What is RDW in CBC Blood Test?

When reading the CBC blood test you find a value in front of RDW line, what does it mean?
Here we understand the clinical meaning and use of the medical term “RDW”.
RDW is an estimated value reported in a blood test called “CBC test”
CBC test means complete blood count or Full blood cells count.
RDW is a medical term for Red Cell Distribution Width.
Red Cell Distribution Width estimation based on the automated counters calculation of the Red cells distribution width, not a difficult as it sound, so lets start the blood story from the beginning.
You go to lab to take a blood sample for “CBC test”, the technician let the automatic counter to suck 50 micro-liter of your sample (1 micro = 1 of 1000 of milliliter), the analyser counts the cells in your blood, then plot a curve of the distribution of RBCs, the width of the curve is the RDW-CV or coefficient of variation.
Should I concern about my RDW value?
RDW is an early marker of several types of anemia, that’s because RDW value means how red cells sizes are distributed in the blood sample, the normal RDW value means that RBCs are in normal size or the same size, high or low RDW value means that the RBCs show variant sizes, in many types of anemia, RBCs appear in abnormal sizes which s detected by RDW value.
Do you know what is anemia?
What is the normal RDW?
RDW normal range is 11.5 – 14.5 % for adult male and female, up to 16 % for neonates
Normal RDW means that RBCs are in the same size, all small or all large or all within the normal diameter of 6 -8 μm, this leads us to use another factor in determining the quality of cell sizes, measured by the RDW coefficient of variation, MCV is the average size of red cells, and measured automatically by the automatic analyzers and can be observed through microscopic examination of a stained blood film from your blood sample.
However the RDW give us an early indicator of abnormal size or shape of RBCs, and before MCV itself.
Don’t foget to compare your results with specific age and sex CBC ranges
What causes High RDW in CBC blood test?
Anisocytosis is the medical term for elevated RDW value above the preset normal range, and means your blood cells are not in equal sizes, which means anemic condition.
Poikilocytosis is the variation in RBCs shapes which also cause elevation in RDW level.
Variation in size means normal size 6-8 micrometer, small size, or large size.
Variation in shape means normal biconcave shape, or abnormal shapes like the spherical, cigar shape, tear drop shape, etc…
Anemias with high RDW value
Iron Deficiency Anemia: causes small RBCs size which means Low MCV, and abnormal sizes as well which means high RDW.
Folate and vitamin B12 deficiency anemia: Vitamin B12 and folate are responsible of cell built up in the bone marrow, lacking the Vitamin B12 and folate means abnormal large cells, thus high RDW but with large sizes (high MCV)
Anemia of mixed causes (Iron, Vit. B12,or folate Deficiency): means variation in MCV values thus high RDW.
Recent Hemorrhage episode: Bleeding typically cause high RDW, because bone marrow hurry up to compensate the blood loss, and produce less immature cells which have different size than the mature cells.

RDW curve

Distribution curve

My doctor say I have anemia but RDW value is normal, what does it mean?
The RDW is prediction of cell’s size, and normal value means the same size, which be understood that cells present in same sizes whether normal or abnormal, thus several anemia may be exist with normal RDW value seen.
Anemias with Normal RDW

  • Anemia of chronic disease: which is a problem of iron deficiency but not affect the iron storage or not at first stages.
  • Hereditary spherocytosis: all cells are in the same normal size but looks spherical.
  • Acute blood loss: such as bleeding or menses.
  • Aplastic anemia: when the bone marrow is unable to produce red blood cells.
  • Thalassemia minor: is a hereditary defect in hemoglobin manufacturing, Mentzer Index is an estimation from CBC report itself to confirm thalassemia.
  • Sickle cell anemia: all RBCs appear as crescent shapes.
  • Polycythemia is a red blood cells disorder and not affect the RBC’s shape, as it causes over production of red cells alone.

Hemoglobin is the primary key for anemia, read about the normal hemoglobin.
What causes Low RDW value
Low RDW is due to false estimation, abnormal counting background can cause false low RDW estimation.
We’re welcome of results interpretation, all interpretations are for guidance purpose only.

Dr.Megan Ralf

A Medical laboratory Scientist who devoted his life to medical and laboratory sciences, writes his everyday expertise dealing with various pathological conditions through laboratory diagnosis of different body fluids, also participating in many workshops for first aids, infection control, and urgent care. Also Dr Megan Ralf coaching many medical teams.

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