RDW - SD

The Red Cell Distribution Width-Standard Deviation (RDW-SD) offers insights into the variability in size among red blood cells, the vital carriers of oxygen throughout the body. 

Normally, red blood cells exhibit uniform dimensions to ensure optimal oxygen transport. However, deviations from this consistency, known as anisocytosis, can signal underlying health issues. The RDW-SD value acts as a vital indicator of this variation, functioning as a quality control measure for the body's oxygen delivery system. 

By assessing the uniformity of red blood cell sizes, the RDW-SD provides valuable information essential for understanding overall health and detecting potential abnormalities in oxygen transport.

What is RDW-SD?

The Red Cell Distribution Width-Standard Deviation (RDW-SD) is a measure used in blood tests to assess the variability in the size of red blood cells. While RDW-CV quantifies the range as a percent, the RDW-SD is measured in femtoliters (fL).

It is determined by calculating the width (in fL) at the 20% height level of the RBC size distribution histogram, independently of the average RBC size (mean corpuscular volume, MCV).

Higher RDW-SD values indicate greater variability in cell size, whereas lower values suggest more uniform cell sizes.

Role of RDW-SD in Complete Blood Count (CBC)

The RDW-SD is typically ordered as part of a complete blood count, or CBC.  This test is commonly run as a part of routine blood work.  

The RDW-SD measures variation in red blood cell size or volume.  Unlike the RDW-CV, which is understood in context with the MCV, the RDW-SD is not interpreted in conjunction with the MCV.  [9.]

RDW is essential in diagnosing anemia as it highlights heterogenous red cell size or the presence of multiple red cell populations. Elevated RDW may suggest conditions like sideroblastic anemia or combined iron deficiency anemia and megaloblastic anemia, where peripheral blood smear review confirms these findings.  [8., 9.]

What Does a High RDW-SD Mean?

A high RDW (Red Cell Distribution Width) indicates greater variability in the size of red blood cells within a blood sample. This variation suggests anisocytosis, where red blood cells are of different sizes. 

A high RDW can be indicative of various conditions including anemia, nutrient deficiencies such as iron, vitamin B12 or folate deficiency, hemolysis (destruction of red blood cells), bone marrow disorders, and chronic diseases.  Additionally, it can be helpful to catch certain nutritional deficiencies early.  [11., 12.]

It can also be a marker of response to treatment or disease severity of other conditions. 

Therefore, when RDW levels are elevated, further investigation is often warranted to determine the underlying cause and appropriate management.

What Does a Low RDW-SD Mean?

A low RDW (Red Cell Distribution Width) typically indicates less variability in the size of red blood cells within a blood sample. This uniformity suggests that most of the red blood cells are of similar size. 

A low RDW is typically considered normal, and is not a concerning finding.   

Testing RDW-SD

Procedure for RDW-SD Test

The RDW-SD test is typically performed as part of a complete blood count (CBC) analysis. During the procedure, a small sample of blood is drawn from a vein in the arm using a needle. The blood sample is then sent to a laboratory for analysis. 

In the laboratory, automated hematology analyzers measure the size variation of red blood cells and calculate the RDW-SD value.

Preparation for RDW-SD Test

No special preparation is usually required for the RDW-SD test. Patients can typically eat and drink normally before the test and do not need to fast. 

Clinical Significance of RDW-SD

Causes of High RDW-SD

Anisocytosis: high RDW-SD values indicate an increased variation in the size of red blood cells, a condition known as anisocytosis.

Iron Deficiency Anemia: elevated RDW-SD levels may be indicative of iron deficiency anemia, a common type of anemia characterized by low levels of iron in the body.  [11.]

Vitamin B12 Deficiency: high RDW-SD values can also be associated with vitamin B12 deficiency anemia, which occurs when the body lacks sufficient vitamin B12 to produce healthy red blood cells.  [5.]

Folate Deficiency: elevated RDW-SD levels may indicate folate deficiency anemia, a condition caused by inadequate levels of folate (vitamin B9) in the body, leading to the production of abnormally large red blood cells.  [5.]

Hemolytic Anemia: high RDW-SD values may be observed in hemolytic anemia, a condition characterized by the premature destruction of red blood cells, resulting in an increased production of new red blood cells of varying sizes.  

Chronic Diseases: certain chronic diseases, such as chronic kidney disease, liver disease, certain cancers, or inflammatory disorders, can lead to high RDW-SD values due to their impact on red blood cell production and turnover.  [7.]

Bone Marrow Disorders: high RDW-SD levels may be associated with bone marrow disorders, such as myelodysplastic syndromes (MDS), which affect the production of red blood cells, leading to abnormal cell sizes and shapes.  [3., 4.]

Causes of Low RDW-SD

A low RDW means that most red blood cells are of similar sizes, and this alone is not typically a cause for concern.  [2.]

Supportive Therapies for High RDW

Treating high RDW-SD begins with understanding the cause, which then guides appropriate treatment and follow up strategies.  Some options to consider include: 

Iron supplementation: addressing iron deficiency can help normalize red blood cell size and reduce RDW.  An iron deficiency requires further assessment and should be monitored by a medical professional.  

‍Vitamin B12 and folate supplementation: correcting deficiencies in these vitamins can treat megaloblastic anemia, a condition associated with high RDW.

‍Blood transfusion: in severe cases of anemia, transfusions may be necessary to increase red blood cell count and improve RDW.

Medications: in certain settings medications including erythropoietin-stimulating agents to stimulate red blood cell production or corticosteroids to reduce inflammation may be considered.  

Treating underlying conditions: identifying and managing underlying causes of anemia, such as chronic kidney disease or bone marrow disorders, can help normalize RDW.

Dietary adjustments: consuming a balanced diet rich in nutrients essential for red blood cell production, such as iron, vitamin B12, and folate, can support the treatment of high RDW.

Testing Options for RDW-CV

Click here to discover a list of panels available to assess RDW-CV.