The size of a red blood cell (RBC), also called an erythrocyte, is fundamentally important for transporting oxygen throughout the body. These cellular components bind oxygen in the lungs and release it into the tissues to fuel metabolism. Any change in the normal dimensions of these cells can compromise their ability to navigate the circulatory system and efficiently carry their oxygen payload. Therefore, measuring and monitoring the size of red blood cells is a standard tool used in medical diagnostics.
The Structure and Normal Dimensions of Red Blood Cells
A healthy, mature red blood cell is characterized by its unique biconcave disc shape, resembling a flattened sphere indented in the center. This specific morphology maximizes the cell’s surface area relative to its volume, enhancing the efficiency of gas exchange. This distinctive shape also provides the cell with flexibility, allowing it to fold and squeeze through the narrowest capillaries. The normal diameter of a human red blood cell typically falls within a narrow range of approximately 6.2 to 8.2 micrometers (µm). At its thickest point along the edge, the cell measures about 2 to 2.5 µm, thinning down to less than 1 µm in the concave center.
Clinical Indices for Measuring Cell Diameter
In routine blood work, specifically the Complete Blood Count (CBC), the size of red blood cells is quantified using several indices that provide a snapshot of their average volume and variability. The Mean Corpuscular Volume (MCV) is the most direct measure of average red blood cell size, reporting the volume of a single cell in femtoliters (fL). A normal MCV value generally ranges between 80 and 96 fL, and it serves as the initial metric for classifying size-related red blood cell disorders.
A second index is the Red Cell Distribution Width (RDW), which quantifies the variation in red blood cell size within a sample, a phenomenon known as anisocytosis. The RDW expresses the uniformity of the cell population as a percentage, unlike the MCV which is an average. A normal RDW typically falls between 11.5% and 14.5%, indicating that most cells are close to the average size.
The RDW is calculated as the coefficient of variation of the red blood cell volume distribution, revealing how spread out the sizes are. When the RDW is elevated, it suggests a mixed population of cells, where some are significantly larger or smaller than the average. Interpreting the MCV and RDW together provides a powerful tool for distinguishing between different underlying causes of conditions affecting red blood cell production.
Interpreting Abnormal Red Blood Cell Sizes
Deviations from the normal size range indicate a problem with the bone marrow’s red blood cell production process, leading to specific diagnostic categories. Microcytosis describes red blood cells that are abnormally small and is indicated by a low MCV value. These smaller cells often result from conditions where there is a defect in the synthesis of hemoglobin, the oxygen-carrying protein. The most common cause of microcytosis is Iron Deficiency Anemia, where inadequate iron prevents the formation of sufficient hemoglobin, causing the cell to finish dividing at a smaller size.
Conversely, Macrocytosis refers to red blood cells that are larger than the normal range, reflected by an elevated MCV. This increase in size is often a sign of ineffective DNA synthesis during red blood cell development in the bone marrow, causing the cells to mature without fully dividing. Macrocytosis is frequently associated with deficiencies in Vitamin B12 or folate, both of which are necessary for DNA synthesis, leading to a condition called megaloblastic anemia.
An elevated RDW, signaling anisocytosis, indicates a greater than normal degree of size variation among red blood cells. This finding is informative because it often appears early in nutritional anemias, sometimes before the average size (MCV) changes significantly. For example, in the early stages of iron deficiency, a mix of newly produced small cells and older, normal-sized cells causes the RDW to rise. This suggests an issue with the consistency of cell production in the bone marrow, prompting further investigation.