The short answer
RDW measures how much your red blood cells vary in size. Labcorp calls up to 15.4% normal. A high RDW is a clue rather than a diagnosis: read with your MCV, it helps show whether low iron, B12 or folate could be the reason.1,2,4
- A good result
- Up to 15.4%. A result under the lab’s range isn’t a concern: it just means your red cells are very even in size.1
- If yours is high
- Check your MCV, then test ferritin, B12 and folate. Low iron, the most common nutrient shortage, is the first thing to rule out.2,9
- Should you test?
- It comes with every complete blood count. Its job is to help explain an abnormal MCV or hemoglobin, so read it with those.2
Check your result
Type the number from your report. It stays on this page.
- Low Under 11.7
- Normal 11.7 to 15.4
- High 15.5 or more
Enter your result to see what it means and what to do next.
Got a result from another lab? Ask Picardo about it. The app explains your results, and a physician can follow up.
Green is Labcorp’s range for women; for men it starts at 11.6%. Below the range is green too, because a low RDW isn’t a concern.1 The bar isn’t to scale.
Below the lab’s range, which is fine
Your red cells are very even in size. That isn’t a problem: in a study of 8,175 US adults, the risk of dying rose steadily from the lowest RDW values up.1 Labcorp’s range starts at 11.6% for men, so your lab may not flag it at all.
Nothing to worry about.
- 1 Nothing to do. There’s no reason to try to raise it.
Normal
Your red cells vary in size about as much as most people’s. RDW drifts up with age, so a slow rise over the years is common.6
Nothing to worry about.
- 1 Nothing to do for this number.
- 2 If your MCV or hemoglobin is outside its range, follow that up on its own.
High: your red cells are mixed in size
Your red cells vary in size more than usual. RDW can’t say why on its own; your MCV narrows it down.2 Common reasons are low iron, even before you’re anemic, and low B12 or folate.2,4,5 In a trial of IV iron, it also rose for the first few weeks of treatment.5
Not urgent on its own. Worth finding the reason over the next few weeks.
- 1 Look at your MCV on the same report. The table further down shows what each combination points to.2,3,4
- 2 Test ferritin, B12 and folate. US hematologists suggest counting a ferritin of 30 ng/mL or under as iron deficiency, and up to 50 if you have symptoms of low iron or a risk factor such as heavy periods, a vegan or vegetarian diet or donating blood.9
- 3 If you started iron in the last month or so, recheck in a few months instead. RDW can rise before it falls.5
- 4 If those tests are normal and you feel well, repeat the blood count at your next checkup.
What is a normal RDW level?
Labcorp calls 11.7% to 15.4% normal for women and 11.6% to 15.4% for men. Only a high result needs a closer look:1,2
Below the lab’s range, which is fine
Your red cells are very even in size. That isn’t a problem: in a study of 8,175 US adults, the risk of dying rose steadily from the lowest RDW values up.1 Labcorp’s range starts at 11.6% for men, so your lab may not flag it at all.
Nothing to worry about.
- 1 Nothing to do. There’s no reason to try to raise it.
Normal
Your red cells vary in size about as much as most people’s. RDW drifts up with age, so a slow rise over the years is common.6
Nothing to worry about.
- 1 Nothing to do for this number.
- 2 If your MCV or hemoglobin is outside its range, follow that up on its own.
High: your red cells are mixed in size
Your red cells vary in size more than usual. RDW can’t say why on its own; your MCV narrows it down.2 Common reasons are low iron, even before you’re anemic, and low B12 or folate.2,4,5 In a trial of IV iron, it also rose for the first few weeks of treatment.5
Not urgent on its own. Worth finding the reason over the next few weeks.
- 1 Look at your MCV on the same report. The table further down shows what each combination points to.2,3,4
- 2 Test ferritin, B12 and folate. US hematologists suggest counting a ferritin of 30 ng/mL or under as iron deficiency, and up to 50 if you have symptoms of low iron or a risk factor such as heavy periods, a vegan or vegetarian diet or donating blood.9
- 3 If you started iron in the last month or so, recheck in a few months instead. RDW can rise before it falls.5
- 4 If those tests are normal and you feel well, repeat the blood count at your next checkup.
What is RDW?
Picture a jar of marbles. MCV tells you their average size; RDW tells you how much the sizes differ. A jar of near-identical marbles has a low RDW. Mix in some small ones and some big ones, and RDW goes up.
RDW stands for red cell distribution width. The counting machine works it out from the same measurements as MCV: it’s the spread of red cell sizes as a percentage of the average size.1
A high RDW usually means something has disturbed how your red cells are made or how long they last.1 It works best as a partner to MCV: together, the two help tell apart causes that MCV alone can’t.2
What causes a high RDW?
- Low iron, which can raise RDW early, even before you’re anemic.2,5
- Low B12 or folate. Mixed sizes are most marked when these are behind large red cells.2,4
- The first weeks of iron treatment.5
- Long-term illness. In a national US study, people with a higher RDW were more likely to have age-related diseases and signs of inflammation, and more likely to smoke or carry extra weight.1
- Age and genes. RDW creeps up as you get older, and genes explain about 29% of the difference between people.6
The MCV guide covers what small or large red cells point to, from thalassemia trait to alcohol and medicines.
How to read RDW with your MCV
| Your MCV | With a high RDW | Test next |
|---|---|---|
| Low (under 79 fL) | Leans toward low iron over thalassemia trait, though the two overlap.3 | Ferritin |
| Normal (79 to 97 fL) | Iron or folate may be starting to run low.2 | Ferritin, B12 and folate |
| High (98 fL or more) | Low B12 or folate is the first thing to rule out.4 | B12 and folate |
These are leanings, not diagnoses. The tests in the last column decide.
Is a high RDW linked to a shorter life?
In large studies, yes. But that doesn’t mean RDW causes it.
In 8,175 US adults aged 45 and over, each 1 point higher RDW went with a 22% higher risk of dying during follow-up. The link held in people who weren’t anemic, and within the normal range.1 A 2025 study of 14,371 healthy adults found the same: people whose usual RDW was higher had higher death rates, even within the normal range, and were more likely to develop atrial fibrillation.7
These are observational studies. They show a link, not a cause, and the reason for it isn’t known; inflammation and the wear of aging are suspects.1,6 Genetics points the same way: a genetic score for RDW didn’t predict who developed disease.6
So a high RDW is a reason to look for a cause, not a number to push down for its own sake.
Can you lower your RDW?
Only by treating its cause. When low iron is behind it, treatment brings it down, though not straight away. In a trial of IV iron for people with heart failure and low iron, RDW rose over the first 4 weeks, then fell below the placebo group’s by 24 weeks.5
If low B12 or folate is the cause, treating that is what matters; the MCV guide covers how. If no cause turns up, there’s nothing to treat.
Who should get an RDW test?
You don’t order RDW on its own: it comes with every complete blood count. It matters most when:
- Your MCV or hemoglobin is outside its range.2
- You’re being checked for low iron, B12 or folate.2
- You’re being treated for low iron. RDW can rise for a few weeks before it falls.5
How to get an RDW test, and what it costs
It comes from the complete blood count, which costs $3.75 through Picardo.
Complete blood count
The test this number comes from.
- Results in 2 to 3 days, in the app
- Physician follow-up if you want one
- Free Labcorp draw, or $75 at home
- No other fees. HSA and FSA eligible.
It’s also part of the Base Core Annual – Men, Base Core Annual – Women, Heart Health, Brain Health Screening, Pregnancy, Postpartum, Fertility, Thyroid & Autoimmunity and Celiac / GI panels, if you’d rather test more at once.
Already have a result? Ask Picardo about itQuestions people ask
Why did my RDW go up after I started iron?
It can happen early on, and on its own it doesn’t mean the iron isn’t working. In a trial of IV iron for people with heart failure and low iron, RDW rose over the first 4 weeks, then fell below the placebo group’s by 24 weeks.5
Can a high RDW be a sign of blood cancer?
Not on its own. It counts only for people already found, through genetic testing, to carry clonal hematopoiesis, where some blood-forming cells have picked up mutations. For them, an RDW of 15% or more is one feature of a risk score for later blood cancer, along with an MCV of 100 or more and age 65 or over. Most people with clonal hematopoiesis land in the low-risk group, where under 1% developed a blood cancer within 10 years.8 The score doesn’t apply to anyone without that genetic result.
Does RDW go up with age?
Yes. RDW creeps up as you get older. Genes explain about 29% of the difference between people, a little more over 60.6
Often checked alongside
Test details
- Labcorp test
- Complete Blood Count (CBC) With Differential, Reflex to Peripheral Smear Review, 005009
- LOINC
- 788-0
- Sample
- Blood
- Fasting
- Not needed
- Typical lab range
- 11.7% to 15.4% (women); 11.6% to 15.4% (men)
- Units
- %
Sources
- 1 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. doi.org/10.1001/archinternmed.2009.11
- 2 Bessman JD, Gilmer PR Jr, Gardner FH. Improved classification of anemias by MCV and RDW. Am J Clin Pathol. 1983;80(3):322–326. doi.org/10.1093/ajcp/80.3.322
- 3 Lima CS, Reis AR, Grotto HZ, Saad ST, Costa FF. Comparison of red cell distribution width and a red cell discriminant function incorporating volume dispersion for distinguishing iron deficiency from beta thalassemia trait in patients with microcytosis. Sao Paulo Med J. 1996;114(5):1265–1269. doi.org/10.1590/S1516-31801996000500005
- 4 Savage DG, Ogundipe A, Allen RH, Stabler SP, Lindenbaum J. Etiology and diagnostic evaluation of macrocytosis. Am J Med Sci. 2000;319(6):343–352. doi.org/10.1097/00000441-200006000-00001
- 5 Van Craenenbroeck EM, Conraads VM, Greenlaw N, et al. The effect of intravenous ferric carboxymaltose on red cell distribution width: a subanalysis of the FAIR-HF study. Eur J Heart Fail. 2013;15(7):756–762. doi.org/10.1093/eurjhf/hft068
- 6 Pilling LC, Atkins JL, Duff MO, et al. Red blood cell distribution width: genetic evidence for aging pathways in 116,666 volunteers. PLoS One. 2017;12(9):e0185083. doi.org/10.1371/journal.pone.0185083
- 7 Foy BH, Petherbridge R, Roth MT, et al. Haematological setpoints are a stable and patient-specific deep phenotype. Nature. 2025;637(8045):430–438. doi.org/10.1038/s41586-024-08264-5
- 8 Weeks LD, Niroula A, Neuberg D, et al. Prediction of risk for myeloid malignancy in clonal hematopoiesis. NEJM Evid. 2023;2(5):EVIDoa2200310. doi.org/10.1056/EVIDoa2200310
- 9 Powers JM, Lim MY, Achebe MO, et al. American Society of Hematology 2026 guidelines for diagnosis of iron deficiency. Blood Adv. 2026;10(21):7353–7374. doi.org/10.1182/bloodadvances.2025015950
Updated 4 Nov 2026.
This is general health information, not medical advice: talk to your clinician about your results and before changing any treatment.