what is rdw in blood count

3 min read 13-03-2025

Red blood cell distribution width (RDW) is a valuable measurement included in a complete blood count (CBC). It reflects the variation in the size of your red blood cells (RBCs). Understanding your RDW can provide crucial insights into your overall health and help diagnose various medical conditions. This article will delve into what RDW is, what it measures, what normal and abnormal ranges are, and what conditions it can help identify.

What Does RDW Measure?

RDW quantifies the variation in the size of your red blood cells. Red blood cells are typically uniform in size, but various health conditions can cause them to become abnormally large or small. RDW expresses this variation as a percentage coefficient of variation. A higher RDW indicates greater variation in RBC size, while a lower RDW suggests more uniform RBC sizes.

Anisocytosis and RDW

The term anisocytosis describes the presence of red blood cells of unequal size. RDW is the quantitative measurement of this anisocytosis. It helps doctors assess the degree of size variation, giving them a more precise understanding of the underlying cause.

Normal RDW Range

The normal range for RDW can vary slightly depending on the laboratory performing the test and the method used. However, a generally accepted normal range is between 11.5% and 14.5%. Always refer to the specific range provided by your doctor or the laboratory that conducted the test.

High RDW: What it Could Mean

An elevated RDW (above the normal range) often indicates that your body is producing red blood cells of varying sizes. This can be a sign of several conditions, including:

Iron deficiency anemia: This is a common cause of high RDW. Iron is crucial for red blood cell production, and a deficiency leads to smaller, paler RBCs (microcytic anemia) and a wide variation in size.
Vitamin B12 or folate deficiency anemia: Similar to iron deficiency, these deficiencies also impair red blood cell production, resulting in abnormally large RBCs (macrocytic anemia) and a high RDW.
Thalassemia: This inherited blood disorder affects hemoglobin production, leading to smaller than normal red blood cells and a high RDW.
Myelodysplastic syndromes (MDS): These are a group of cancers affecting the bone marrow's ability to produce healthy blood cells, often resulting in abnormal RBC size and shape.
Sickle cell anemia: In this inherited disorder, abnormal hemoglobin causes red blood cells to become rigid and sickle-shaped, affecting size and increasing RDW.
Chronic diseases: Conditions like kidney disease, liver disease, and hypothyroidism can also contribute to high RDW.

Low RDW: What it Could Mean

A low RDW (below the normal range) is less common than a high RDW. It often suggests a more uniform red blood cell size, but can sometimes be associated with:

Iron deficiency anemia (in later stages): Paradoxically, in the later stages of iron deficiency, the RDW may decrease as the bone marrow struggles to produce even fewer red blood cells, resulting in more uniform, smaller cells.
Certain types of thalassemia: Depending on the specific type, thalassemia can sometimes show a low or normal RDW.

It's crucial to understand that a low RDW alone is rarely diagnostic. It often requires further investigation in conjunction with other blood test results.

How is RDW Measured?

RDW is measured as part of a complete blood count (CBC) using an automated hematology analyzer. This machine counts and measures the size of red blood cells in a blood sample. The result is a percentage that reflects the variation in size.

When to Consider RDW Testing

Your doctor may order an RDW test if you are experiencing symptoms suggestive of anemia or other blood disorders, such as:

Fatigue
Weakness
Shortness of breath
Pale skin
Dizziness

RDW, in conjunction with other blood tests like hemoglobin and hematocrit levels, helps your doctor determine the type and severity of anemia or other blood disorders.

Conclusion

RDW is a valuable tool in diagnosing various hematological conditions. While a high RDW often points towards significant variations in red blood cell size, indicating potential underlying diseases, a low RDW is less indicative and usually requires further testing. Always consult your doctor to interpret your RDW results and understand their implications for your health. Remember, this information is for educational purposes only and does not constitute medical advice. Always seek professional medical guidance for diagnosis and treatment.