When reviewing a Complete Blood Count (CBC) panel, two values relate directly to red blood cell health: Red Blood Cell count (RBC) and Red Cell Distribution Width (RDW). The CBC is a routine laboratory test that examines the cellular components circulating in the bloodstream. RBC and RDW offer distinct but related information about the quantity and physical characteristics of the cells responsible for oxygen transport. Understanding these metrics helps medical professionals interpret blood work and differentiate between various conditions affecting blood production.
Red Blood Cells and Their Function
Red Blood Cells (RBCs) are the most numerous cell type in the blood, primarily tasked with moving oxygen from the lungs to body tissues. The RBC count measures the total number of these cells present in a specific volume of blood. A low RBC count often suggests anemia, a condition where the body lacks enough healthy red cells to carry adequate oxygen. Conversely, a count that is too high, known as polycythemia, can cause blood flow issues due to increased blood thickness.
While the RBC count measures quantity, the Mean Corpuscular Volume (MCV) measures the average size of the red blood cells. MCV is an index used to classify anemias based on cell size: small-celled (microcytic), normal-sized (normocytic), or large-celled (macrocytic). A low MCV indicates the average red cell is smaller than the normal range. Because this average measurement can mask underlying issues, the RDW measurement is also included on the panel.
Red Cell Distribution Width (RDW): A Measure of Variation
Red Cell Distribution Width (RDW) measures the degree of variation in the size of circulating red blood cells. Unlike MCV, which reports the average size, RDW quantifies the uniformity of the cell population. This value is expressed as a percentage, indicating how much cell sizes deviate from the average. A high RDW value signifies significant variation in cell size, a condition known as anisocytosis.
A normal RDW suggests that most red blood cells are similar in size, typically falling within a standard range of 11% to 15%. An elevated RDW suggests the body is producing and maintaining red cells of vastly different sizes. This variation often occurs when the bone marrow compensates for a deficiency or disease by releasing new cells structurally different from older cells. The presence of both small and large cells increases the RDW, even if the overall MCV average remains normal.
RDW is a sensitive indicator because its value can increase early in the development of certain conditions, sometimes before the MCV or RBC count changes noticeably. For example, when a nutritional deficiency begins, the bone marrow starts producing new, misshapen cells that quickly raise the RDW. This early elevation signals a problem before the average cell size is significantly affected.
Clinical Interpretation and Diagnostic Patterns
The RDW is most valuable when interpreted alongside the MCV, allowing professionals to differentiate between various causes of anemia. This joint analysis creates specific diagnostic patterns that help narrow down possibilities. For instance, a low MCV combined with a normal RDW suggests a condition where all cells are uniformly small. This pattern is characteristic of genetic conditions, such as heterozygous thalassemia, where red cells are consistently smaller than normal, but size variation is low.
A different pattern emerges when the MCV is low and the RDW is high. This combination strongly suggests Iron Deficiency Anemia, especially in early stages. The deficiency leads to the production of small red cells (low MCV). However, because the deficiency develops over time, the blood contains a mix of older, normal-sized cells and newer, smaller cells, resulting in high size variation (high RDW).
When the MCV is high, indicating large red blood cells, the RDW helps refine the diagnosis. A high MCV paired with a high RDW suggests a nutritional deficiency involving Folate or Vitamin B12, leading to megaloblastic anemia. These deficiencies cause the bone marrow to produce abnormally large and variably sized cells. Conversely, a high MCV with a normal RDW may point toward other causes of macrocytosis, such as some forms of liver disease.
The RDW is useful for identifying two distinct populations of red blood cells, a situation the average MCV alone would fail to detect. This occurs in “dimorphic” anemia, such as when a patient with a pre-existing macrocytic anemia develops a new microcytic one. Interpreting RDW and RBC count alongside MCV guides the need for specific follow-up tests, allowing for a more targeted determination of the underlying health issue.