The Red Cell Distribution Width (RDW) is a measurement included in a standard complete blood count (CBC) panel. This test provides information about red blood cells, which are responsible for carrying oxygen from the lungs to all tissues of the body. A CBC measures the different components of blood, and the RDW value helps create a broader picture of a person’s health by focusing on the physical characteristics of red blood cells.
What the RDW Blood Test Measures
The RDW test measures the degree of variation, or anisocytosis, in the size of your red blood cells. Red blood cells, produced in the bone marrow, normally maintain a uniform size and shape, appearing as discs with a diameter of about 6 to 8 micrometers. The RDW value quantifies how much your red blood cells deviate from this standard.
Imagine red blood cells as a collection of marbles. A low RDW would be like a bag where all the marbles are nearly identical in size. Conversely, a high RDW is like a bag containing a mix of small, medium, and large marbles. The test does not measure the average size of the cells; it only assesses the range of different sizes present in a blood sample.
This measurement is displayed on a graph called a histogram. When red blood cells are similar in size, they group together, resulting in a low RDW percentage. If there is a wide variety of cell sizes, the data points on the histogram are more spread out, leading to a higher RDW value. This size variability can be an early indicator of certain health conditions.
Understanding RDW Levels
Interpreting RDW levels begins with comparing the result to a reference range, which is around 11% to 14.5% for adults. A result within this range is considered normal and indicates the red blood cells are mostly uniform in size. However, a normal RDW does not rule out a health issue, as the cells could still be consistently smaller or larger than ideal.
A high RDW result signifies considerable variation in the size of red blood cells. This suggests that the body may be having trouble producing red blood cells consistently, leading to a mix of differently sized cells circulating in the bloodstream. Health providers see this as a signal to investigate potential underlying causes.
A low RDW result is uncommon and not a cause for medical concern. It means the red blood cells are very uniform in size, which is the expected state for healthy cells. A low RDW value does not indicate a specific disease or abnormality and is not monitored.
Conditions Associated with a High RDW
An elevated RDW is frequently associated with various types of anemia, a condition where the body lacks enough healthy red blood cells to carry adequate oxygen. A common cause is iron deficiency anemia. When the body is low on iron, it struggles to produce hemoglobin, the protein in red blood cells that binds to oxygen. This can lead to the production of smaller red blood cells, and as the deficiency progresses, the variation in size increases, raising the RDW.
Deficiencies in other nutrients, such as vitamin B12 or folate, can also lead to a high RDW. These deficiencies result in megaloblastic anemia, where the bone marrow produces unusually large, immature red blood cells. The presence of these large cells alongside any remaining normal-sized cells creates significant size variation, increasing the RDW. The RDW can become elevated in these nutritional deficiencies before other blood cell parameters change.
Beyond nutritional anemias, other conditions can contribute to a high RDW. Thalassemia, an inherited blood disorder that affects hemoglobin production, can cause severe anemia. The body’s response to significant blood loss, such as after surgery or trauma, can also temporarily increase RDW as the bone marrow rapidly produces new red blood cells of varying sizes. Conditions like chronic liver disease or certain autoimmune disorders have also been linked to higher RDW values.
How RDW and MCV are Used Together
Medical professionals rarely interpret the RDW value in isolation. Its diagnostic utility increases when viewed with the Mean Corpuscular Volume (MCV), another standard component of a CBC. The MCV measures the average size of the red blood cells, complementing the RDW’s measurement of size variation.
The combination of these two values helps narrow the potential causes of anemia. For example, a high RDW paired with a low MCV (smaller than average red blood cells) often points toward iron deficiency anemia. This pattern reflects a state where the body is producing progressively smaller cells due to a lack of iron, increasing the overall size variation.
Conversely, a high RDW combined with a high MCV (larger than average red blood cells) is a classic indicator of megaloblastic anemia. This combination suggests the presence of large, abnormal cells alongside other cells, creating high variation. In contrast, a normal RDW with a low MCV might suggest thalassemia or anemia related to a chronic disease, where the cells are consistently small but uniform.