Red Blood Cell Distribution Width (RDW)

Overview

Red Blood Cell Distribution Width (RDW) is a measure of the variation in size and volume of red blood cells (RBCs) in the bloodstream. It reflects how uniform or varied the red blood cells are in size, which is important because RBCs carry oxygen from the lungs to tissues throughout the body. RDW is primarily used as part of a complete blood count (CBC) to help diagnose and differentiate types of anemia and other blood disorders. Tracking RDW can provide insights into nutrient deficiencies, bone marrow function, and underlying health conditions that affect red blood cell production and lifespan.¹²

Scientific Background

RDW quantifies the heterogeneity in red blood cell volume, often reported as RDW-CV (coefficient of variation) or RDW-SD (standard deviation). Normal RBCs have a volume range of about 80–100 femtoliters, but some variation is normal. RDW increases when there is a wider range of RBC sizes, a condition called anisocytosis. This can occur due to mixed populations of small and large cells, often seen in nutrient deficiencies such as iron, vitamin B12, or folate deficiency. The bone marrow regulates RBC production, and disruptions in this process or increased RBC destruction can alter RDW. RDW is closely interpreted alongside mean corpuscular volume (MCV) to help identify the cause of anemia, distinguishing between single-cause and mixed-cause anemias.³⁴

Measurement and Testing

RDW is measured using automated hematology analyzers as part of a CBC test. It is calculated from the distribution of RBC volumes, often displayed graphically as a histogram. Factors such as recent blood transfusions, hydration status, and laboratory methods can affect RDW results. Testing is recommended when anemia or other blood disorders are suspected, or to monitor treatment response. RDW alone is not diagnostic but provides valuable context when combined with other CBC parameters like hemoglobin and MCV.¹⁵

Reference Ranges

Typical reference ranges for RDW-CV in adults are approximately 11.5% to 15.4%, though ranges may vary slightly by laboratory. RDW-SD values are also used but less commonly standardized. Reference ranges can differ based on age, sex, and population. Values within the normal range indicate uniform RBC size, while values above the range suggest increased size variability. Interpretation must consider demographic factors and clinical context, as normal RDW does not exclude all blood disorders.³⁵

High Values

Elevated RDW indicates greater variation in RBC size and is commonly caused by nutrient deficiencies such as iron, vitamin B12, or folate deficiency. It is also seen in mixed anemia types, hemolytic anemia, recent blood loss, and bone marrow disorders. High RDW is associated with increased risk of cardiovascular diseases and can reflect underlying inflammation or chronic illness. Symptoms related to high RDW often overlap with anemia symptoms, including fatigue, weakness, and pallor. Elevated RDW helps clinicians narrow down causes of anemia and monitor disease progression.¹³⁴

Low Values

Low RDW values are less common and generally indicate very uniform red blood cell sizes. This is typically not associated with specific health risks and is often considered normal. However, a low RDW in the presence of anemia may suggest a single cause anemia such as thalassemia minor or anemia of chronic disease, where RBC size variation is minimal. Low RDW alone is rarely a clinical concern but should be interpreted in the full clinical context.³⁵

Improving Biomarker Levels

Improving RDW values involves addressing underlying causes of red blood cell size variation. Lifestyle modifications include ensuring adequate nutrition with sufficient iron, vitamin B12, and folate intake through diet or supplements. Managing chronic diseases and reducing inflammation can also normalize RDW. In cases of anemia, targeted treatment such as iron supplementation or vitamin therapy is essential. Regular monitoring helps assess response to interventions. Consultation with healthcare providers is important before starting supplements or treatments.¹³

Importance of Tracking

Monitoring RDW provides valuable information about red blood cell health and helps detect early signs of anemia and other hematologic conditions. It aids in differentiating types of anemia, guiding appropriate treatment decisions. Tracking RDW over time can reveal changes in bone marrow function or nutritional status and may indicate risk for cardiovascular complications. Regular RDW assessment as part of CBC testing supports comprehensive health evaluation and timely intervention.²⁵

Disclaimer

The information provided in this document is for educational purposes only and does not constitute medical advice. It is essential to consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.