RDW or erythrocyte distribution amplitude

What is RDW? 

What does the abbreviation RDW? RDW stands for Red Cell Distribution Width, and indicates the width of the erythrocyte distribution. It represents one of the parameters of the blood count, that is the most classic blood test.

What is RDW used for?

RDW allows you to evaluate the degree of anisocytosis, i.e. changes in the diameter of erythrocytes (red blood cells).
Anisocytosis can be associated with conditions of alteration of the morphology of erythrocytes in the blood and, therefore, will take the name of anisopoikilocytosis.
There poikilocytosis, in fact, it is indicative of an alteration in the shape of the erythrocytes, which is often associated with a pathological condition.
To this end, therefore, these parameters are fundamental to distinguish between the various forms of anemia.

What does anemia mean? L'anemia it is a pathological condition that consists in the reduction of hemoglobin below the normal values. The values considered indicative of anemia are a Hb less than 12 g / dl in women and a Hb <13 g / dl in men.

How are red blood cells formed?

Red blood cells are formed in the bone marrow as a result of a stimulus represented by erythropoietin (EPO).

They originate from the myeloid stem cell (CFU-GEMM: units forming colonies of Granulocytes, Erythrocytes, Monocytes and Megakaryocytes). They therefore share a common origin with granulocytes, monocytes is platelets.
Red blood cells have a characteristic morphology, which is essential for them to perform their function as transport of oxygen to the tissues is transport of carbon dioxide from the tissues to the lungs, in order to expel it through respiratory exchanges.
Mature red blood cells have no nucleus and cytoplasmic organelles, a feature that also differentiates them from reticulocytes - which do not contain the nucleus, but still have ribosomes.

Alterations in the shape of erythrocytes, therefore, can occur as a result of changes in the ionic concentration of the plasma:

  • Hypotonicity: decrease in the ionic component, which leads to an increase in the intake of water in the cells, which results in a swelling of the same.
  • Hypertonicity: it manifests itself as a wrinkling of the erythrocytes, secondary to a leakage of water - consequent to the increase of the ionic concentration of the plasma.

Variations in size, on the other hand, are secondary to alterations in red blood cell production.

  • Microcytosis: red blood cells of small size. This situation is often related to iron deficiency, which is essential to ensure the red blood cell formation process. If so, they can be reduced both in terms quantities (i.e. observe a reduction in the number of red blood cells), either qualitative (the red blood cells that will form will have smaller dimensions, caused by the scarcity of the substrate that does not allow their correct constitution).
  • Macrocytosis: red blood cells of large size. This situation is often related to megaloblastic anemia, in which the term "megaloblastic" indicates an increase in the size of the erythrocyte. It is a condition that occurs as a result of the difficulty of erythrocyte precursors in reaching complete maturation. It is associated with a deficiency of vitamin B12 or folic acid; some drugs can also cause megaloblastic anemia.
Image of the graph indicating the RDW, which is the mean erythrocyte amplitude.

RDW is the blood count parameter that measures the distribution width of the erythrocyte volume. It gives the idea of the presence of red blood cells of uniform size or not.

RDW: when to take the exam?

Your doctor may require you to perform this parameter to distinguish between the various forms of anemia or for to monitor any anisocytosis.

Ex. In the presence of Low or normal MCV and high RDW, the diagnosis will be oriented towards one iron deficiency (iron deficiency anemia), a shortage of folate (megaloblastic anemia) or one deficiency of vitamins B12 (megaloblastic anemia).

Ex. In the presence of Low MCV and normal RDW, the diagnostic suspicion is oriented towards the thalassemia.

Therefore, the evaluation of the RDW makes it possible to distinguish between various forms of macrocytosis - condition linked to an increase in the size of red blood cells (e.g. with normal RDW) or more specific forms, linked to nutritional deficiencies such as folate and vitamins B12 (high RDW).
This parameter also helps to distinguish between forms of heterozygous thalassemia is nutritional deficiency anemia.

To understand the etiology of anemia, a diagnostic algorithm is followed which first involves evaluating the reticulocytes, thus allowing to distinguish the forms of altered production (e.g. aplastic anemia) from those of excessive elimination (in this case, in fact, the marrow will be stimulated in the production of a greater quantity of red blood cells, which can be analyzed through the dosage of reticulocytes, or "young" red blood cells).

After this, the evaluation of the MCV (mean corpuscular volume), which allows to evaluate the mean volume of erythrocytes, and as a function of this to narrow down the clinical suspicion. However, this may not yet be conclusive, therefore it is deemed necessary to carry out further investigations, including the assessment ofRDW (eg. Thalassemia and iron deficiency anemias have the same MCV, but different RDW).
Another parameter that can be considered is the MCH. The presence of MCH low supports the hypothesis of anemia, being associated with a condition of reduction of the average amount of hemoglobin in each red blood cell.

RDW normal values 

Normal blood RDW values vary according to the sex andage.

RDW normal values in men


  • Age 2 years: 12 - 14.5%
  • Age 3 - 5 years: 12 - 14%
  • Age 6 - 11 years: 12 - 14%
  • Age 12 - 15 years: 11.6 - 13.8%


  • Age> 16 years: 11.9 - 15.5%

RDW normal values in women 

Little girls

  • Age 2 years: 12 - 14.5%
  • Age 3 - 5 years: 12 - 14%
  • Age 6 - 11 years: 11.6 - 13.4%
  • Age 12 - 15 years: 11.2 - 13.5%


  • Age> 16 years: 11.9 - 15.5 %

The RDW count, depending on the analysis laboratory where it is performed, can be RDW cv and RDW sd:

  • RDW sd: when you go to evaluate the standard deviation. It measures the true magnitude of the size distribution of red blood cells in the blood. It is therefore not affected by the size of other blood cells.
  • RDW cv: when you go to evaluate the coefficient of variation. It is expressed in terms of % and is calculated both taking into account the standard deviation and the MCV, that is the mean corpuscular volume of erythrocytes.

NB the choice of one or the other variant is the responsibility of the analysis laboratory and does NOT involve any difference in the clinical - diagnostic and / or follow - up field. The results are therefore superimposable.

 CBC and other parameters

CBC is a commonly used laboratory test. It allows you to evaluate the parameters related to blood cells. The blood sample taken corresponds to a few milliliters, but guarantees multiple information:

  • White blood cells or leukocytes (per μL or mm ^ 3)
    • Neutrophils
    • Lymphocytes
    • Monocytes
    • Eosinophils
    • Basophils
  • Red blood cells or erythrocytes (μL or mm ^ 3)
    • Normal erythrocytes: 4,000,000 - 5,500,000
    • High erythrocytes:> 5,500,000
    • Low erythrocytes: <4,000,000
  • Hemoglobin (g / L)
  • Hematocrit: represents the percentage of blood occupied by blood cells
  • Average corpuscular volume (MCV): represents the average volume of red blood cells
  • Average cellular hemoglobin content (MCH): represents the average hemoglobin content in each red blood cell
    • Normal MCH: 27 - 31 pg
    • High MCH:> 31 pg
    • Low MCH: <27 pg
  • Average cellular concentration of hemoglobin (MCHC): represents the average concentration of hemoglobin per dL of red blood cell
    • Normal MCHC: 32 - 36 g / dL
    • High MCHC:> 36 g / dL
    • Low MCHC: <32 g / dL
  • Amplitude of the erythrocyte distribution (RDW)
  • Reticulocytes: represents the percentage of reticulocytes circulating in the blood.
  • Platelets: represents the number of platelets per mm ^ 3
  • Average platelet volume

 High RDW

A High RDW it is indicative of a high variability in the size of red blood cells (ie the contextual presence of large cells and small cells).

High RDW values 

 Values of high rdw cv and high rdw sd in the blood correspond to:

  • Adults (men):> 15.5%
  • Adults (women):> 15.5 %
  • Children (boys):> 14%
  • Girls (female):> 14%

Causes of high RDW

 High RDW values can be caused by:

  • Alcohol abuse
  • Hemolytic anemia
  • Pernicious anemia
  • Sickle cell anemia
  • Iron deficiency
  • Folic acid deficiency

To better understand the causes that lead to an increase in RDW, it is advisable to evaluate both the RDW parameter and the MCV parameter.

If RDW high and MCV normal

  • Iron deficiencies
  • Deficiencies of folic acid
  • Deficiencies of vitamin B12
  • Sickle cell anemia
  • Myelodysplastic syndrome
  • Chronic liver disease

If RDW high and MCV high

  • Folate deficiencies
  • Deficiencies of vitamin B12
  • Autoimmune hemolytic anemia
  • Myelodysplastic syndrome
  • Chronic liver disease
  • Chemotherapy (drug-related side effect)

If RDW high and MCV low

  • Iron deficiencies
  • Beta-thalassemia

High RDW Symptoms

Symptoms associated with reduced rdw values vary according to the underlying cause. More frequently, in the presence of high RDW values, we find:

  • Pallor
  • Asthenia
  • Increased sensitivity to cold
  • Weakness
  • Dizziness
  • Headache
  • Easy fatigue
  • Irritability

 Low RDW

A Low RDW it is indicative of a reduced variability in the size of red blood cells (i.e. presence of a majority of red blood cells of the same size). The red blood cells, in this case, therefore, will be homogeneous.

Low values 

Low rdw cv and low rdw sd values in blood correspond to:

  • Adults (men): <11.9 %
  • Adults (women): <11.9 %
  • Children (boys): <12 %
  • Girls (girls): <12 %


If low RDW and low MCV:

  • Homozygous thalassemia
  • Beta-thalassemia
  • Chronic diseases

If low RDW and high MCV:

  • Aplastic anemia
  • Chronic liver disease
  • Alcohol abuse
  • Chemotherapy drugs
  • Antiviral drugs

Low RDW Symptoms

Symptoms associated with low erythrocyte distribution amplitude values vary according to the underlying cause. More frequently, in the presence of low RDW values, non-specific symptoms are found, linked to the condition of anemia:

  • Pallor
  • Asthenia
  • Increased sensitivity to cold
  • Weakness
  • Dizziness
  • Headache
  • Easy fatigue
  • Irritability

Exam preparation

Preparation for the exam includes the same precautions performed for all common blood tests. It is advisable to go to the analysis laboratory at morning it's at fasting.

NB Often the evaluation of the blood count (including rdw) can be associated with other laboratory investigations, which may require special precautions.

Factors that can influence the examination

The outcome of the examination can be influenced byage and / or from transitory situations of inhomogeneity of red blood cells.



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