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Written By: B. Dame
Updated On: January 11, 2025
Red cell distribution width (RDW) is a red blood cell parameter that measures the variability in red cell volume or size, a condition referred to as anisocytosis. RDW is commonly used in blood tests to assess overall red blood cell health and is reported in two forms depending on the hematology analyzer:
RDW-SD is reported in femtoliters (fL) and measures the width of the red cell size distribution histogram. Specifically, it calculates the width at the 20% height level of the histogram, which reflects the variability in red blood cell sizes.
Since RDW-SD focuses on the distribution of sizes, the average red blood cell size, measured as mean corpuscular volume (MCV), does not influence this parameter.
RDW-CV is expressed as a percentage and is calculated using the following formula:
RDW-CV (%) = (Standard Deviation ÷ Mean Corpuscular Volume) × 100
This parameter provides a relative measure of red blood cell size variability in relation to the average cell size (MCV).
The normal reference ranges for RDW vary slightly depending on the method used:
Abnormal RDW values can indicate a variety of conditions, such as anemia, nutrient deficiencies (iron, vitamin B12, or folate), or chronic diseases. High RDW suggests increased variability in red blood cell size, which often warrants further investigation.
The Red Cell Distribution Width (RDW) test is conducted using a whole blood sample. This specimen is typically collected in a tube containing an anticoagulant, such as EDTA, to prevent the blood from clotting.
The analysis is performed as part of a Complete Blood Count (CBC), which measures various components of the blood, including red blood cells, white blood cells, and platelets. The RDW is specifically calculated based on the red blood cell size variability observed in this sample.
RDW provides valuable insights into red blood cell health and variability, helping healthcare providers identify potential underlying conditions. Understanding both RDW-SD and RDW-CV can give a clearer picture of what your blood test results mean.
A high red cell distribution width (RDW) indicates significant variability in the size of red blood cells (RBCs). This variability can signal various health conditions. To identify the underlying cause of high RDW, it is often compared to the mean corpuscular volume (MCV)—the average size of red blood cells.
MCV represents the average space occupied by each red blood cell. The relationship between RDW and MCV provides key insights into potential medical conditions.
When both RDW and MCV are elevated, the following conditions may be present:
Liver disease impacts RDW-SD due to several factors:
In this condition, RDW-SD increases due to the destruction of RBCs and the body's compensatory response:
A high RDW with low MCV suggests smaller-than-average RBCs with significant size variability. Common causes include:
Iron deficiency reduces hemoglobin production, leading to smaller RBCs and higher RDW.
This genetic blood disorder impairs hemoglobin production, causing:
Elevated RDW with normal MCV often signals early stages of conditions such as:
A high RDW provides essential insights into red blood cell size variability and possible health issues. Understanding RDW in relation to MCV helps pinpoint conditions such as anemia, liver disease, or nutrient deficiencies.
If your RDW levels are high, consult a healthcare provider for further evaluation. Additional tests can help confirm the cause and guide appropriate treatment to address the underlying issue.
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Red Cell Distribution Width-SD (RDW-SD) is a parameter in blood tests that measures the variation in the size (or volume) of red blood cells (RBCs). It is part of a complete blood count (CBC) and helps identify abnormalities in red blood cell size, often used to detect anemia and related disorders.
While high RDW-SD values are commonly linked to medical conditions, low RDW-SD results can also provide valuable insights. This article explores what a low RDW-SD result might indicate, associated symptoms, and possible treatment options.
RDW-SD measures the variability in RBC size. A low RDW-SD result suggests that red blood cells are more uniform in size. This finding can be associated with several conditions:
In some cases, low RDW-SD is normal and indicates healthy red blood cell production. A balanced diet and adequate nutrient levels often result in uniform RBC sizes, requiring no further investigation.
Conditions like iron deficiency anemia or thalassemia can lead to uniformly small red blood cells, causing a low RDW-SD. Other parameters, such as mean corpuscular volume (MCV), would typically also be abnormal in these cases.
Rarely, low RDW-SD may signal bone marrow suppression, where the bone marrow produces fewer or less diverse RBCs. This could result from certain medications, chemotherapy, or chronic diseases.
Chronic illnesses or inflammatory conditions may cause the body to produce red blood cells of uniform size, reflecting a suppressed bone marrow response.
While low RDW-SD itself often causes no symptoms, underlying conditions may present with:
If these symptoms occur, seek medical advice to determine the underlying cause.
To pinpoint the reason for a low RDW-SD, healthcare providers evaluate other blood parameters:
The treatment for low RDW-SD depends on the underlying cause:
Iron deficiency anemia may improve with iron-rich foods (e.g., red meat, spinach, lentils) or iron supplements.
Treatment may involve blood transfusions, chelation therapy to remove excess iron, or medications to boost hemoglobin production.
Stopping medications, using drugs to stimulate blood production, or in severe cases, bone marrow transplantation may be necessary.
Treating the underlying condition with medications, lifestyle changes, or therapy can improve RBC health.
A low RDW-SD result isn’t always a cause for concern. However, consult a healthcare provider if you experience symptoms like fatigue, weakness, or shortness of breath, or if you have a family history of blood disorders.
Further testing may reveal the root cause, and your doctor can recommend appropriate treatment or monitoring.
A low RDW-SD result reflects less variability in red blood cell sizes, which is generally less concerning than high RDW-SD. However, it can indicate conditions such as microcytic anemia, bone marrow suppression, or chronic illness.
Regular health check-ups, a balanced diet, and timely medical evaluation of symptoms can help maintain healthy red blood cell levels and overall well-being.
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