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Know Your Blood Parameter : Red Cell Distribution Width

Updated: Dec 29, 2025


1. Parameter Name 

Red Cell Distribution Width (RDW)

The Red Cell Distribution Width (RDW) is a crucial hematological parameter that measures the variation in size (volume) of red blood cells (erythrocytes) in a blood sample. It is an important indicator used in the diagnosis and management of various types of anemias and other blood disorders. By analyzing the RDW, healthcare professionals can gain insights into the underlying causes of anemia, as well as other conditions affecting red blood cell production and morphology.


2. Test Type 

Blood

The RDW test is performed on a blood sample, typically drawn from a vein in the arm. This test is often conducted as part of a complete blood count (CBC), which provides a comprehensive overview of an individual's hematological health. The blood sample is analyzed using automated hematology analyzers that measure various parameters, including the RDW, to provide accurate and reliable results. The RDW can be reported in two different ways: RDW-CV (coefficient of variation) and RDW-SD (standard deviation), each offering unique insights into red blood cell variability.


3. Normal Range

  • RDW-CV: 11.5% – 14.5%

  • RDW-SD: 39 – 46 fL (some labs may vary)

The normal range for RDW-CV is between 11.5% and 14.5%. This percentage reflects the degree of variability in red blood cell sizes and is calculated based on the standard deviation of red blood cell volume divided by the mean cell volume. A higher RDW-CV indicates greater anisocytosis, or variation in red blood cell size, which can be associated with various medical conditions, including iron deficiency anemia, thalassemia, and vitamin B12 deficiency.


On the other hand, the RDW-SD is measured in femtoliters (fL) and typically ranges from 39 to 46 fL, although specific reference ranges may differ slightly from one laboratory to another due to variations in equipment and methodologies. RDW-SD provides a direct measurement of the width of the red blood cell volume distribution curve and is particularly useful in assessing the overall population of red blood cells in terms of size. An elevated RDW-SD can indicate similar conditions as an elevated RDW-CV, emphasizing the importance of this parameter in clinical diagnostics.


Understanding the RDW values in conjunction with other blood indices can assist healthcare providers in making informed decisions regarding further diagnostic testing, treatment options, and monitoring of patients with hematological disorders.


4. High Levels Indicate 

High levels of RDW suggest increased variability in the size of red blood cells, which can be indicative of several underlying health issues. This increased variability is often observed in conditions such as iron deficiency anemia, where the body does not have enough iron to produce healthy red blood cells. Additionally, high RDW can be associated with vitamin B12 or folate deficiency, which are critical for red blood cell production. Other potential causes include recent blood loss, which can lead to a mix of old and new red blood cells, and bone marrow disorders like myelodysplastic syndromes, where the bone marrow fails to produce blood cells properly. The presence of high RDW can be a crucial diagnostic marker for healthcare providers when evaluating a patient's hematological status.


5. Low Levels Indicate 

Low levels of RDW are typically not considered clinically significant on their own. They may indicate a uniformity in red blood cell size, which could be normal but might also point to a pathological condition if the uniformly sized cells are all abnormally small, as seen in microcytic anemia. In microcytic anemia, the red blood cells are smaller than normal, which can occur in conditions such as thalassemia or chronic disease. Therefore, while low RDW levels may not warrant immediate concern, they should be interpreted in the context of other hematological parameters and clinical findings.


6. Why It's Tested 

The RDW test is performed primarily to assess the variation in size of red blood cells, which is an important factor in diagnosing and classifying various types of anemia. Anemia can result from a multitude of causes, and the RDW provides valuable information that helps differentiate between these causes. For instance, a high RDW may suggest a mixed population of red blood cells due to different underlying conditions, while a normal RDW in the context of anemia may suggest a more uniform cause. Thus, the RDW test is a critical component of a complete blood count (CBC) and serves as a valuable tool in the diagnostic process.


7. Associated Symptoms (if abnormal) 

When RDW levels are abnormal, particularly when elevated, patients may experience a range of symptoms that are often associated with anemia. Common symptoms include fatigue, which can stem from inadequate oxygen transport due to insufficient healthy red blood cells. Weakness and pallor are also frequently reported, as the body struggles to maintain energy levels and proper blood circulation. Dizziness may occur due to reduced blood flow to the brain. It is important to note that the specific symptoms experienced can vary significantly depending on the underlying cause of the anemia, making it essential for healthcare providers to conduct a thorough evaluation to determine the root of these symptoms.


8. What To Do If Abnormal 

If RDW levels are found to be abnormal, it is crucial to follow up with additional tests to further evaluate the underlying causes. This may include iron studies to assess iron levels and storage, as well as vitamin B12 and folate levels to check for deficiencies that could be contributing to the red blood cell abnormalities. A reticulocyte count may also be ordered to evaluate bone marrow function and the body's response to anemia. It is essential to consult a physician for an appropriate diagnosis and to discuss potential treatment options, which may include dietary changes, supplementation, or more comprehensive medical interventions depending on the identified cause.


9. Common Conditions Related 

Several common conditions are associated with abnormal RDW levels, particularly high RDW. These include iron deficiency anemia, which is one of the most prevalent forms of anemia globally, and pernicious anemia, a condition resulting from vitamin B12 deficiency due to malabsorption. Thalassemia, a genetic blood disorder characterized by abnormal hemoglobin production, can also present with abnormal RDW levels. Chronic liver disease is another condition that may be related, as it can affect the production and lifespan of red blood cells. Understanding these conditions is vital for healthcare providers in diagnosing and managing patients effectively.


10. Nutrition or Lifestyle Connection 

Maintaining adequate nutrition is essential for healthy red blood cell production and overall hematological health. A balanced diet rich in iron, vitamin B12, and folate is crucial for preventing deficiencies that can lead to abnormal RDW levels and various forms of anemia. Foods such as lean meats, leafy green vegetables, legumes, and fortified cereals are excellent sources of these nutrients. In some cases, individuals may require supplementation if dietary intake is insufficient or if they have specific health conditions that impair nutrient absorption. Therefore, understanding the nutrition and lifestyle factors that contribute to red blood cell health is vital for both prevention and management of anemia and related disorders.


11. References (APA Format)

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