How the RDW Blood Test
Works
The RDW (Red Cell Distribution Width) blood test is one of the most informative markers within the Complete Blood Count (CBC) panel. At RDW Blood Test, we believe every patient deserves to fully understand what happens at each stage of the process, from how the test is ordered to how results are calculated and what they mean for your health. This page walks you through the complete journey in clear, clinically accurate detail.
Your Doctor Orders the Test
The RDW blood test is almost always ordered by a GP, hospital doctor, or specialist as part of a Complete Blood Count (CBC) or Full Blood Count (FBC). It is not a standalone test. Your doctor may order it if you present with symptoms such as fatigue, breathlessness, pale skin, or dizziness, or as part of a routine health check, pre-operative screening, or ongoing monitoring of a chronic condition.
You Attend the Phlebotomy Appointment
You attend a clinic, hospital phlebotomy department, or private laboratory for your blood draw. On arrival, a trained phlebotomist confirms your identity, reviews the test request, and checks for any known allergies, medications, or bleeding disorders. You are seated or positioned comfortably and the phlebotomist selects the most accessible vein, usually the median cubital vein in the inner elbow.
The Skin Is Cleaned and the Needle Inserted
The phlebotomist applies a tourniquet to your upper arm to make the vein more prominent, then thoroughly cleans the skin with a 70% isopropyl alcohol antiseptic swab and allows it to dry. A sterile, single-use needle connected to a vacuum collection tube is swiftly inserted into the vein. Most patients experience only a brief, mild pinch. The tourniquet is released once blood begins to flow into the collection tube.
Blood Sample Is Collected and Labelled
Approximately 3 to 5 millilitres of blood is drawn into an EDTA-coated vacuum tube. EDTA (ethylenediaminetetraacetic acid) is an anticoagulant that prevents the blood from clotting during transport and analysis, ensuring the haematology analyser can measure individual red blood cells accurately. The needle is withdrawn, gentle pressure is applied to the puncture site, and a bandage is placed. The tube is immediately labelled with your patient details and dispatched to the laboratory.
The Haematology Analyser Processes Your Sample
In the laboratory, your blood sample is loaded into a fully automated haematology analyser. Using electrical impedance or laser-based flow cytometry, the machine individually measures the volume and size of every single red blood cell passing through a narrow channel in the flow cell. Thousands of cells are measured within seconds, generating a histogram of red blood cell size distribution that forms the basis of the RDW calculation.
The RDW Value Is Calculated
Using the full distribution of red blood cell volumes measured by the analyser, the RDW-CV (Coefficient of Variation) is calculated by dividing the standard deviation of red cell volume by the mean corpuscular volume (MCV), then multiplying by 100 to express the result as a percentage. A wider distribution of cell sizes produces a higher percentage. Some analysers also report RDW-SD (Standard Deviation), measured directly in femtolitres (fL), which can detect size variation differently from RDW-CV.
Your Results Are Reviewed and Reported
The completed CBC report, including your RDW value alongside haemoglobin, MCV, white cell count, platelet count, and all other indices, is reviewed by a laboratory haematologist or biomedical scientist before being released. For NHS samples this typically takes 24 to 48 hours. Your GP or referring clinician then reviews the results in the context of your symptoms and medical history before communicating them to you via phone, letter, or your NHS online account.
Your Doctor Interprets and Acts on Results
Your GP or specialist reviews your RDW alongside all other CBC values, particularly MCV and haemoglobin, to reach a clinical interpretation. An elevated RDW with a low MCV suggests iron deficiency anaemia. A high RDW with a high MCV points to B12 or folate deficiency. A low RDW with a low MCV raises suspicion for thalassaemia. Based on this interpretation, your doctor will either reassure you, request further targeted blood tests, or initiate treatment for the identified cause.
Once your RDW value is reported, it falls into one of three categories. Each category has different clinical implications depending on what your other CBC values show alongside it.
Significant Red Cell Size Variation
A high RDW means your red blood cells vary considerably in size. This is the most commonly abnormal RDW finding and indicates the body is producing cells of inconsistent size due to a deficiency, disease, or compensatory process.
- Iron deficiency anaemia (most common cause globally)
- Vitamin B12 or folate deficiency anaemia
- Mixed nutritional deficiency
- Haemolytic anaemia and recent blood transfusion
- Liver disease and chronic inflammatory conditions
Uniform Red Blood Cell Sizing
A normal RDW indicates that your red blood cells are relatively consistent in size. This is a reassuring finding but must be read alongside haemoglobin and MCV. A normal RDW does not rule out all blood conditions.
- Healthy blood profile in most cases
- Anaemia of chronic disease can show normal RDW
- Aplastic anaemia may present with normal RDW
- Early nutritional deficiency before RDW has shifted
- Always interpret alongside full CBC values
Unusually Uniform Cell Sizing
A low RDW means red blood cells are very consistent in size. While this sounds positive, it can indicate conditions where the bone marrow produces uniformly abnormal cells, most notably thalassaemia trait.
- Thalassaemia trait (key distinguishing finding)
- Anaemia of chronic disease
- Aplastic anaemia with reduced but uniform cells
- Microspherocytosis (rare inherited disorder)
- Isolated low RDW without other changes is rarely alarming
Understanding how the RDW percentage is actually derived from your blood sample helps explain why it is such a sensitive and early indicator of red blood cell abnormality.
When your blood sample reaches the haematology analyser, it is passed through a flow cell where each red blood cell is individually measured for its volume using either electrical impedance (the Coulter principle) or laser-based flow cytometry. The former measures the change in electrical resistance as each cell passes through a tiny aperture; the latter measures the light scattered by each cell as it crosses a laser beam.
Both methods produce a precise volume measurement for each cell in femtolitres (fL). From tens of thousands of individual measurements, the analyser constructs a red cell volume distribution histogram — a curve showing how many cells fall within each size range.
The width of this distribution curve is the RDW. A narrow, steep curve indicates uniform cell sizes and a low RDW. A wide, flat curve indicates high size variability and an elevated RDW. Even before a patient develops frank anaemia or obvious symptoms, a widening of this distribution curve can signal emerging nutritional deficiency or bone marrow stress, making RDW a valuable early warning marker within the CBC.
Where Standard Deviation is derived from the full distribution of individual red blood cell volume measurements, and MCV is the average red blood cell volume in femtolitres. A higher standard deviation relative to the MCV produces a higher RDW percentage, indicating greater size variation. The normal RDW-CV range is 11.5% to 14.5% for most adults.
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Ready to Understand Your Results?
Now that you know how the RDW blood test works, explore our detailed guides on interpreting high RDW, low RDW, and normal results, or visit our FAQ page for answers to the most common patient questions about the RDW blood test.
Disclaimer from RDW Blood Test: All content on this page is provided for general educational purposes only. It does not constitute medical advice, diagnosis, or treatment. Always consult a qualified GP or haematologist to interpret your specific blood test results in the context of your complete blood count, symptoms, and full medical history.
