The Red Cell Distribution Width (RDW) is one of the measurements reported as part of a standard Complete Blood Count (CBC) test. The RDW specifically measures the variation in the size of these oxygen-carrying cells circulating throughout the body. A result outside the typical range, often referred to as an alarming count, indicates that the cells are not uniform in size, which can be an early sign of an underlying health issue. This measurement offers a valuable preliminary clue that guides further medical investigation into the cause of the size disparity.
Decoding the RDW Measurement
The RDW is expressed as a percentage that quantifies anisocytosis, the presence of red blood cells of unequal size. A normal RDW percentage suggests that the majority of red blood cells are similar in volume, typically falling within a narrow range. While laboratory reference ranges can vary slightly, a typical normal range for RDW is generally between 11.5% and 14.5%. An alarming RDW count is a result significantly higher than this threshold, signifying a greater disparity in cell size. This means the blood sample contains a mix of smaller and larger red blood cells. The RDW value is more informative when elevated, reflecting a disruption in the body’s red cell production and maturation processes.
Mechanisms Behind an Elevated RDW
An elevated RDW arises from physiological processes that lead to the simultaneous presence of new, differently sized cells alongside older, existing cells in the bloodstream. One primary mechanism involves nutritional deficiencies, particularly a lack of the necessary building blocks like iron, Vitamin B12, or folate. When these nutrients are scarce, the bone marrow cannot properly construct new red cells, leading to the production of cells that are abnormally large or small, which increases the size variation.
Another common cause is the rapid production of new red blood cells following an acute event, such as recent blood loss from hemorrhage. The body quickly releases immature red cells, called reticulocytes, which are often larger than mature cells, creating a temporary mixture of cell sizes. Conditions causing ineffective red blood cell production, where cells are prematurely released or destroyed, also elevate the RDW due to the resulting heterogeneity of cell volumes. Chronic inflammation or high levels of oxidative stress can also contribute by impairing iron metabolism and reducing the lifespan of red blood cells.
RDW as a Key Diagnostic Clue
The RDW is interpreted in conjunction with other red cell indices from the CBC, most notably the Mean Corpuscular Volume (MCV). The MCV measures the average size of all red blood cells, whereas the RDW measures the variation in that size. Analyzing the pattern of these two values helps medical professionals narrow down the potential causes.
For instance, an elevated RDW combined with a low MCV often points toward iron deficiency anemia. The body struggles to make hemoglobin due to low iron, resulting in many small cells (low MCV). The RDW is high because the population is a mix of small, new cells and older, normal-sized cells. Conversely, a high RDW paired with a high MCV suggests a deficiency in Vitamin B12 or folate. These deficiencies impair DNA synthesis, causing the bone marrow to produce abnormally large red blood cells, known as megaloblasts, which increases both the average size and the variation.
When the RDW is high, but the MCV remains normal, it can indicate an early stage of a nutritional deficiency, before the average cell size changes significantly. This pattern can also suggest a mixed anemia, such as concurrent iron and Vitamin B12 deficiencies, where the effects on cell size cancel each other out in the average MCV measurement. The RDW is often a more sensitive and earlier indicator of conditions like nutritional anemia than the MCV alone.
Next Steps After an Alarming Result
An alarming RDW result is a non-specific finding, meaning it suggests a problem exists but does not identify the specific cause. The most important next step is consulting a healthcare provider for a thorough review of the entire clinical picture, including the full CBC panel, medical history, and physical examination. Self-diagnosis based solely on the RDW number is strongly discouraged due to its lack of specificity.
To pinpoint the source of the elevated RDW, a provider typically orders additional, more specific laboratory tests. These follow-up tests often include:
- A comprehensive iron panel, which measures levels of serum ferritin and transferrin saturation to assess iron stores.
- Testing for Vitamin B12 and folate levels is also common to rule out these specific nutritional deficiencies.
- A reticulocyte count, which measures the number of immature red cells to determine if the bone marrow is responding appropriately.
- A peripheral blood smear, which allows for a visual assessment of red blood cell shape and size variation.