What is the RDW (Red Cell Distribution Width) Test?
An RDW (Red Cell Distribution Width) Test measures the variation in the size and volume of your red blood cells (RBCs). Healthy red blood cells are generally similar in size, while a high RDW indicates greater variation in red blood cell size, a condition known as anisocytosis. Measuring RDW helps healthcare providers evaluate how evenly red blood cells are being produced and can provide important clues about certain blood disorders.
The RDW (Red Cell Distribution Width) Test is usually performed as part of a Complete Blood Count (CBC). It is interpreted together with other red blood cell measurements, such as Mean Corpuscular Volume (MCV), hemoglobin, and hematocrit. It is commonly used to help diagnose and differentiate different types of anaemia, including iron deficiency anaemia, vitamin B12 deficiency anaemia, and folate deficiency anaemia. The test may also assist in evaluating chronic diseases, liver disease, and certain bone marrow disorders. Your healthcare provider will interpret the results in the context of your symptoms, medical history, and other laboratory findings.
How is the Test Performed?
Preparation
- Preparing for an RDW (Red Cell Distribution Width) Test is generally simple.
- Fasting is not usually required unless the test is performed with other blood tests that require fasting.
- Continue taking your medications unless instructed otherwise by your healthcare provider.
- Inform your healthcare provider about any prescription medications, supplements, or existing blood disorders.
- Drink water before the test unless advised otherwise.
Collection
- The test requires a blood sample, which is collected by a trained healthcare professional.
- The skin over a vein, usually inside the elbow, is cleaned with an antiseptic.
- A sterile needle is inserted into the vein to collect a blood sample into an EDTA (lavender top) tube.
- Once enough blood has been collected, the needle is removed, and gentle pressure is applied to the puncture site to minimise bleeding.
- The procedure usually takes only a few minutes.
Lab Analysis
The blood sample is analysed using an automated hematology analyser, which calculates the Red Cell Distribution Width (RDW) by measuring the variation in red blood cell size. Because RDW alone cannot diagnose a specific condition, it is interpreted together with other Complete Blood Count (CBC) parameters, particularly MCV, haemoglobin, haematocrit, and the red blood cell count. This combined information helps healthcare providers identify different types of anaemia and evaluate other blood-related conditions.
What are the Symptoms that Prompt this Test?
An RDW (Red Cell Distribution Width) Test may be recommended if you have symptoms suggesting anemia or another condition affecting red blood cell production.
Symptoms of Anemia
Your healthcare provider may recommend an RDW (Red Cell Distribution Width) Test if you experience:
- Persistent fatigue
- Weakness
- Shortness of breath
- Dizziness or light-headedness
- Pale skin
- Rapid or irregular heartbeat
- Cold hands and feet
- Headaches
Conditions That May Require This Test
An RDW (Red Cell Distribution Width) Test may also be performed if your healthcare provider suspects or is monitoring:
- Iron deficiency anemia
- Vitamin B12 deficiency
- Folate deficiency
- Mixed nutritional deficiencies
- Chronic liver disease
- Bone marrow disorders
- Certain inherited blood disorders
- Monitoring response to treatment for anemia
What are Normal Ranges for This Test?
The RDW (Red Cell Distribution Width) normal range is usually reported as a percentage (%), representing the variation in the size of red blood cells.
The commonly accepted reference range is:
Normal RDW: 12%- 15%
Low RDW: Below 12%
High RDW: 15% or more
“The reference ranges vary depending on the laboratory and the methodology used. Individuals must consult their healthcare provider for correct interpretation of the results.”
What Happens if Test Results Show Abnormal Levels?
An abnormal RDW (Red Cell Distribution Width) Test result does not by itself diagnose a medical condition.
High RDW
A high RDW means there is a greater variation in the size of your red blood cells. This may be associated with:
- Iron deficiency anaemia
- Vitamin B12 deficiency
- Folate deficiency
- Mixed nutritional deficiencies
- Recent blood loss
- Hemolytic anemia
- Chronic liver disease
- Certain bone marrow disorders
Low RDW
A low RDW means there is very little variation in the size of your red blood cells. In most cases, a low RDW is not considered clinically significant and, by itself, does not usually indicate a medical condition. It is generally interpreted together with other Complete Blood Count (CBC) parameters, particularly Mean Corpuscular Volume (MCV), haemoglobin, and haematocrit.
Depending on your overall blood test results and symptoms, your healthcare provider may recommend additional investigations such as a Complete Blood Count (CBC) review, iron studies, vitamin B12 and folate tests, reticulocyte count, or bone marrow tests to determine whether any underlying blood disorder is present. Treatment, if needed, depends on the underlying condition rather than the RDW value alone.
Need an RDW (Red Cell Distribution Width) Test? Book your appointment today for accurate blood analysis and expert evaluation of your red blood cell health.
