A complete blood count (CBC) is a common blood test that provides information about a person’s health. This test analyzes blood components, including red blood cells, white blood cells, and platelets. It is used as a screening tool to detect a wide range of conditions, from infections to anemia. Two measurements related to red blood cells are the red blood cell (RBC) count and the Red Cell Distribution Width (RDW).
The Role of Red Blood Cells
Red blood cells, or erythrocytes, are responsible for transporting oxygen from the lungs to all other tissues and organs. This is accomplished by a protein within the cells called hemoglobin, which binds to oxygen and gives blood its red color. Healthy red blood cells are uniform in size and shape, resembling a biconcave disc. This shape maximizes their surface area for efficient oxygen exchange. The RBC count measures the number of these cells to determine the body’s oxygen-carrying capacity.
Defining Red Cell Distribution Width
Red Cell Distribution Width (RDW) measures the variation in the size of your red blood cells. While the RBC count tells you the number of red blood cells, the RDW provides information about their uniformity. A high degree of variation in red blood cell size is a condition called anisocytosis. In a blood test, this value is expressed as a percentage, with a normal range between 12% and 15%, though this can vary by laboratory.
What High RDW Indicates
An RDW value above the normal range signifies that there is a considerable variation in the size of red blood cells circulating in the bloodstream. This finding is not a diagnosis in itself but can point towards a number of potential underlying medical conditions. It suggests that the body may be having difficulty producing a consistent supply of healthy red blood cells.
One of the most common causes of a high RDW is iron deficiency anemia. In this condition, the body lacks the iron needed to produce hemoglobin, leading to the creation of smaller-than-normal red blood cells. These new, smaller cells mix with older, normal-sized cells, resulting in a high degree of size variation and, consequently, a high RDW.
Other conditions can also lead to an elevated RDW. Anemias caused by a deficiency in vitamin B12 or folate can result in the production of unusually large red blood cells, which, when mixed with normal cells, also increases the RDW. Certain types of liver disease can also impact the production and health of red blood cells, leading to increased size variation.
The Significance of Low and Normal RDW
A low RDW, which would indicate that the red blood cells are extremely uniform in size, is uncommon and generally not considered a sign of a health problem. A normal RDW, however, requires more careful interpretation, as it does not automatically indicate the absence of a health issue. The RDW is most informative when considered alongside another value from the CBC called the Mean Corpuscular Volume (MCV), which measures the average size of the red blood cells.
The interplay between RDW and MCV can provide a more nuanced picture of blood health. For instance, a person might have a normal RDW but a low MCV. This combination, where the cells are uniformly small, can be suggestive of a condition like thalassemia, a genetic disorder that affects hemoglobin production.
Conversely, a normal RDW paired with a high MCV, meaning the cells are consistently larger than normal, could point towards aplastic anemia. In this condition, the bone marrow does not produce enough new blood cells. By analyzing RDW and MCV together, healthcare providers can gain more specific insights into the potential causes of any abnormalities found in a CBC.