High Red Cell Distribution Width: What It Means and What You Should Know
Red cell distribution width (RDW) is a key component of a complete blood count (CBC) that measures the variation in size of your red blood cells. In healthy individuals, red blood cells are typically uniform in size, resulting in a normal RDW value. This consistency ensures efficient oxygen transport throughout the body.
Understanding Elevated RDW Levels
When RDW levels are significantly elevated, it indicates a condition known as anisocytosis, where red blood cells vary widely in size. This inconsistency can signal an underlying health issue affecting red blood cell production or survival. An increased RDW often serves as an early warning sign, prompting further diagnostic evaluation.
Common Causes of High RDW
One of the most frequent causes of high RDW is iron deficiency anemia. When the body lacks sufficient iron, it struggles to produce hemoglobin, leading to the creation of smaller-than-normal red blood cells (microcytic cells). However, the bone marrow may still release older or irregularly sized cells, increasing the overall size variability and raising the RDW.
In such cases, healthcare providers also examine the mean corpuscular volume (MCV)—a measure of average red blood cell size. A low MCV combined with high RDW strongly suggests iron deficiency or certain inherited disorders like thalassemia. While both conditions result in small red blood cells, iron deficiency tends to cause greater size variation, making RDW a useful differentiator.
Macrocytic Anemias and Nutritional Deficiencies
Conversely, when MCV is high—indicating larger-than-average red blood cells—and RDW is also elevated, the likely culprit is megaloblastic anemia. This type of anemia is commonly caused by deficiencies in essential nutrients such as folic acid (vitamin B9) and vitamin B12. These vitamins are critical for DNA synthesis and proper red blood cell maturation. Without them, red blood cells grow abnormally large and immature, contributing to increased size diversity.
Vitamin B12 deficiency can stem from poor dietary intake, malabsorption (such as in pernicious anemia), or gastrointestinal disorders. Folate deficiency may result from inadequate nutrition, alcoholism, or certain medications. Identifying and addressing these deficiencies through blood tests and targeted supplementation is crucial for restoring normal hematologic function.
Clinical Significance and Next Steps
An elevated RDW alone isn't diagnostic but acts as a valuable clue in the broader clinical picture. It helps doctors narrow down potential causes of anemia and guides further testing, including serum ferritin, vitamin B12, folate levels, and sometimes genetic testing for inherited blood disorders.
Moreover, recent studies suggest that a high RDW may be associated with chronic inflammation, cardiovascular diseases, and even adverse outcomes in hospitalized patients. Therefore, monitoring RDW over time can provide insights into overall health and disease progression beyond just anemia.
If your blood work shows an abnormal RDW, don't panic—but do follow up. Consult with a healthcare professional who can interpret your results in context, evaluate symptoms, and recommend appropriate interventions. Early detection and management of the root cause can lead to better long-term health outcomes.