The red blood cells are usually flat and lenticular (disc-shaped) with a diameter of around 7.5 µm (micrometer).
The sole function of red blood cells is to transport oxygen to different parts of the body.
Let’s describe a few terms first:
Blood Vessels:
Blood vessels are anything that carries blood. There are three major types of blood vessels:
- arteries,
- veins, and
- capillaries.
Arteries are blood vessels responsible for carrying oxygen-rich blood away from the heart to the body. Veins are blood vessels that carry blood low in oxygen from the body back to the heart for re-oxygenation.
Capillaries are tiny blood vessels connecting arteries to veins. These blood vessels carry oxygen and nutrients to individual cells throughout the body.
Blood cells must squeeze through the body’s smallest blood vessels, the above described capillaries, to do their job, but capillaries often become narrower than the cells in their normal disc shape. Capillaries can be as small as 4 µm in diameter. So the cells must deform and “curl up” to fit through those capillaries. Remember a normal red blood cell is around 7.5 µm in diameter.
Only when this process of “squeezing” through capillaries can happen, the supply of oxygen is guaranteed throughout the whole body.
Some red blood cells are however not optimally formed. To a certain extent this is normal as there are 2 million red blood cells formed per second. Usually there are around 85% to 89% of red blood cells developed properly.
By looking at the red cell distribution width value one can see how many red blood cells have a deviating form factor. If 85% to 89% are usually normally developed the RDW value then should range between 11% and 15%.
RDW-SD and RDW-CV are parameters that are analyzed as part of the red blood cell blood analysis:
RDW-CV = Coefficient of Variation
RDW-SD = Standard Deviation
One important factor to mention now is that when we are looking at the red cell distribution width, we are not looking at the actual width (or diameter) of the individual red blood cells. We are looking at the width of the curve that the blood analyzing machine spits out.
So a higher or lower RDW-CV value would not indicate how much bigger or smaller the actual red blood cells are (=diameter), but rather how big the variation in cell size and volume is.
Mathematically, the RDW-CV is calculated with the following formula:
RDW-CV = (Standard deviation of MCV ÷ MCV) × 100.
Mean corpuscular volume (=MCV), also called mean cell volume, is a measure of the average volume of a red blood cell. The normal range of a MCV value ranges between 80 to 96 fl (=femtoliter). This means that on average a red blood cell volume is, let’s say, 90 fl. If now the standard deviation from this curve is, let’s say, 10 fl then the RDW-CV value would be 11.1% and therefore in the normal range.
The RDW-SD is an actual measurement of the width of the red cell distribution curve in femtoliters (fL) and not percent. The width of the distribution curve is measured at the point that is 20% above the baseline (aka frequency level). Since the RDW-SD is an actual measurement, it is not influenced by the MCV and more accurately reflects the red cell size variance. The RDW-SD normal value, which is the width of the volume curve, is between 39 and 46 fL.
A person with a normal result may still have an underlying condition. Doctors often compare RDW results with those of a mean cell volume (MCV) test.
To confirm a diagnosis, a doctor will compare the results of the RDW test with those of the MCV measurement.
Please also refer to our Blog post titled: The Relationship Between RDW and Autoimmune Diseases: What You Need to Know.
- BAIN, B. J. Diagnosis from the blood smear. N Engl J Med, v. 353, n. 5, p. 498-507, 2005.
- BARNES, P. W. et al. The international consensus group for hematology review: suggested criteria for action following automated CBC and WBC differential analysis. Lab Hematol, v. 11, n. 2, p. 83-90, 2005.
- BESSMAN, J. A. Red cells. In: BESSMAN, J. A. (Ed.). Automated blood counts and differentials: a practical guide. Baltimore: The Johns Hopkins University Press, 1986. p. 5-56.
- BUTTARELLO, M.; PLEBANI, M. Automated blood cell counts: state of the art. Am J Clin Pathol, v. 130, n. 1, p. 104-16, 2008.
- CAPORAL, F. A.; COMAR, S. R. Evaluation of RDW-CV, RDW-SD, and MATH-1SD for the detection of erythrocyte size heterogeneity observed by optical microscopy. Int J Lab Hematol, v. 35, suppl. 1, p. 44, 2013.
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If your red blood cells are unequal in size this is called anisocytosis. “Aniso” means unequal, and “cytosis” refers the characteristics, features and the number of cells. Anisocytosis itself is a nonspecific term, as there are several different ways in which cells can be unequal. The condition is prominent in cases of iron deficiency anemia. Iron is mostly stored in red blood cells, which help carry and store oxygen in the blood. A lack of iron in the blood leads to a reduction of red blood cells.
When the shape and size of red blood cells are not correct, then oxygen is not being transported around the body as efficiently as it should be.
- tiredness
- shortness of breath
- dizziness
- headache
- cold hands and feet
- pale skin
- chest pain
→ High RDW and normal MCV. This suggests a deficiency of iron, B-12, or folate. It may also indicate chronic liver disease.
→ High RDW and low MCV. This suggests iron deficiency or microcytic anemia.
→ High RDW and high MCV. This indicates a lack of B-12 or folate. It can also suggest macrocytic anemia or chronic liver disease.
Because several factors can cause a high RDW result, lowering the variation in red blood cell size varies according to each case. A doctor can advise how best to do this, depending on the root cause.
For those who are susceptible to a high RDW, it may help by:
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RDW measures the variation in size among your red blood cells (RBCs) and is part of a routine Complete Blood Count (CBC) test. It can be expressed in two ways:
Normal RDW-CV Range: 11.5%–14.5% (may vary by laboratory).
A low RDW indicates that your red blood cells are highly uniform in size, showing minimal variation. Imagine them as consistently sized circles, rather than a mix of different sizes.
Unlike a high RDW, which may signal conditions such as anemia, recent blood transfusions, or bone marrow disorders, a low RDW is generally not a cause for concern—especially if other CBC parameters (such as hemoglobin and MCV) are normal.
A low RDW may sometimes be seen in specific types of anemia where red blood cells are uniformly sized:
In these cases, the RDW is low because the cells are consistently sized—whether they are all large or all small.
While a low RDW is rarely indicative of a serious medical issue, it’s important to:
Your healthcare provider can determine whether additional tests or further evaluation are necessary based on your complete clinical picture.
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Healthmatters Pro Member since 2024
As a PRO member and medical practitioner, Healthmatters.io has been an invaluable tool for tracking my clients' data. The layout is intuitive, making it easy to monitor trends and spot patterns over time. The ability to customize reports and charts helps me present information clearly to my clients, improving communication and outcomes. It's streamlined my workflow, saving me time and providing insights at a glance. Highly recommended for any practitioner looking for a comprehensive and user-friendly solution to track patient labs!
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Haemoglobin (g/L), Hematocrit (Female range), Hematocrit (HCT) / Packed Cell Volume (PCV), Hemoglobin, Hemoglobin (Female range), Mean Cell Volume, Mean Corpuscular Hemoglobin (MCH), Mean Corpuscular Hemoglobin Concentration (MCHC), Mean Corpuscular Volume (MCV), Mean Platelet Volume (MPV), Mean RBC Iron, Mean RBC Iron Concentration, Mean RBC Volume, MXD #, MXD %, Platelet count / Platelets, Platelet Distribution Width (PDW), Platelet-large cell ratio (P-LCR), RBC (Female range), RBC Distribution Width, RDW-CV (Red Cell Distribution Width) in %, RDW-SD (Red Cell Distribution Width) in fL, Red Blood Cells (Erythrocytes / RBC), White blood cells (Leukocytes / WBC)
