A complete blood count (CBC) is a common blood test that measures the quantity and characteristics of red blood cells, white blood cells, and platelets circulating in the bloodstream. The Red Cell Distribution Width (RDW) is a specific measurement within the red blood cell panel.
The RDW is a laboratory value that quantifies the variation in size and volume among a person’s red blood cells. Red blood cells (erythrocytes) transport oxygen from the lungs to the body’s tissues. The RDW value reports the degree of uniformity in the size of these cells.
Understanding RDW and the 15.7 Result
The RDW is typically reported as a percentage, and a result of 15.7% is considered elevated based on standard reference ranges. The typical range for RDW in healthy adults often falls between 11.5% and 14.5%, though this can vary by laboratory. A value of 15.7% suggests a degree of cell size variation greater than what is observed in the average population.
This increased variation in red blood cell size is referred to as anisocytosis. An elevated RDW means the red cell sample contains a wider mix of cell sizes, including cells that are smaller and/or larger than average. This is a non-specific finding, indicating a disruption in the normal process of red blood cell production or survival rather than a single disease.
The elevated RDW suggests that newly created red blood cells are maturing differently than older, existing cells. For instance, the bone marrow might be releasing a mix of cells that are prematurely large or abnormally small due to a physiological stressor. This variance in size can be an early indicator of a problem, sometimes appearing before other red blood cell measurements change significantly.
Common Causes of Elevated RDW
An elevated RDW, such as 15.7%, reflects underlying issues that interfere with the body’s ability to produce uniformly sized red blood cells. One frequent category of causes relates to nutritional deficiencies, which are necessary building blocks for healthy red cell formation. For example, iron deficiency anemia causes the bone marrow to produce smaller-than-normal red blood cells, leading to a mixture of small and normal-sized cells in circulation and raising the RDW.
Deficiencies in Vitamin B12 or folate also disrupt red blood cell production, but they typically result in the creation of abnormally large cells. When the body lacks these B vitamins, it can lead to a condition known as macrocytic anemia, where the presence of these large cells mixed with normal cells increases the overall RDW value. An elevated RDW is often one of the first markers to become abnormal in the early stages of these nutritional deficits.
Beyond nutritional causes, chronic inflammation and disease can also lead to an increased RDW. Conditions such as chronic liver disease, kidney disease, diabetes, and certain cardiovascular diseases are associated with higher RDW values. The chronic inflammatory state can interfere with bone marrow function, inhibit the production of red blood cells, or affect their lifespan, resulting in a less uniform cell population.
Certain genetic conditions, or hemoglobinopathies, may also present with an elevated RDW. Conditions like Thalassemia or Sickle Cell Disease can cause red blood cell fragmentation or abnormal cell shapes, contributing to increased size variability. RDW elevation is a single piece of evidence suggesting a need for further investigation, not a definitive diagnosis of any specific condition.
Interpreting RDW Alongside Other Blood Markers
The RDW is rarely evaluated in isolation and gains its greatest clinical utility when considered alongside other red blood cell indices from the CBC. The Mean Corpuscular Volume (MCV) is the measurement most frequently paired with RDW, reporting the average size of the red blood cells. Combining the RDW (cell size variation) with the MCV (average cell size) helps a healthcare provider narrow down the potential cause of the cell size abnormality.
A high RDW (15.7%) combined with a low MCV suggests the presence of many small cells and is the pattern most commonly seen in iron deficiency anemia. In this scenario, the body is struggling to incorporate enough iron into the cells, resulting in smaller cells being produced.
Conversely, a high RDW paired with a high MCV typically points toward deficiencies in Vitamin B12 or folate. The lack of these nutrients leads to the production of fewer, larger red blood cells.
A third pattern involves a high RDW with a normal MCV, which can indicate an early-stage nutritional deficiency, a mixed deficiency, or the presence of two distinct populations of cells. This scenario shows that while the average cell size is within the normal range, the amount of size variation is still elevated. Given the complexity of these interpretations, the 15.7% RDW result should prompt a consultation with a healthcare professional for further testing to determine the exact underlying cause.