Red cell distribution width (RDW)
RDW measures how much red blood cells vary in size. US survey ranges for adults start near 11.4% and end between 14.5% and 16.3% by sex and age. Higher values were linked to more deaths, heart disease and cancer in large cohorts, about 14% more deaths per 1 point. Results differ between analyzer brands.
- Status
- Part of the complete blood count, ordered by a clinician and also sold direct to consumers by some US laboratories. It is calculated by the automated cell counter, a type of device FDA places in class II [1].
- Measures
- How much a person's red blood cells differ from one another in size, calculated as the standard deviation of red cell volume divided by the mean cell volume, times 100 [2].
- Also called
- RDW, RDW-CV, RDW-SD, red cell distribution width, red blood cell distribution width, erythrocyte distribution width, anisocytosis
- Tested for
- anemia workup, iron deficiency, telling iron deficiency from thalassemia trait, heart disease risk, longevity, biological age, general checkup
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Linked in research4
- ModerateLinked to more deaths at higher RDW in community cohorts, about 14% more per 1 point in pooled data on 11,827 older adults.
- ModerateLinked to more coronary disease at higher RDW, both in volunteers free of disease and in people who had already had a heart attack.
- ModerateLinked to more deaths in people with heart failure, about 10% more per 1 point of RDW across 9 pooled studies.
- ModerateLinked to more new cancers and cancer deaths at higher RDW in two sets of cohorts, including 37% more cancers at 15% or above in UK volunteers.
Reported risks0
No risks sourced yet.
Reference ranges
Each range is the one its source published, in %. Where sources differ, each is shown.
US national survey laboratory procedure manual, 2017 to 2018, men aged 19 to 65
Men aged 19 to 65, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.4 % | No name given |
| 11.4 to 14.5 % | "reference range" |
| above 14.5 % | No name given |
The 2.5th to 97.5th percentiles of everyone measured in the US national survey from 1999 to 2004, calculated by ranking, without screening out people with iron deficiency or illness. The manual is for a Beckman Coulter analyzer, and the 1998 US recommendations state that the RDW value obtained depends on the instrument used [2][3].
US national survey laboratory procedure manual, 2017 to 2018, men aged 66 and older
Men aged 66 and older, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.8 % | No name given |
| 11.8 to 16.2 % | "reference range" |
| above 16.2 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3]. A study of 8,089 people on a different analyzer found a strong association between RDW and age and none with sex, with RDW 6% higher in the oldest age class than the youngest [4].
US national survey laboratory procedure manual, 2017 to 2018, women aged 19 to 65
Women aged 19 to 65, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.4 % | No name given |
| 11.4 to 16.3 % | "reference range" |
| above 16.3 % | No name given |
Derived the same way from the 1999 to 2004 survey sample, which was not screened for iron deficiency. The upper limit is 1.8 points above the limit for men of the same age [3].
US national survey laboratory procedure manual, 2017 to 2018, women aged 66 and older
Women aged 66 and older, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.6 % | No name given |
| 11.6 to 16.3 % | "reference range" |
| above 16.3 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3].
US national survey laboratory procedure manual, 2017 to 2018, boys aged 1 to 5
Boys aged 1 to 5, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.4 % | No name given |
| 11.4 to 15.8 % | "reference range" |
| above 15.8 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3].
US national survey laboratory procedure manual, 2017 to 2018, boys aged 6 to 18
Boys aged 6 to 18, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.4 % | No name given |
| 11.4 to 14 % | "reference range" |
| above 14 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3].
US national survey laboratory procedure manual, 2017 to 2018, girls aged 1 to 5
Girls aged 1 to 5, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.3 % | No name given |
| 11.3 to 15.4 % | "reference range" |
| above 15.4 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3].
US national survey laboratory procedure manual, 2017 to 2018, girls aged 6 to 18
Girls aged 6 to 18, as the reference range used by the survey laboratory · [3]
| Range | What the source calls it |
|---|---|
| below 11.3 % | No name given |
| 11.3 to 14.8 % | "reference range" |
| above 14.8 % | No name given |
Derived the same way from the 1999 to 2004 survey sample [3].
US Centers for Disease Control and Prevention recommendations, 1998
People being assessed for iron deficiency · [2]
| Range | What the source calls it |
|---|---|
| 14 or below % | No name given |
| above 14 % | "high" |
The source states that a high RDW is generally set at greater than 14.0%, the 95th percentile of the reference population in the third US National Health and Nutrition Examination Survey, and that the cutoff is specific to the instrument and may not apply in all laboratories. It uses RDW after a low MCV: a low MCV with an RDW above 14.0% is read as pointing to iron-deficiency anemia, and a low MCV with an RDW of 14.0% or below as pointing to thalassemia minor. It gives no lower cutoff [2].
Reference range used in a US national survey analysis, Patel and colleagues, 2009
Adults aged 45 and older in the US National Health and Nutrition Examination Survey, 1988 to 1994 · [5]
| Range | What the source calls it |
|---|---|
| below 11 % | No name given |
| 11 to 15 % | "reference range" |
| above 15 % | No name given |
The range the authors call the reference range for RDW when testing whether the link with death held inside it. It held: RDW stayed strongly associated with death among people within 11% to 15% who had no iron, folate or vitamin B12 deficiency [5].
How it compares with the reference method
Mixed results1
Variation from one draw to the next in one person
Shown to vary about 1.7% from one draw to the next within the same person, in a pooled estimate for the RDW-SD form of the test.
The numbers and sources
- Compared with
- The same person's results on repeated venous draws
- Effect
- The European Federation of Clinical Chemistry and Laboratory Medicine database gives a pooled within-person variation of 1.7% for RDW expressed as a standard deviation (RDW-SD), with lower and upper limits of 1.4% and 3.7%, from 5 included estimates. Variation between people is 4.3% [12].
- Population
- Healthy adults in the studies the database pools [12].
- Sources
- [12]
- Caveats
- The database lists RDW-SD, a form of the test that leaves out the division by mean cell volume [4], and has no pooled entry for the percent form, so the figure may not carry over exactly. It states no threshold for agreement, so the result is given as numbers only [12]. The US survey manual lists a very high white cell count, clumped platelets, two distinct populations of red cells and clumped or rigid red cells as things that can distort the reading [3].
Did not match the reference method1
Agreement between analyzer brands
One study found RDW values differed widely between four analyzers run on the same samples, beyond the laboratory quality goal for bias.
The numbers and sources
- Compared with
- The same blood samples measured on four hematology analyzers
- Effect
- Within-run imprecision was 0.3% to 1.2%, so each analyzer repeated its own result closely. Values differed broadly between the four analyzers, with bias exceeding the desirable quality specification. The authors conclude that RDW results are not yet harmonized between analyzers [11]. The 1998 US recommendations also state that the RDW value obtained depends on the instrument used [2].
- Population
- 132 inpatient samples and 3 sample pools on Abbott Sapphire, Mindray BC6800, Siemens Advia 2120 and Sysmex XE-5000 analyzers [11].
- Sources
- [11], [2]
- Caveats
- One laboratory study from 2014 with samples from hospital inpatients. The abstract does not give the size of the differences in RDW points [11].
What studies tie it to
Linked in research4
Death from any cause
Linked to more deaths at higher RDW in community cohorts, about 14% more per 1 point in pooled data on 11,827 older adults.
The numbers and sources
- Effect
- In a pooled analysis of individual data from 7 community studies of older adults, each 1 point higher RDW went with a 14% higher risk of death (adjusted hazard ratio 1.14, 95% CI 1.11 to 1.17). Hazard ratios were 1.15 for cardiovascular death, 1.13 for cancer death and 1.13 for other causes. Among 1,603 people without major age-associated diseases it was 1.32 (1.21 to 1.44) [6]. In a US national sample aged 45 and older, the top fifth of RDW had a hazard ratio of 2.1 (1.7 to 2.6) against the bottom fifth, and each 1 point went with 22% more deaths (1.22, 1.15 to 1.30) [5]. In UK Biobank volunteers free of major disease and anemia, an RDW of 15% or above had a hazard ratio for death of 3.10 (2.57 to 3.74) against below 12.5% [7].
- Population
- 11,827 older adults over 68,822 person-years [6]. 8,175 community-dwelling US adults aged 45 and older, followed through 2000 [5]. 240,477 volunteers aged 40 to 70, followed up to 9 years [7].
- Sources
- [6], [5], [7]
- Caveats
- Observational. Median RDW differed between studies, from 13.2% to 14.6% [6]. The sources do not test whether changing RDW changes the risk of death, and the pooled analysis describes the biology behind the link as not established [6].
Heart attack and other artery disease
Linked to more coronary disease at higher RDW, both in volunteers free of disease and in people who had already had a heart attack.
The numbers and sources
- Effect
- In UK Biobank volunteers free of coronary disease at the start, an RDW of 15% or above went with more new coronary artery disease than an RDW below 12.5% (subhazard ratio 1.67, 95% CI 1.40 to 1.99), and with more heart failure, peripheral vascular disease, atrial fibrillation and stroke [7]. In people with a prior heart attack, each 1 point higher RDW had a hazard ratio for death of 1.14 (1.05 to 1.24). The top quarter had a hazard ratio of 1.78 (1.28 to 2.47) against the bottom quarter, and higher RDW also went with more coronary death or nonfatal heart attack, new heart failure and stroke [8]. Among older adults in community studies, the hazard ratio for cardiovascular death was 1.15 per 1 point (1.12 to 1.25) [6].
- Population
- 240,477 UK volunteers aged 40 to 70 [7]. 4,111 participants with a prior heart attack and no heart failure in a pravastatin trial, median follow-up 59.7 months [8]. 11,827 older adults in 7 studies [6].
- Sources
- [7], [8], [6]
- Caveats
- Observational. The analysis in people with a prior heart attack was planned after the trial ended [8]. None of the sources shows that RDW itself changes risk.
Outcomes in people with heart failure
Linked to more deaths in people with heart failure, about 10% more per 1 point of RDW across 9 pooled studies.
The numbers and sources
- Effect
- Among 36 routine laboratory values in a heart failure trial program, higher RDW showed the strongest association with illness and death (adjusted hazard ratio 1.17 per standard deviation). In a second cohort it was 1.29 per standard deviation for death, second only to age [9]. A meta-analysis found that each 1 point higher RDW at baseline had a hazard ratio for death of 1.10 (95% CI 1.07 to 1.13), pooling 9 of its 17 studies [10].
- Population
- 2,679 people with chronic heart failure in a trial program and 2,140 in a hospital database [9]. 18,288 people with heart failure in 17 cohort studies [10].
- Sources
- [9], [10]
- Caveats
- Observational. The meta-analysis found evidence of publication bias among the pooled studies [10].
Cancer
Linked to more new cancers and cancer deaths at higher RDW in two sets of cohorts, including 37% more cancers at 15% or above in UK volunteers.
The numbers and sources
- Effect
- In UK Biobank volunteers free of cancer at the start, an RDW of 15% or above went with more new cancer than an RDW below 12.5% (subhazard ratio 1.37, 95% CI 1.21 to 1.55), including colorectal cancer (1.92, 1.36 to 2.72) and leukemia (2.85, 1.63 to 4.97) [7]. In pooled community studies of older adults, the hazard ratio for cancer death was 1.13 per 1 point (1.07 to 1.20) [6].
- Population
- 240,477 UK volunteers aged 40 to 70, followed up to 9 years [7]. 11,827 older adults in 7 studies [6].
- Sources
- [7], [6]
- Caveats
- Observational. A cancer not yet diagnosed may itself alter red cells, although the UK associations held 4.5 years or more after the blood test [7].
Sources
- US Food and Drug Administration. 21 CFR 864.5200, Automated cell counter. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/section-864.5200
- Centers for Disease Control and Prevention. Recommendations to Prevent and Control Iron Deficiency in the United States. MMWR Recommendations and Reports. 1998;47(RR-3). https://www.cdc.gov/mmwr/preview/mmwrhtml/00051880.htm
- Centers for Disease Control and Prevention, National Center for Health Statistics. National Health and Nutrition Examination Survey 2017-2018 Laboratory Procedure Manual: Complete Blood Count, whole blood, Beckman Coulter DxH 800 analyzer. Performed by the NHANES Mobile Examination Center. https://wwwn.cdc.gov/nchs/data/nhanes/public/2017/labmethods/CBC-J-MET-508.pdf
- 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). Clinical Chemistry and Laboratory Medicine. 2015. https://doi.org/10.1515/cclm-2015-0155
- Patel KV, Ferrucci L, Ershler WB, et al. Red Blood Cell Distribution Width and the Risk of Death in Middle-aged and Older Adults. Archives of Internal Medicine. 2009. https://doi.org/10.1001/archinternmed.2009.11
- Patel KV, Semba RD, Ferrucci L, et al. Red Cell Distribution Width and Mortality in Older Adults: A Meta-analysis. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences. 2010. https://doi.org/10.1093/gerona/glp163
- Pilling LC, Atkins JL, Kuchel GA, et al. Red cell distribution width and common disease onsets in 240,477 healthy volunteers followed for up to 9 years. PLOS ONE. 2018. https://doi.org/10.1371/journal.pone.0203504
- Tonelli M, Sacks F, Arnold M, et al. Relation Between Red Blood Cell Distribution Width and Cardiovascular Event Rate in People With Coronary Disease. Circulation. 2008. https://doi.org/10.1161/CIRCULATIONAHA.107.727545
- 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. Journal of the American College of Cardiology. 2007. https://doi.org/10.1016/j.jacc.2007.02.067
- Huang YL, Hu ZD, Liu SJ, et al. Prognostic Value of Red Blood Cell Distribution Width for Patients with Heart Failure: A Systematic Review and Meta-Analysis of Cohort Studies. PLOS ONE. 2014. https://doi.org/10.1371/journal.pone.0104861
- Lippi G, Pavesi F, Bardi M, Pipitone S. Lack of harmonization of red blood cell distribution width (RDW). Evaluation of four hematological analyzers. Clinical Biochemistry. 2014. https://doi.org/10.1016/j.clinbiochem.2014.06.003
- European Federation of Clinical Chemistry and Laboratory Medicine. EFLM Biological Variation Database, meta-analysis estimates, read 2026-11-13. https://biologicalvariation.eu/