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Red Cell Distribution Width (RDW-SD)

How wide the spread of red blood cell sizes is, in femtoliters (fL). A higher number means a wider mix of small and large cells.

AI-verified against cited sources. Awaiting expert reviewAlso called RDW-SD, RDW SDLOINC 115742-9

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What it is

RDW-SD stands for red cell distribution width, standard deviation. A blood analyzer measures the size of thousands of your red cells and plots them on a graph. RDW-SD is the actual width of that graph, reported in femtoliters (fL), the same unit as MCV, the average cell size. Some labs print it with the complete blood count (CBC), either alongside RDW-CV, which gives the spread as a percentage, or instead of it. Some labs print it, for example hospital labs that use Sysmex analyzers.

Why it matters

RDW-SD is an early clue when your clinician looks for the cause of anemia or a low nutrient level. It tends to rise when iron, vitamin B12 or folate are low, or when red cells of different ages and sizes are mixed together after bleeding or a transfusion. It can also be a little high for reasons that have nothing to do with anemia, and it creeps up slightly with age. Because it is a direct width, it doesn't depend on average cell size the way RDW-CV does, but that also makes it less suitable for comparing people whose average cell sizes are very different. It is read alongside MCV and hemoglobin.

If it’s high

A high RDW-SD means your red cells vary more in size than usual. Common reasons include low iron, low vitamin B12 or folate, and recent bleeding or a transfusion. It also rises slightly with age, and it can read high simply because of the analyzer your lab uses, so compare it with the range on your own report. Long-term illness, inflammation and liver, kidney or heart conditions have also been linked to a higher RDW. Your clinician reads it together with MCV, hemoglobin and your other results.

Common causes

  • Low iron
  • Low vitamin B12 or folate
  • Recent blood loss or a recent blood transfusion
  • Older age
  • Differences between analyzers and labs
  • Inherited blood conditions such as thalassemia or sickle cell disease
  • Long-term illness or inflammation
  • Liver, kidney or heart conditions
  • Some medicines, including chemotherapy

What can help

  • Ask your clinician whether iron, B12 and folate levels should be checked
  • Ask whether MCV, hemoglobin or a repeat test would help explain the pattern
  • Eat a balanced diet that includes iron, B12 and folate foods, and keep up with routine checkups

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 37.4–52.4 fL. Source: Vanderbilt Medical Laboratories, OHSU Laboratory Services (Oregon Health & Science University), PLoS One, Anemia, Scandinavian Journal of Clinical and Laboratory Investigation, Clinical Chemistry and Laboratory Medicine.

What can affect your result

  • The analyzer and method: RDW-SD limits differ between manufacturers and labs
  • Age: values tend to rise with age
  • Sex: some studies find higher values in women, others find no difference
  • Recent blood transfusion or blood loss
  • Iron, vitamin B12 and folate status
  • Pregnancy
  • How long the blood sat before testing
  • A recent blood transfusion mixes red cells of different sizes and can raise RDW-SD.
  • Cold agglutinins and red cell clumping can distort the size measurements.
  • RDW-SD depends on the analyzer, so results from different labs or machines are not directly comparable.
  • Blood that sits for many hours before testing can change red cell size measurements slightly.

Changes over weeks to months, because it follows red cell turnover, and red cells live for roughly 3 to 4 months.

When to talk to a clinician

There is no urgent-call limit for RDW-SD. Contact a clinician if a high RDW-SD comes with tiredness, shortness of breath, pale skin, a fast heartbeat or unexplained weight loss. Call 911 or your local emergency number for chest pain, severe shortness of breath, fainting or confusion.

Questions you could ask

  • Has my RDW-SD changed compared with my earlier results from the same lab?
  • Could low iron, B12 or folate explain my RDW-SD?
  • Should we look at my MCV and hemoglobin together, or repeat the test?

Questions people ask

What is the difference between RDW-CV and RDW-SD?

Both measure how much red cell size varies. RDW-CV is a percentage and is tied to the average cell size (MCV). RDW-SD is measured in femtoliters (fL) and is a direct width. Most US labs print RDW-CV, while many labs in other countries print RDW-SD or both. The two have different ranges and can't be swapped for each other.

Why does the range for RDW-SD differ between labs?

Each analyzer measures the width a little differently, and labs set ranges for their own machines and patients. For example, one US hospital lab prints 37.4 to 52.4 fL, while a study of healthy adults on another type of analyzer found 37.7 to 45.5 fL. Use the range on your own report, and compare results from the same lab over time.

My lab only prints RDW-SD. Can I compare it with an RDW-CV from another lab?

No. RDW-SD is in fL and RDW-CV is a percentage, so the numbers are not interchangeable. If you want to follow your result over time, compare RDW-SD with other RDW-SD results, ideally from the same lab.

Show the science and sources

Units

  • 1 fL = 1 fL
  • 1 fL = 1 µm³

Is a change real?

RDW-SD naturally varies about 1.51% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 5% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Systematic review and meta-analysis of within-subject and between-subject biological variation estimates of 20 haematological parameters (Coskun A et al., EFLM WG-BV).

For clinicians and researchers

RDW-SD is the width, in fL, of the red cell volume distribution curve at the 20% height level, so it is an absolute measure of size spread and is not influenced by MCV. RDW-CV is the standard deviation divided by MCV, times 100, so it is partly driven by MCV (BMC Nephrol 2025; BMJ Open Diabetes Res Care 2024; Clin Chem Lab Med 2015). Because RDW-SD does not scale with MCV, it is less suitable for comparing anemias with markedly different MCV values (BMC Nephrol 2025). Limits depend on the analyzer and the population: Vanderbilt (Sysmex) prints 37.4–52.4 fL, OHSU prints 35.1–46.3 fL, healthy Russian adults on Beckman Coulter analyzers had 37.7–45.5 fL (PLoS One 2024), and in Cameroon the manufacturer range of 37–54 fL on a URIT 3300 placed 80% of healthy men and 95% of healthy women outside the range (Anemia 2021). In 8,089 people measured on an Abbott CELL-DYN Sapphire, RDW-SD rose nearly 15% from the youngest to the oldest age class, compared with 6% for RDW-CV, because MCV also rises with age, and the authors suggested that studies of prognosis focus on RDW-SD and take age into account (Clin Chem Lab Med 2015). In 26,242 men and 11,934 women in Wuhan, RDW-SD increased with age and was higher in women (Scand J Clin Lab Invest 2020). In a mid-pregnancy study, RDW-SD was higher in women with iron deficiency anemia than in controls while RDW-CV was not (area under the curve 0.84 versus 0.58), but in alpha and beta thalassemia both were poor discriminators (Open Life Sci 2021; Clin Lab 2021). Studies linking a higher RDW to long-term outcomes mostly used RDW-CV. RDW-SD has also been linked to outcomes in specific patient groups, such as dilated cardiomyopathy (BMJ Open Diabetes Res Care 2024), but that does not describe individual risk. LOINC 21000-5 and 30384-2 are deprecated; the current code is 115742-9.

LOINC 115742-9 (primary), 21000-5, 30384-2. Specimen: whole-blood.

Sources

  1. Sysmex Complete Blood Count Reference Ranges (March 2024), Vanderbilt Medical Laboratories (2024)
  2. Heme Reference Ranges (updated 6/5/18), OHSU Laboratory Services (Oregon Health & Science University) (2018)
  3. Derivation of Russian-specific reference intervals for complete blood count, iron markers and related vitamins, PLoS One (2024)
  4. Establishment of Hematological Reference Values among Healthy Adults in Bamenda, North West Region of Cameroon, Anemia (2021)
  5. Reference intervals of systemic immune-inflammation index, neutrophil to lymphocyte ratio, platelet to lymphocyte ratio, mean platelet volume to platelet ratio, mean platelet volume and red blood cell distribution width-standard deviation in healthy Han adults in Wuhan region in central China, Scandinavian Journal of Clinical and Laboratory Investigation (2020)
  6. 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)
  7. Impact of red blood cell distribution width on response to HIF-PH inhibitor in improving anemia and clinical outcomes in patients with chronic kidney disease, BMC Nephrology (2025)
  8. Association between RDW-SD and prognosis across glycemic status in patients with dilated cardiomyopathy, BMJ Open Diabetes Research & Care (2024)
  9. Red blood cell distribution width-standard deviation but not red blood cell distribution width-coefficient of variation as a potential index for the diagnosis of iron-deficiency anemia in mid-pregnancy women, Open Life Sciences (2021)
  10. Discrimination of Iron Deficiency, Alpha and Beta Thalassemia on the Basis of Red Cell Distribution Width and Reticulocyte Indices, Clinical Laboratory (2021)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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