Help the development of the site, sharing the article with friends!

The RDW SD indicator in the blood count shows the volume differences between the individual red blood cells of a given patient. RDW SD - interpreted together with other blood count parameters - is used primarily in the search for the causes of anemia. What does low or elevated RDW SD mean? What are the standards for this test?

The term RDW SDis an abbreviation of red blood cell distribution width, standard deviation. In Polish, RDW SD is referred to aserythrocyte volume distribution index .

Red blood cells (erythrocytes) circulating in human blood are similar in size, but they are not identical. The RDW SD index in morphology shows exactly what are the differences between the erythrocyte volumes in the examined patient. The determination of the WFD may allow to determine whether the patient has anisocytosis, i.e. a significant variation in the size of individual red blood cells.


  1. RDW SD indicator in morphology: how is it marked?
  2. RDW SD: indications for the test
  3. RDW SD: norm
  4. RDW SD increased. What does high RDW mean?
  5. RDW SD lowered. What does low RDW mean?

RDW SD indicator in morphology: how is it marked?

RDW is determined when performing blood counts. The test is performed on the basis of a blood sample taken from the patient. The patient should report for the examination on an empty stomach.

It should be emphasized that patients do not only measure the RDW SD index. In order to be able to fully interpret the determined value of RDW SD, it is necessary to analyze this indicator together with other blood count parameters - among them the most important in this case are, inter alia, erythrocyte count, hemoglobin level and MCV index.

RDW SD: indications for the test

A closer look at the RDW value is recommended, first of all, in the diagnosis of anemia - analyzing the value of this indicator (together with the assessment of MCV in particular) makes it possible to suspect the probable cause of anemia in the patient.


RDW SD: standard

RDW SD values ​​are given in femtoliters (fl). Generally, 36-47 fl is considered to be the RDW SD standard. Inin the case of men, the correct values ​​are sometimes considered to be slightly lower than the given values. It is worth noting that individual laboratories may have a slightly different standard of the RDW SD indicator in morphology - always before analyzing the test result, you should read the standard in force in a given institution.

The WFD standard is sometimes given as a percentage - then the correct value is usually taken as 11.5-14.5%. If the test result is given as a percentage, then the RDW CV indicator was determined, not the RDW SD.

RDW SD increased. What does high RDW mean?

A patient whose blood countRDW SDiselevatedhas red blood cells larger than typical in his blood. Such a situation may occur in the case of megaloblastic anemia (resulting, for example, from vitamin B12 or folic acid deficiency), but it also happens in patients with iron deficiency anemia.High RDWis also observed in patients with chronic inflammation, as well as in the case of various neoplastic diseases (such as, for example, myelodysplastic syndrome or when metastases of a tumor appear in the bone marrow) .

An excessively high RDW, however, can also be caused by a blood transfusion. Increasing this index may also be due to a recent haemorrhage.

Based on the RDW result alone, it is not possible to make a suspicion, which has led, for example, to the patient's anemia. For this reason, RDW SD is always analyzed together with other blood count parameters, the most important of which is probably MCV. An increase in RDW with low MCV may suggest iron deficiency anemia, while an increase in RDW and high MCV may be due to vitamin B12 and folate deficiency anemia.

RDW SD lowered. What does low RDW mean?

Lowered RDWSD usually has no diagnostic significance - if it is the only abnormality in the patient's blood count, then there is probably no cause for concern. However, if the decreased RDW value is accompanied by some other deviations (e.g. a decrease in the hemoglobin value), it may already indicate the existence of e.g. microcytic anemia (i.e. a form of anemia in which the patient has smaller than typical erythrocytes).

About the authorBow. Tomasz NęckiA graduate of medicine at the Medical University of Poznań. An admirer of the Polish sea (most willingly strolling along its shores with headphones in his ears), cats and books. In working with patients, he focuses on always listening to them andgive them as much time as they need.

Help the development of the site, sharing the article with friends!