RDW Blood Test: Normal Range, High & Low Levels Explained
The RDW (Red Cell Distribution Width) blood test is a key part of your complete blood count. It measures how much red blood cells vary in size, helping doctors detect anemia, nutritional deficiencies, and other serious conditions — often before symptoms appear.
01 What Is an RDW Blood Test?
The RDW blood test — short for Red Cell Distribution Width — is a laboratory measurement that tells your doctor how much the size and volume of your red blood cells (RBCs) vary from one another. It is a standard component of the complete blood count (CBC), the most commonly ordered blood panel in medicine.
Medical Definition: Red Cell Distribution Width (RDW) quantifies the degree of variation in red blood cell size (anisocytosis) within a blood sample, expressed as a coefficient of variation (RDW-CV) in percentage or as standard deviation (RDW-SD) in femtoliters.
Under normal circumstances, red blood cells are roughly the same size — approximately 6.2 to 8.2 micrometres in diameter. When the body struggles to produce healthy red blood cells due to nutritional deficits, disease, or bone marrow dysfunction, the cells begin to vary widely in size. The RDW test detects that variation and flags it as a percentage.
What makes this test especially valuable is its early-warning capability. RDW can rise before other markers of anemia — such as haemoglobin or MCV — move out of the normal range, making it one of the first detectable signs that something is going wrong with red blood cell production.
Figure 1: The RDW blood test measures size variation across your red blood cells as part of the CBC panel.
02 Why Is an RDW Blood Test Ordered?
Doctors order the RDW test in a variety of clinical situations. Because RDW is automatically included in every CBC test, it is often checked as part of a routine health examination — even when no specific symptoms are present. However, there are situations where the RDW result becomes particularly important.
Routine Screening
During a standard annual check-up or pre-operative blood work, a CBC is almost always ordered. Your RDW is reported automatically alongside other red blood cell indices. A routine RDW result outside the normal range can uncover silent conditions before symptoms develop.
Suspected Anaemia
If you present with symptoms such as persistent fatigue, weakness, shortness of breath, dizziness, cold hands and feet, or pale skin, your doctor will strongly consider anaemia as a cause. RDW helps determine what type of anaemia is present and guides further investigation.
Monitoring Chronic Conditions
People managing ongoing conditions — including kidney disease, liver disease, autoimmune disorders, HIV, or cancer — often have RDW checked regularly. These conditions can affect red blood cell production and lifespan, and RDW reflects those changes over time.
Family History of Blood Disorders
If you have a family history of conditions such as sickle cell disease, thalassaemia, or hereditary spherocytosis, your doctor may monitor your RDW alongside other haematological markers as part of an ongoing surveillance plan.
- Unexplained fatigue or persistent weakness
- Pale or yellowed skin (jaundice)
- Frequent dizziness or fainting episodes
- Rapid or irregular heartbeat
- Shortness of breath during routine activity
- History of blood transfusions or bone marrow disorders
- Monitoring nutritional deficiency treatment progress
- Before and after major surgery
03 How Is the RDW Blood Test Done?
The RDW test is performed on a standard blood sample. There is nothing unique about the collection procedure itself — it is drawn the same way as any other blood test. The analysis of RDW values happens in the laboratory using specialised equipment called a haematology analyser.
Preparation Before the Test
For a standalone RDW or CBC test, no special preparation is required. You can eat, drink, and take medications as normal. However, if your doctor has ordered additional tests alongside the CBC — such as iron studies, glucose, or lipid panels — you may need to fast for 8 to 12 hours beforehand. Always follow your doctor's specific instructions.
Step-by-Step: What Happens During the Blood Draw
Are There Any Risks?
Blood tests are extremely safe. The most common side effects are minor — a brief sting at the needle site, slight bruising that fades within a few days, or feeling lightheaded immediately after the draw. In rare cases, some people experience fainting. If this happens, stay seated or lie down until the feeling passes. Serious complications such as infection or excessive bleeding are very uncommon.
04 RDW Normal Range: What the Numbers Mean
RDW results are reported as a percentage. The value reflects how much variation exists in the volume and size of your red blood cells. A lower percentage means your red blood cells are more uniform; a higher percentage indicates greater variation.
It is important to note that reference ranges can vary slightly between different laboratories and haematology analysers, so always interpret your results in the context of the reference range printed on your specific lab report.
| RDW Level | Range (Adults) | What It Indicates | Clinical Significance |
|---|---|---|---|
| Low | Below 11.5% | Red blood cells are uniformly small and consistent in size | Not typically associated with disease on its own. May appear alongside liver disease, kidney disease, or in very healthy individuals. |
| Normal | 11.5% – 14.5% | Healthy variation — red blood cells are similar in size | Generally reassuring. A normal RDW does not rule out all forms of anaemia — some types produce normal RDW with abnormal haemoglobin. |
| High | Above 14.5% | Significant variation in red blood cell size (anisocytosis) | Most commonly linked to iron deficiency, B12 or folate deficiency, mixed anaemia, liver disease, or bone marrow disorders. Requires further investigation. |
RDW in Children vs Adults
Reference ranges for RDW differ by age group. Newborns naturally have higher RDW values due to the transition from foetal haemoglobin to adult haemoglobin. By the time a child reaches school age, their RDW typically falls within adult reference ranges. Always ensure your doctor interprets your child's results using age-appropriate laboratory reference ranges.
05 What Does a High RDW Mean?
A high RDW — any value above 14.5% — means that your red blood cells vary considerably in size. This condition is medically known as anisocytosis. Some cells may be much smaller than normal (microcytes), others may be much larger (macrocytes), and the mixture creates the elevated RDW reading.
It is crucial to understand that a high RDW is a clue, not a diagnosis. Your doctor will always interpret a raised RDW alongside other blood test results — particularly the MCV, haemoglobin, white cell count, and platelet count — before reaching any conclusion.
Figure 2: High RDW (anisocytosis) showing wide variation in red blood cell sizes across the blood sample.
Common Causes of High RDW
High RDW and Heart Disease
Research has shown a significant association between elevated RDW and outcomes in cardiovascular disease. Studies suggest that a high RDW in patients with heart failure is linked to worse outcomes, including higher mortality rates. The precise mechanism is thought to involve chronic inflammation, oxidative stress, and neurohormonal activation — all of which contribute to both red blood cell abnormalities and cardiac dysfunction. Importantly, RDW has been proposed as an inexpensive prognostic marker in cardiac care that goes far beyond its traditional role in haematology.
High RDW and COVID-19
Emerging research following the COVID-19 pandemic has identified elevated RDW as a potential marker of severity and mortality in SARS-CoV-2 infection. Patients admitted to hospital with COVID-19 who showed high RDW values on admission were observed to have significantly worse clinical outcomes. Researchers believe this is related to cytokine storm, hyperinflammation, and secondary anaemia common in severe COVID-19 cases.
RDW Levels in High RDW: How Elevated Is Too Elevated?
06 What Does a Low RDW Mean?
A low RDW (below 11.5%) means that your red blood cells are very uniform in size. Unlike a high RDW, a low value is generally not considered clinically concerning on its own. It does not indicate anaemia and is rarely a sign of a serious underlying condition.
However, a low RDW can occasionally be observed in:
- Thalassaemia trait — where red blood cells are consistently small but uniform in size, producing a low MCV alongside a normal or even low RDW
- Iron supplementation therapy — as iron stores are replenished, newly produced cells become more uniform, potentially lowering RDW temporarily
- People with very healthy and balanced nutritional status
- Aplastic anaemia in some presentations
The distinction between low RDW with low MCV (which may suggest thalassaemia trait) and high RDW with low MCV (which is characteristic of iron deficiency anaemia) is one of the most clinically useful comparisons your doctor will make when reviewing your CBC results.
07 RDW and MCV Combined: The Key to Accurate Diagnosis
The most powerful way to use RDW in clinical practice is not in isolation, but alongside the Mean Corpuscular Volume (MCV) — a measure of the average size of your red blood cells. Together, the RDW and MCV create a diagnostic matrix that helps doctors narrow down the cause of anaemia with much greater precision than either marker alone.
| RDW | MCV | Likely Diagnosis | Next Steps |
|---|---|---|---|
| High | Low (Microcytic) | Iron Deficiency Anaemia | Serum ferritin, serum iron, TIBC |
| Normal | Low (Microcytic) | Thalassaemia Trait | Haemoglobin electrophoresis, genetic testing |
| High | Normal (Normocytic) | Early Iron or B12 Deficiency, Mixed Anaemia | Ferritin, B12, folate, peripheral smear |
| Normal | Normal (Normocytic) | Anaemia of Chronic Disease, Early Marrow Suppression | Inflammatory markers, kidney function |
| High | High (Macrocytic) | Megaloblastic Anaemia (B12 or Folate Deficiency) | Serum B12, red blood cell folate, homocysteine |
| Normal | High (Macrocytic) | Liver Disease, Alcohol Use, Hypothyroidism | Liver function tests, TSH, alcohol history |
This RDW-MCV matrix is one of the most practical tools in clinical haematology. A peripheral blood smear — where red blood cells are examined under a microscope — is often added to visually confirm the patterns the numbers suggest.
08 RDW-CV vs RDW-SD: Understanding Both Measurements
Most modern haematology analysers report two separate RDW values on your lab results: RDW-CV and RDW-SD. Both measure red blood cell size variation, but they do so differently and have different strengths in clinical practice.
Reported as a percentage. Measures the size variation of red blood cells relative to their average size (MCV). It is the more commonly reported value and is the one most people see on their lab results.
Normal range: 11.5% – 14.5%
Limitation: Can be misleading when MCV is unusually high or low, because it is calculated relative to the average size.
Reported in femtoliters (fL). Measures the actual width of the red blood cell size distribution histogram — the spread between the smallest 20% and the largest 20% of cells in the sample.
Normal range: 37 – 54 fL
Advantage: More reliable when MCV is abnormal, because it does not depend on the average cell size. Particularly useful for detecting early iron deficiency or liver damage.
In most routine clinical settings, RDW-CV is sufficient for initial anaemia workup. RDW-SD becomes especially useful when red blood cells are dramatically oversized or undersized, and the CV calculation may misrepresent the true degree of variation.
09 How to Treat Abnormal RDW Levels
There is no treatment for an abnormal RDW value itself — because RDW is a measurement, not a condition. The goal is to identify and treat the underlying cause that is producing the abnormal result. Once the root problem is corrected, RDW typically normalises over several weeks to months as new healthy red blood cells replace the abnormal ones.
Lifestyle Tips to Support Healthy RDW Levels
- Eat a varied, balanced diet rich in iron, B12, and folate
- Include vitamin C with iron-rich meals to boost absorption
- Avoid excessive alcohol, which impairs folate absorption and damages bone marrow
- Quit smoking — cigarette smoking has been shown to elevate RDW through chronic inflammation and oxidative stress
- Stay well-hydrated to support healthy red blood cell production
- Manage chronic conditions proactively with your healthcare team
- Take nutritional supplements only under medical guidance — excess iron or B12 can cause harm
10 When to See a Doctor About Your RDW
If your RDW comes back abnormal on a routine blood test and you have no symptoms, your doctor will likely request a repeat test or additional investigations before taking action. A single abnormal result does not automatically indicate a serious condition.
However, you should seek prompt medical attention if you have an elevated RDW alongside any of the following symptoms:
- Severe shortness of breath at rest
- Chest pain or rapid, irregular heartbeat
- Extreme fatigue preventing daily activities
- Significant unexplained weight loss
- Yellowing of the skin or eyes (jaundice)
- Dark, tea-coloured, or blood-tinged urine
- Numbness or tingling in hands and feet
- Persistent unexplained fatigue lasting several weeks
- Ongoing dizziness or difficulty concentrating
- Pale skin, brittle nails, or hair loss
- Known poor diet or suspected nutritional deficiency
- Abnormal RDW on two consecutive blood tests
- Family history of blood disorders
- Starting or completing a course of iron or B12 supplements
11 Frequently Asked Questions About the RDW Blood Test
12 Medical Sources and References
The information presented on this page has been compiled using peer-reviewed medical literature and guidance from internationally recognised health organisations. Our editorial team reviews all health content regularly to ensure accuracy and clinical relevance.
- Cleveland Clinic — RDW Blood Test: What It Is, Procedure & Results
- MedlinePlus (NIH) — RDW: Red Cell Distribution Width
- WebMD — RDW Blood Test: Red Cell Distribution Width
- Medical News Today — RDW blood test: What is it, preparation, and results
- Medscape — Red Cell Distribution Width (RDW) Test
- Pagana KD, Pagana TJ, Pagana TN. Mosby's Diagnostic & Laboratory Test Reference. 17th ed. 2024.
- Wen Y et al. Red Blood Cell Distribution Width (RDW) as an Important Predictor of Outcome in Different Clinical Settings. BioMed Research International. 2018.
