Red blood cell distribution width

Complete blood count and differential

Red Blood Cell Distribution Width (RDW) is a laboratory value that represents the variation in size and volume of red blood cells (RBCs) within a blood sample. It is reported as part of a standard complete blood count (CBC), a common blood test performed for a variety of diagnostic purposes. There are two types of RDW: RDW-SD and RDW-CV. RDW-CV (coefficient of variation) measures the width of red cell size distribution, while RDW-SD (standard deviation) provides the actual size of red cells.

Overview

In healthy individuals, red blood cells have an average volume of 80-100 femtoliters, but individual cell volumes can vary. Some health conditions can cause a significantly increased variation in cell size, leading to a higher RDW value. The normal reference range of RDW-CV in human red blood cells is 11.5–15.4%.

Clinical Significance

RDW values are often used in conjunction with mean corpuscular volume (MCV) results to determine the potential causes of anemia. For example, if a patient's MCV is low (indicating smaller than average red blood cells, or microcytosis) and the RDW is high, this could suggest iron deficiency anemia or thalassemia, both conditions which can cause significant variation in red blood cell size.

RDW is primarily used to differentiate between different types of anemia, but elevated RDW values can also be associated with conditions such as liver disease and heart failure.

Interpretation

Higher RDW values indicate greater variation in red blood cell size, and thus more potential for underlying pathology. A low RDW is not typically clinically significant, as it simply indicates a uniform size of red blood cells. If anemia is observed, the specific RDW result can help clinicians distinguish between different types of the condition. It is especially useful in differentiating between anemias of mixed causes and those of a singular cause.