The Red Cell Distribution Width (RDW) test is a standard component of a Complete Blood Count (CBC), which is a common blood panel ordered by healthcare providers. A high RDW result often causes concern because it measures the variation in size of your red blood cells, which can signal an underlying health issue. While many people search for a connection between a high RDW and serious illnesses like cancer, this measurement provides a quick snapshot of the uniformity of red blood cells, offering valuable data for diagnosis.
Understanding the RDW Measurement
The RDW test quantifies the variation in volume and size among a person’s red blood cells, a phenomenon technically known as anisocytosis. Healthy red blood cells are relatively uniform in size, and a normal RDW typically falls within a range of about 12% to 15%. A high result simply indicates that the red blood cells in the sample are not all the same size, with a mixture of both smaller and larger cells present.
The calculation is generally performed in two ways: RDW-CV (Coefficient of Variation) and RDW-SD (Standard Deviation). RDW-CV, expressed as a percentage, is a ratio that incorporates the average size of the red blood cells, known as the Mean Corpuscular Volume (MCV). Conversely, RDW-SD is an absolute measurement, in femtoliters (fL), that represents the actual width of the red cell volume distribution curve. An elevated RDW value means there is a greater spread on this distribution curve, reflecting a problem in the production or survival of red blood cells within the bone marrow.
Common Reasons for Elevated RDW
An elevated RDW is most often a sign of a nutritional deficiency or a common, non-cancer related condition. The body requires specific resources to produce new, healthy red blood cells, and a shortage of these can immediately lead to size variations. Iron deficiency anemia is a frequent cause, resulting in the production of smaller-than-normal red blood cells (microcytes) alongside existing, normal-sized cells.
Similarly, a deficiency in Vitamin B12 or folate can also cause a high RDW, but for a different reason. These deficiencies lead to the production of abnormally large red blood cells (macrocytes). In the case of an early or mixed deficiency, the simultaneous presence of both small and large cells can cause the RDW to rise, sometimes before other blood parameters drop below the normal range.
Beyond nutritional causes, various chronic diseases can disrupt the bone marrow’s ability to maintain a uniform red blood cell population. For instance, chronic liver disease and kidney disease are both associated with an increased RDW. These conditions interfere with the regulation of red blood cell production or their lifespan, leading to a heterogeneous mix of cells in circulation.
Other common causes include hemolytic anemias, where red blood cells are destroyed prematurely, and recent blood loss, where the bone marrow rapidly releases immature, slightly larger cells (reticulocytes) to compensate. Certain inherited blood disorders, such as thalassemia, may also present with an elevated RDW. Therefore, a high RDW is a general signal of hematological stress rather than a specific disease.
High RDW and the Cancer Connection
While an elevated RDW is not a diagnostic test for cancer, research has established a correlation between a high RDW and certain malignancies. This connection is largely understood through the lens of chronic systemic inflammation, which often accompanies the development and progression of cancer. The inflammatory state can interfere with the bone marrow’s function and the process of red blood cell maturation, leading to the observed variation in cell size.
Scientists have investigated RDW primarily for its prognostic value, which means its ability to predict the likely outcome or course of a disease. Studies have consistently shown that an elevated RDW before treatment is associated with poorer overall survival and disease-free survival in patients already diagnosed with various types of cancer. This association has been observed across a range of cancers, including colorectal, lung, hepatic, and breast cancer.
The higher RDW may reflect the body’s overall physiological stress response, which includes ongoing inflammation and the presence of nutritional deficiencies that are often secondary to the cancer itself. An elevated RDW is considered an inexpensive and widely available biomarker that indicates a more aggressive disease or a patient who is less able to cope with the physical demands of the illness. It is a marker of poor general health status, which overlaps with the effects of a developing or existing malignancy.
Clinical Interpretation and Next Steps
A high RDW result is rarely interpreted in isolation and is instead used as a piece of a larger diagnostic puzzle. Healthcare providers always examine the RDW alongside the MCV, as the relationship between these two values helps narrow down the potential cause of the cell size variation. For example, a high RDW combined with a low MCV strongly suggests iron deficiency anemia, while a high RDW with a high MCV points toward a Vitamin B12 or folate deficiency.
If a high RDW is detected, the next steps involve a targeted series of follow-up tests to determine the precise cause. These may include specific serum tests for ferritin (iron storage), Vitamin B12, and folate levels to confirm or rule out nutritional deficits. If the initial bloodwork is inconclusive, or if the patient presents with symptoms suggestive of a chronic condition, further investigation into potential inflammatory, liver, or kidney disorders will be initiated.
The RDW provides a general signal, not a specific diagnosis. A physician will integrate the RDW result with a complete patient history, physical examination findings, and all other laboratory values before determining the appropriate course of action. Self-diagnosis based solely on an RDW number should be avoided, and all concerns should be discussed with a qualified healthcare professional.