.6 to 15.5% | At or just above the limit | Borderline; early iron depletion is a common reason, and a ferritin check is reasonable |
| About 15.5 to 18% | Clearly high | Usually a real mixed population: iron, B12 or folate shortfall, a recent bleed or recovery |
| Above about 18% | Markedly high | Often an established deficiency anemia, a transfusion, or hemolysis (red cells breaking down early); read with hemoglobin |
These bands are a reading guide, not lab standards: each lab sets its own cutoffs, and in general the higher the value, the more likely a clear cause is present 15.
Read a high RDW with your MCV
In 1983 Bessman and colleagues proposed sorting anemias by MCV and RDW together, and that grid is still the fastest way to read a high RDW 2. Each row below starts from a high RDW and changes only the MCV.
| High RDW plus | Likely causes | Usual next test |
|---|---|---|
| Low MCV (small cells) | Iron deficiency; less often an inherited hemoglobin disorder such as HbH disease | Ferritin, iron panel |
| Normal MCV | Early iron or B12/folate deficiency, mixed deficiency, recent bleed, liver disease | Ferritin, B12, folate, reticulocytes |
| High MCV (large cells) | B12 or folate deficiency, alcohol, liver disease, hemolysis, some marrow disorders | B12 with methylmalonic acid (MMA), folate, reticulocytes, blood smear |
Reticulocytes are brand-new red cells; MMA is a blood or urine test that confirms low B12 when the B12 level is borderline; a smear is a lab check of your cells under a microscope.
In the low-MCV row, iron deficiency usually gives small cells with a high RDW, while thalassemia trait (an inherited, usually harmless reduction in hemoglobin production) often gives small cells with a normal RDW 2. RDW alone does not reliably separate the two 1, so ferritin is checked first; if iron is normal, a doctor may order hemoglobin electrophoresis, and the two can also occur together. The low MCV page and high MCV page take each branch further.
Read these beside your RDW
The common causes of a high RDW
Iron deficiency is the most common cause. RDW often rises early in iron deficiency, before the MCV drops below range, because new smaller cells circulate alongside older normal ones 1. Low iron can also push the platelet count up, so a high platelet count on the same report fits the picture. Iron deficiency always has a reason. In men and in women past menopause, a doctor will usually look for slow blood loss from the gut before or alongside treatment, so iron should not be started just to fix the number without finding why it is low. The iron deficiency guide explains why a normal blood count can still miss low iron.
- Vitamin B12 or folate deficiency, which typically gives a high RDW with a high MCV 2. See B12 deficiency and folate deficiency.
- Mixed deficiency: low iron plus low B12 or folate can leave the MCV in range while RDW climbs, because small and large cells average out 1.
- Recent bleeding, blood donation, hemolysis or a transfusion, each of which adds a population of a different size 1.
- Recovery from anemia, or the first weeks of iron or B12 treatment 1.
- Alcohol, liver or kidney disease, and long-running inflammation 1.
- Medicines that enlarge red cells, such as hydroxyurea, methotrexate and some chemotherapy. If you take one of these, do not stop or change it because of an RDW result; raise it with the doctor who prescribes it.
Supplements that bear on a high RDW
Strongest evidenceIronUsed under a clinician's direction to replace iron once low ferritin confirms deficiency and the cause has been looked for; not for a high RDW without confirmed low iron, and not for thalassemia traitIn our shop 
The causes not to miss
Less often, a high RDW comes with ongoing hidden bleeding from the gut or with a marrow problem such as myelodysplasia (a disorder in which the marrow makes faulty blood cells) 1. These rarely show up as an isolated RDW flag; they usually bring other changes on the same blood count or a falling trend across several tests.
Symptoms a high RDW can and cannot cause
A high RDW causes no symptoms by itself. Symptoms come from the cause behind it: tiredness, breathlessness on stairs and pale skin when anemia develops; tingling, numbness or a sore tongue with B12 deficiency; restless legs or hair shedding with low iron. The fatigue guide covers tiredness and the tests that check it.
High RDW with normal hemoglobin: should you worry?
A high RDW with a normal hemoglobin and normal MCV is common, and a common explanation is early iron depletion, so ferritin is usually the first test a doctor checks. RDW also drifts up with age: in a study of 8,089 people, RDW-CV rose by about 6% and RDW-SD by nearly 15% from the youngest to the oldest age group 6. A repeat count in a few months shows whether a borderline value is stable.
Why RDW is also read as a general health signal
In 8,175 US adults aged 45 or older in the NHANES III survey, each 1% rise in RDW went with a 22% higher risk of death over follow-up, even in people without anemia or iron, folate or B12 deficiency 3. Patel and colleagues concluded that RDW "is a strong predictor of mortality in the general population of adults 45 years or older" 3. In chronic heart failure, RDW had the strongest link with illness and death of 36 routine lab values, with a hazard ratio of 1.17 per standard deviation 4. Higher RDW has also been linked with diabetes, heart, liver and kidney disease and other chronic conditions 15. These are associations; the reason is not settled and may involve inflammation, poor nutrition and aging 1. No trial shows that lowering RDW itself changes those outcomes.
Tests usually checked next
The next tests follow the MCV pattern. Ferritin and transferrin saturation check iron; B12, with methylmalonic acid if B12 is borderline, and folate check the large-cell causes; a reticulocyte count shows whether the marrow is answering a bleed or hemolysis; CRP flags inflammation. If hemolysis is suspected, a doctor adds LDH, bilirubin and haptoglobin, and unexplained iron deficiency often prompts a celiac screen. The iron studies guide shows how the iron panel reads together, and the CBC guide covers the rest of the count.
Usually checked after a high RDW
How to lower a high RDW
RDW falls when the cause is treated, on the timetable of red cell turnover. Red cells live about 120 days, so once iron, B12 or folate is replaced under a doctor's direction, new normal cells mix with the old abnormal ones and RDW often rises for the first few weeks before it falls 1. A repeat blood count with iron or vitamin markers after about 8 to 12 weeks gives a fair read. Cutting alcohol helps when alcohol is the cause.
Citations
- Salvagno GL, Sanchis-Gomar F, Picanza A, Lippi G. Red blood cell distribution width: a simple parameter with multiple clinical applications. Crit Rev Clin Lab Sci. 2015;52(2):86-105.
- Bessman JD, Gilmer PR Jr, Gardner FH. Improved classification of anemias by MCV and RDW. Am J Clin Pathol. 1983;80(3):322-326.
- Patel KV, Ferrucci L, Ershler WB, Longo DL, Guralnik JM. Red blood cell distribution width and the risk of death in middle-aged and older adults. Arch Intern Med. 2009;169(5):515-523.
- Felker GM, Allen LA, Pocock SJ, et al. Red cell distribution width as a novel prognostic marker in heart failure: data from the CHARM Program and the Duke Databank. J Am Coll Cardiol. 2007;50(1):40-47.
- MedlinePlus, US National Library of Medicine. RDW (Red Cell Distribution Width) test.
- Hoffmann JJ, Nabbe KC, van den Broek NM. Effect of age and gender on reference intervals of red blood cell distribution width (RDW) and mean red cell volume (MCV). Clin Chem Lab Med. 2015;53(12):2015-2019.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
