The Red Cell Distribution Width (RDW) is a measurement included in a routine blood test called a Complete Blood Count (CBC). This measurement helps assess the variation in the size and volume of your red blood cells. Understanding your RDW ratio can offer insights into your overall health, particularly concerning conditions that affect red blood cell production.
The RDW Measurement and Normal Ranges
The RDW test specifically quantifies the degree of variation in the size of red blood cells, a condition referred to as anisocytosis. Red blood cells typically maintain a relatively uniform size to efficiently transport oxygen throughout the body. When there is a wider range of sizes, it can indicate issues with red blood cell formation or destruction.
There are two primary ways RDW is reported: RDW-CV (Coefficient of Variation) and RDW-SD (Standard Deviation). RDW-CV is expressed as a percentage and is influenced by the average size of red blood cells, known as Mean Corpuscular Volume (MCV). RDW-SD, measured in femtoliters (fL), directly reflects the width of the red blood cell size distribution, independent of MCV.
Typical normal ranges for RDW-CV are between 11.5% and 15.4%. However, these reference ranges can vary slightly depending on the laboratory that performs the test. It is always advisable to refer to the specific normal range provided on your individual lab report for the most accurate interpretation of your results.
Causes and Implications of a High RDW
A high RDW indicates that your red blood cells vary more in size than what is considered typical. This increased variation often signals an underlying medical condition affecting red blood cell production or survival. A common cause for an elevated RDW is anemia, a condition where the blood lacks healthy red blood cells.
When a high RDW is observed alongside a low Mean Corpuscular Volume (MCV), it suggests iron deficiency anemia. In this scenario, the body struggles to produce adequate hemoglobin due to insufficient iron, leading to smaller red blood cells that also vary in size. This combination is a frequent diagnostic pattern for this specific type of anemia.
Alternatively, a high RDW combined with a high MCV indicates anemias caused by deficiencies in vitamin B12 or folate. These vitamins are necessary for proper red blood cell maturation, and their absence can result in larger, abnormally shaped red blood cells with significant size variation.
Other conditions can elevate RDW, including chronic liver disease, heart disease, diabetes, and kidney disease. Certain cancers, particularly colorectal cancer and blood cancers, may also be associated with a high RDW. Recent episodes of significant blood loss, such as hemorrhages, can also result in a temporary increase in RDW as the body produces new, often varied-sized, red blood cells.
Significance of a Low or Normal RDW
A low RDW is an uncommon finding and does not indicate a health concern. It simply means that your red blood cells are very uniform in size, showing little variation. This uniformity suggests a consistent red blood cell production process.
A normal RDW, however, does not automatically rule out the presence of a health issue. Other blood markers must be considered in conjunction with a normal RDW to gain a comprehensive understanding of your health status.
For instance, a normal RDW combined with a low MCV might suggest conditions like anemia of chronic disease or certain hereditary disorders such as thalassemia. In these cases, the red blood cells are consistently smaller than average, but they maintain a similar size among themselves, leading to a normal RDW.
Similarly, a normal RDW accompanied by a high MCV could indicate conditions such as aplastic anemia. Here, the bone marrow produces fewer red blood cells, which may be larger than typical, yet they remain relatively uniform in size, resulting in a normal RDW.
When to See a Doctor
The RDW ratio is not a standalone diagnostic tool but rather a component of a comprehensive blood analysis. An abnormal RDW result, whether high or even a normal RDW in the context of other abnormal findings, should always be discussed with a healthcare provider. Self-diagnosing based solely on RDW levels is not recommended.
Your doctor will interpret your RDW result alongside all other values from your Complete Blood Count, such as hemoglobin, hematocrit, and particularly MCV, to provide an accurate assessment. They can offer context for your specific results, determine if further investigation is necessary, and advise on any appropriate next steps or treatments.