Red Cell Distribution Width, or RDW, is a measurement included in routine blood tests. It assesses the variation in size among red blood cells within a blood sample, providing insight into their uniformity. The RDW test is a common component of a complete blood count.
What RDW Measures
Red blood cells are uniform in size, but various health conditions can cause these cells to differ significantly in volume and size. The medical term for this variation in red blood cell size is anisocytosis. Measuring this variation reflects potential issues in the bone marrow’s production of red blood cells or their lifespan. A higher RDW value indicates greater variability in red blood cell size. This measurement helps healthcare providers understand red blood cell production.
Understanding Your RDW Results
RDW results are reported as a percentage. For adults, a normal RDW range falls between 11.5% and 14.5%, though this can vary slightly depending on the laboratory. A normal RDW suggests that red blood cells are relatively uniform in size. Even with a normal RDW, other red blood cell parameters might be abnormal, indicating an underlying condition.
A high RDW means there is greater variation in the size of red blood cells. This increased variability can be an early indicator of certain conditions, sometimes even before other blood parameters change. This occurs because the body may be producing red blood cells of different sizes in response to a health issue. For instance, a high RDW can signal a nutrient deficiency or certain types of anemia.
A low RDW is less common and indicates that red blood cells are very uniform in size. This result is not associated with specific blood disorders. A low RDW implies stable red blood cell production, which is a sign of healthy bone marrow function. While a low RDW is not a concern, healthcare providers interpret all results in the context of a patient’s overall health and other blood tests.
RDW in Medical Diagnosis
Healthcare professionals interpret RDW results in conjunction with other parameters from a complete blood count (CBC), particularly the Mean Corpuscular Volume (MCV). MCV measures the average size of red blood cells, and combining this with RDW’s variation measurement provides a more comprehensive picture. This combined assessment is useful in differentiating between various types of anemia.
For example, a high RDW coupled with a low MCV can suggest iron deficiency anemia. In this scenario, the body produces both small and normally sized red blood cells. Conversely, a high RDW with a high MCV might indicate deficiencies in vitamin B12 or folate, or certain types of macrocytic anemia. Here, the red blood cells are, on average, larger.
RDW can also assist in diagnosing mixed anemias with multiple contributing factors. For instance, a normal MCV but a high RDW could occur in early stages of iron, vitamin B12, or folate deficiencies, or in dimorphic anemia. Beyond anemias, an elevated RDW has been linked to conditions such as chronic liver disease, kidney disease, heart disease, and certain cancers, reflecting broader systemic issues. RDW guides further investigation into a patient’s health.