RDW: Definition and importance in blood test

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RDW, acronym for Red Cell Distribution Width, is an index of erythrocyte anisocytosis and which represents the heterogeneity of distribution of the size of red blood cells. This size variation is called anisocytosis, and the value considered normal in adults and children varies between 11.6 and 14.5%.

The RDW index is generally used to detect different types of anemia, because in the automated blood count, it represents the presence of anisocytosis or variation in the size of erythrocytes. Such an alteration can occur for several reasons, from anemia to the presence of chronic diseases, leukemia and use of medications.

According to Bessman, et.al.; RDW may contribute to the differentiation between microcytic and hypochromic anemias. The evaluation should be done in conjunction with other indices such as MCV (Mean Corpuscular Volume), MCH (Mean Corpuscular Hemoglobin), MCHC (Mean Corpuscular Hemoglobin Concentration) and RDW.

Various hematology analyzers such as the Erba 3/5 PDA series are often used to determine the distribution width of red blood cells, such as the RDW-CV, reported as Red Blood Cell Distribution Width measured as Coefficient of Variation (CV) and the RWD-SD, measured as standard deviation (SD), calculated from mean corpuscular volume (MCV).


                         

Image 1: Elite 580 Erba Mannheim

The hemogram evaluates the average size of erythrocytes (MCV) through impedance, which are the electrical impulses converted into fentoliters (fL), emitted by the passage of each cell individually in a flow, whose intensity will be proportional to the size of the cells. Additionally, the RDW-CV will be calculated in relation to the mean erythrocyte size (MCV) and the RDW-SD, calculated based on the mean size of erythrocytes distributed up to 20% above the base of the erythrogram histogram.

The RDW can be considered as increased when there are a variety of red blood cells with different sizes, not forgetting to take into account, the age and the reference values within the population study covered by the laboratory. The main causes of high RDW are associated with deficiency anemia, such as iron, folate or vitamin B deficiency, β-thalassemia, sickle cell anemia, hereditary spherocytosis, hemolytic anemia, among others. In addition, people undergoing chemotherapy or using some antiviral may also have increased RDW.

A low RDW does not normally have clinical significance when interpreted in isolation, however, if other alterations are verified in the blood count, such as in the MCV, also presenting low values, we may be facing anemia caused by chronic diseases or even facing a picture of heterozygous thalassemia.

It is worth mentioning that all exams must be requested and interpreted by physicians, as they will take into account other factors such as family history, age and the request for complementary exams to follow up on the clinical conduct.

Author: Juliana Oliveira, Scientific Advisor and Master in Pathology-UFF

Bibliography

·         Bessman JD, Gilmer PR Jr., Gardner FH. Improved classification of anemias by MCV and RDW. Am J Clin Pathol. 1983;80(3):322-326

·         Bessman JD, Feinstein DI. Quantitative anisocytosis as a discriminant between iron deficiency and thalassemia minor. Blood 1979; 53(2):288-93.

·         Fitzsimons EJ, Brock JH. The anemia of chronic disease. BMJ 2001;322(7290):811-12.

·  Jayaranee S, Sthaneshwar P. Serum soluble transferrin receptor in hypochromic microcytic anemia. Singapore Med J. 2006;47(2): 138-42.

·         Nobili B, Perrotta S, Matarese SMR, Conte ML, Giudice EM. Evaluation of body iron status in Italian carriers of beta-thalassemia trait. Nourish Res. 2001; 21:55-60.

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