High RDW on Your Blood Test
What red cell distribution width means, how to interpret RDW in combination with MCV, and what the patterns tell your GP about iron, B12, and folate status.
The Quick Answer
RDW stands for red cell distribution width — a measurement of how much the sizes of your red blood cells vary from one another. When all red cells are a similar size, RDW is low or normal. When cells come in many different sizes, RDW is high. This is called anisocytosis.
The Australian normal range is approximately 11.5–14.5% (some labs use 11.6–14.6%). A result above this means the bone marrow is producing an inconsistent mix of red cell sizes. The most common reason is a nutrient deficiency — usually iron, vitamin B12, or folate — that has disrupted the normal red cell production process.
The key insight: RDW is most useful when read alongside MCV (mean corpuscular volume — the average red cell size). Together, the two numbers tell a remarkably specific story about what is going wrong with red cell production. See the patterns table below.
Interpreting RDW + MCV Together
The combination of RDW and MCV is far more informative than either value alone. This is one of the most clinically useful pattern-recognition tools in haematology.
High RDW + Low MCV
Small, size-varying red cells
Common causes: Iron deficiency anaemia (most likely). Also: sickle cell trait, some haemoglobinopathies.
High RDW + Normal MCV
Normal average size, but wide variation
Common causes: Early iron or B12/folate deficiency. Mixed deficiency (iron + B12 offsetting each other). Post-transfusion. Early myelodysplastic syndrome.
High RDW + High MCV
Large, size-varying red cells
Common causes: B12 or folate deficiency (megaloblastic anaemia). Also: haemolysis, liver disease.
Normal RDW + Low MCV
Uniformly small red cells
Common causes: Thalassaemia trait (most likely). Anaemia of chronic disease. Helps distinguish thalassaemia from iron deficiency.
Normal RDW + High MCV
Uniformly large red cells
Common causes: Alcohol use. Hypothyroidism. Liver disease. Some medications. Drug-related macrocytosis often has normal RDW.
Causes of High RDW
Causes are ordered roughly by frequency in Australian general practice.
Iron deficiency
Insufficient iron for haemoglobin production → small, pale red cells mixed with normal-sized cells
The most common cause of high RDW worldwide. Even early iron deficiency raises RDW before the MCV falls or haemoglobin drops. Check serum ferritin (most sensitive) and iron saturation.
Vitamin B12 deficiency
Impaired DNA synthesis → large, oval macrocytes mixed with normal cells
B12 deficiency from pernicious anaemia, diet (vegans), gastric surgery, or long-term metformin/PPI. Can cause permanent nerve damage if missed. Check serum B12 and active B12 (holotranscobalamin) if borderline.
Folate deficiency
Impaired DNA synthesis → megaloblastic macrocytes
Poor diet (low leafy greens), alcohol, pregnancy, coeliac disease, methotrexate, phenytoin. Always check alongside B12 — the two deficiencies often coexist and B12 must be excluded first before treating folate.
Mixed deficiency (iron + B12/folate)
Iron pushes MCV down, B12/folate pushes it up — net effect is normal MCV despite very high RDW
The classic "dimorphic blood film" — a mixture of microcytes and macrocytes that average out to a normal MCV. High RDW with normal MCV and anaemia should always trigger testing for both iron and B12/folate. Particularly common after gastric sleeve or bypass surgery.
Post-transfusion
Donor red cells differ in size from patient's own cells, temporarily widening the distribution
A high RDW after a blood transfusion can simply reflect the mixture of patient and donor red cells. Usually resolves within 1–2 months as transfused cells are cleared.
Haemolytic anaemia
Rapid red cell destruction stimulates bone marrow to release large young reticulocytes (young red cells are larger)
The presence of reticulocytes (large, newly produced cells) alongside fragmented old cells creates size variation. Check LDH, bilirubin, haptoglobin, and reticulocyte count if haemolysis is suspected.
Myelodysplastic syndrome (MDS)
Abnormal bone marrow clone produces red cells of erratic size and shape
An important cause to exclude when RDW is persistently elevated without a nutritional explanation, especially in people over 60. Often associated with unexplained anaemia, low WBC, or low platelets. Requires haematology review.
Sickle cell disease / trait
Sickle cells vary in size and shape compared with normal red cells
Sickle cell disease causes significant RDW elevation alongside chronic haemolytic anaemia. Sickle cell trait is usually clinically mild but may show mildly elevated RDW. Haemoglobin electrophoresis confirms the diagnosis.
Thalassaemia + iron deficiency
Adding iron deficiency to a thalassaemia background worsens size variation
Thalassaemia trait alone usually gives a normal or only mildly raised RDW. If RDW is markedly elevated in someone with thalassaemia, coexisting iron deficiency should be excluded before attributing the result to the thalassaemia alone.
Symptoms Associated With High RDW
High RDW itself does not cause symptoms — the symptoms come from the underlying cause. Many people with mildly elevated RDW feel completely well and the result is found on a routine blood test. Symptoms become more likely when anaemia is also present.
Fatigue and reduced exercise tolerance
The most common symptom when RDW elevation is accompanied by anaemia from iron or B12 deficiency. Red cells that vary widely in size and shape carry oxygen less efficiently as a collective.
Pale skin, lips, and inner eyelids
Pallor from associated anaemia. Best assessed inside the lower eyelids (conjunctival pallor) and in the nail beds in good natural light.
Tingling or numbness in hands and feet
A red-flag symptom specifically for B12 deficiency, which can cause peripheral neuropathy. Requires urgent investigation and treatment to prevent permanent nerve damage.
Sore, smooth tongue (glossitis)
The tongue loses its papillae and becomes painful in B12 and folate deficiency. Classic finding in megaloblastic anaemia.
Cravings for non-food substances (pica)
Craving ice, dirt, clay, chalk, or starch is strongly associated with iron deficiency. The mechanism is not fully understood but pica resolves with iron replacement.
Shortness of breath on exertion
Occurs when associated anaemia becomes significant enough to reduce oxygen delivery. Climbing stairs or carrying shopping may trigger breathlessness that was not previously present.
Restless legs at night
A strong association exists between iron deficiency (even without significant anaemia) and restless legs syndrome — an uncomfortable urge to move the legs at rest, especially at night.
No symptoms at all
High RDW with early or mild nutritional deficiency, or with thalassaemia trait, can be completely asymptomatic and discovered only on a routine blood test. Investigation is still warranted.
Red Flags — When to See Your GP Promptly
High RDW with tingling, numbness, or balance problems
These are neurological signs of B12 deficiency and can become permanent if untreated. Seek GP review within days, not weeks.
High RDW with haemoglobin below 80 g/L
Severe anaemia alongside anisocytosis usually needs urgent assessment. Symptoms of breathlessness at rest, chest pain, or palpitations warrant same-day medical review.
High RDW with low WBC and low platelets (pancytopenia)
Failure of all three cell lines points to bone marrow disease. Requires urgent haematology referral — this is not a nutritional deficiency pattern.
High RDW rising on serial tests despite supplementation
If RDW is not responding to treatment within 6–8 weeks of B12 or iron supplementation, the diagnosis or the delivery of treatment should be reviewed. Consider malabsorption (coeliac disease, Crohn's disease, or other GI issues).
High RDW in combination with abnormal cells on blood film
Blast cells, schistocytes (fragmented red cells), or significant numbers of nucleated red cells reported by the laboratory suggest a serious underlying diagnosis requiring urgent review.
The Diagnostic Workup for High RDW
Identify the MCV + RDW pattern
The first step is looking at both RDW and MCV together. A high RDW with low MCV points to iron deficiency. High RDW with high MCV points to B12/folate deficiency. High RDW with normal MCV suggests early deficiency or mixed deficiency. This pattern guides the next tests.
Iron studies: ferritin, iron, TIBC, transferrin saturation
Serum ferritin is the most sensitive marker of iron stores — low ferritin confirms iron deficiency even before anaemia develops. Transferrin saturation and iron-binding capacity confirm the picture. Note: ferritin is an acute-phase reactant and can be falsely normal in the presence of infection or inflammation — check CRP alongside.
Vitamin B12 and folate
Serum B12, active B12 (holotranscobalamin), and red cell folate are checked when RDW is high with normal or raised MCV. Borderline B12 (150–220 pmol/L) can be confirmed as truly deficient using methylmalonic acid (MMA) levels. Always check B12 before treating folate.
Blood film
A trained laboratory scientist examines the red cell shapes under the microscope. A dimorphic film (mixture of microcytes and macrocytes) is the classic appearance of mixed deficiency. Oval macrocytes and hypersegmented neutrophils confirm megaloblastic change. Target cells, sickle cells, or schistocytes point to other diagnoses.
Reticulocyte count
Reticulocytes (young red cells) are normally 0.5–2% of the total RBC count. Elevated reticulocytes with high RDW suggest haemolysis or active recovery from haemorrhage. Low reticulocytes with anaemia and high RDW suggest bone marrow underproduction.
Haemoglobin electrophoresis if thalassaemia suspected
When RDW is mildly elevated with a persistently low MCV in someone of relevant ethnic background (South-East Asian, South Asian, Middle Eastern, Mediterranean, African), thalassaemia trait should be considered and tested with Hb electrophoresis or HPLC.
Haematology referral if unexplained
Persistently elevated RDW without nutritional deficiency, with other abnormal cell lines, or with abnormal cells on the blood film in a patient over 60 warrants haematology referral to exclude MDS, chronic haemolytic anaemia, or other bone marrow pathology.
Treatment — What to Expect
Iron deficiency
Oral iron supplementation (ferrous sulfate, ferrous fumarate, or ferric carboxymaltose — choice depends on tolerability). Take on an empty stomach or with vitamin C for best absorption, and avoid taking with dairy, antacids, or tea. Side effects (constipation, dark stools, stomach upset) are common — tell your GP if they are intolerable, as dose adjustment or intravenous iron can be considered. RDW takes 4–8 weeks to begin normalising, with full normalisation over 3–6 months. The underlying cause of iron deficiency (diet, heavy periods, GI bleeding) must also be addressed.
B12 or folate deficiency
B12 injections (hydroxocobalamin) for malabsorption (pernicious anaemia, gastric surgery), or high-dose oral B12 (1000–2000 mcg daily) for dietary deficiency in vegans. Folic acid 5 mg daily for folate deficiency. Always confirm B12 is treated or normal before giving folate alone. RDW begins to fall within 2–4 weeks of treatment, with normalisation by 2–3 months.
Monitoring response
A repeat FBC at 6–8 weeks of treatment should show a falling RDW, rising reticulocyte count (within the first 1–2 weeks — this is the earliest response), and recovering haemoglobin. If RDW has not improved after 8 weeks of supplementation, a malabsorption condition (coeliac disease, inflammatory bowel disease) or incorrect diagnosis should be considered.
Foods That Support Consistent Red Cell Production
Lean red meat (beef, lamb)
Haem iron, B12Haem iron from meat is absorbed at 15–35% efficiency, far superior to plant iron (1–5%). Eating red meat 3–4 times per week significantly supports iron stores.
Liver (chicken, lamb)
Iron, B12, folateThe single densest source of iron, B12, and folate in one food. Avoid in pregnancy due to high vitamin A content.
Leafy greens (spinach, kale, silverbeet)
Folate, non-haem ironThe word "folate" comes from foliage. Lightly cooked rather than raw maximises folate bioavailability. Pair with vitamin C to enhance iron absorption.
Legumes (lentils, chickpeas)
Folate, non-haem ironExcellent plant sources of both folate and iron. Soaking and rinsing dried legumes reduces phytates that inhibit iron absorption.
Eggs and dairy
B12Important B12 sources for lacto-ovo vegetarians. One egg provides about 0.5 mcg B12; a glass of milk about 1 mcg.
Citrus fruits and berries
Vitamin CVitamin C dramatically improves non-haem iron absorption when consumed at the same meal. A glass of orange juice with iron-rich plant foods can double iron uptake.
Fortified cereals and bread
Folic acid, iron, B12Australian wheat flour is mandatorily fortified with folic acid. Many cereals are also fortified with iron and B12 — a practical option for those on plant-based diets.
Seafood (oysters, sardines, tuna)
Iron, B12, omega-3Excellent sources of both haem iron and B12. Sardines with bones also provide calcium. Canned fish is a convenient and affordable option.
Related Reading
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This page provides general educational information about red cell distribution width (RDW) and anisocytosis. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your GP about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care.