Red cell distribution widthRDW
A measure of how varied your red blood cells are in size.
Red cell distribution width (RDW) measures how much your red blood cells vary in size. It arrives automatically with every full blood count: the analyser that counts your cells also records the spread of their volumes, and RDW is that spread expressed as a single number, usually a percentage. Where MCV (mean corpuscular volume) tells you the average size of your red cells, RDW tells you how tightly the population clusters around that average: a low value means your cells are close to uniform, a higher one means new, old, large and small cells are mixed together.
The classic reason a clinician looks at it is anaemia. Red cells live about 120 days, so the cells in a sample are a rolling four-month archive of what your bone marrow was doing, and when the marrow's supply of iron or vitamins changes, the new cells come out a different size from the old ones and the spread widens before the average moves. That is why RDW is read alongside MCV: the pairing helps distinguish, for example, early iron deficiency (spread widens, cells trend small) from other patterns, a use formalised in haematology since Bessman's 1983 classification of anaemias by MCV and RDW together.
What has made RDW interesting well beyond anaemia is an accidental discovery from cardiology. Cohort studies kept finding that RDW predicted outcomes in people whose counts were otherwise unremarkable. The largest general-population analysis, from the US NHANES survey of 8,175 adults aged 45 and over, found that people in the highest fifth of RDW had roughly twice the all-cause mortality of those in the lowest fifth over about a decade of follow-up, and the association held even in participants who were not anaemic, had RDW within the reference range, and had no measured iron, folate or B12 deficiency. A community cohort published the same year reported the same pattern independently.
Nobody fully knows why. RDW correlates with inflammatory markers such as CRP and ESR: inflammation interferes with red-cell production and survival, so a widened spread may be a readout of low-grade inflammation that other routine numbers miss. It also rises with anything that stresses the marrow's supply lines: nutrient shortfall, kidney function, oxidative stress, recent bleeding or a blood transfusion, all of which mix cell generations of different sizes. A widened RDW is therefore best read as a general, non-specific signal that something is perturbing red-cell production: informative precisely because it is unspecific, and uninterpretable on its own for the same reason.
This is also why RDW appears in biological-age scoring. The Phenotypic Age model, derived from mortality follow-up in the same NHANES population, selected RDW as one of its nine blood biomarkers, alongside better-known values like glucose and CRP, because the spread of red-cell sizes carried independent information about ageing that the others did not. If you have an RDW value from a blood test, it is one of the inputs our Phenotypic Age calculator uses.
Some practical context for reading your own report. RDW is reported in two forms, a coefficient of variation (RDW-CV, the common percentage form) and a standard deviation (RDW-SD, in femtolitres), and the two are not interchangeable, so a value only means something against the same lab's own reference interval for the same form. A mildly raised RDW is common and has many mundane explanations, recent blood donation among them. And the direction of the evidence matters: the cohort studies show that higher RDW accompanies higher risk on average across thousands of people, not that a given individual's number carries a verdict; RDW is a flag that invites the question "what is the marrow responding to?", which is a conversation for whoever ordered the test, alongside the rest of the panel.
Sources
- Bessman JD, et al. Improved classification of anemias by MCV and RDW. Am J Clin Pathol 1983;80:322-326
- Patel KV, et al. Red blood cell distribution width and the risk of death in middle-aged and older adults. Arch Intern Med 2009;169:515-523
- Perlstein TS, et al. Red blood cell distribution width and mortality risk in a community-based prospective cohort. Arch Intern Med 2009;169:588-594
- Lippi G, et al. Relation between red blood cell distribution width and inflammatory biomarkers in a large cohort of unselected outpatients. Arch Pathol Lab Med 2009;133:628-632
- Salvagno GL, et al. Red blood cell distribution width: a simple parameter with multiple clinical applications. Crit Rev Clin Lab Sci 2015;52:86-105
- Levine ME, et al. An epigenetic biomarker of aging for lifespan and healthspan. Aging 2018;10:573-591 (clinical Phenotypic Age)
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
Red cell distribution width has no published interval on this page yet. We hold one, and the dashboard draws it beside a reader’s own result, but it has not been read back against the laboratory’s own page since it was collected. A figure printed beside somebody’s blood result has to have been seen at its source, not remembered, so this one waits.
On your own blood test?
Red cell distribution width is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, and your results are explained together in one place.
Frequently asked questions
- What does a high RDW mean?
- It means your red blood cells vary more in size than usual, which normally indicates the bone marrow has recently been producing cells under changed conditions — most often a shortage of iron, B12 or folate, or recovery after blood loss. A raised RDW is a prompt to look at the rest of the blood count rather than a diagnosis on its own, and it is common enough that it frequently turns out to be nothing serious. Interpreting it is a job for the clinician who ordered the test and can see your full picture.
- Is a low RDW anything to worry about?
- No. A low RDW means your red blood cells are unusually uniform in size, and that has no established clinical significance — essentially all the research on RDW concerns the raised end. Laboratories flag it only because their software flags anything outside the printed interval.
- What is a normal RDW?
- There is no single universal figure, and we deliberately do not print one. Analysers report RDW either as a percentage (RDW-CV) or in femtolitres (RDW-SD), and each laboratory sets its interval from its own instrument and population, so the two are not interchangeable and one laboratory's range can misrepresent another's result. Use the reference range printed beside the result on your own report.
- Does a high RDW mean I am anaemic?
- Not by itself. Anaemia is defined by haemoglobin, not by RDW. What RDW adds is a clue to the cause when anaemia is present, and occasionally an early hint that a deficiency is developing while haemoglobin still looks normal — a widened RDW with a still-normal MCV is the classic version of that.
- Why does RDW appear in biological age calculators?
- Because it predicts mortality across several unrelated disease categories rather than tracking any single one, which makes it behave like a general measure of physiological strain. That is the property biological-age scores are trying to capture, so RDW sits in them alongside albumin, creatinine, the white cell count and other routine markers.