Frequently Asked
Questions About RDW

RDW stands for Red Cell Distribution Width. It is a measurement that quantifies the degree of variation in size among the red blood cells (RBCs) in your blood sample. A higher RDW value indicates greater variation in cell size, a condition known as anisocytosis. A lower value indicates that your red blood cells are more uniform in size.

The RDW is always reported as a percentage and is one of several indices included in a Full Blood Count (FBC) or Complete Blood Count (CBC), the most commonly ordered blood test in clinical medicine worldwide.

Doctors order the RDW blood test primarily to help diagnose, classify, and monitor anaemia. When combined with MCV (Mean Corpuscular Volume), the RDW allows clinicians to distinguish between different types of anaemia such as iron deficiency anaemia, vitamin B12 deficiency anaemia, and thalassaemia, which can present with similar symptoms but require very different treatments.

Beyond anaemia, research has shown that elevated RDW values are associated with cardiovascular disease, chronic kidney disease, liver disease, inflammatory conditions, and even certain cancers. This makes it a valuable broad-spectrum screening marker within the CBC panel.

RDW-CV (Coefficient of Variation) is the most commonly reported form. It is calculated as the standard deviation of red cell volume divided by the mean corpuscular volume (MCV), then multiplied by 100 to give a percentage. The normal range is typically 11.5% to 14.5%.

RDW-SD (Standard Deviation) is an absolute measurement expressed in femtolitres (fL) rather than a percentage. It directly measures the width of the red cell size distribution curve at a fixed point. The normal range for RDW-SD is approximately 39 to 46 fL. Most standard blood reports in the UK use RDW-CV. If you are unsure which variant appears on your report, ask your GP or the laboratory that processed your sample.

Yes. The RDW blood test is not performed in isolation. It is always included as one of the red blood cell indices within a Full Blood Count (FBC) or Complete Blood Count (CBC). Other values in the same panel include haemoglobin, haematocrit, red blood cell count, MCV, MCH (Mean Corpuscular Haemoglobin), MCHC (Mean Corpuscular Haemoglobin Concentration), white blood cell count, and platelet count.

Because RDW only becomes clinically meaningful when interpreted alongside MCV and haemoglobin, it is never used as a standalone test.

In the UK, you can access a private CBC including RDW without a GP referral through private diagnostic services such as Medichecks, Blue Horizon, or private hospital phlebotomy services. These typically cost between 25 and 60 GBP for a full blood count panel.

On the NHS, an RDW test as part of a CBC is ordered by your GP when clinically indicated. You cannot self-refer for NHS blood tests. If you have symptoms of anaemia such as fatigue, breathlessness, or pale skin, your GP can request the test free of charge on your behalf.

The RDW test itself does not require fasting. However, if your CBC is being ordered alongside other tests that do require fasting, such as a fasting glucose, HbA1c, or a lipid panel, your doctor may ask you to fast for 8 to 12 hours before the blood draw.

Always follow the specific instructions provided by your GP, hospital, or private laboratory. If no fasting instructions are given, it is generally safe to eat and drink normally before the test. Staying well hydrated with water before your appointment will help make the blood draw easier.

The standard normal RDW range for most adults is 11.5% to 14.5% when measured as RDW-CV. This range applies to both men and women across all age groups. Results within this range suggest that your red blood cells are relatively uniform in size, which is a healthy finding.

However, it is important to understand that reference ranges can vary slightly between laboratories and testing facilities. Some labs may use a slightly different upper threshold. Always compare your result to the reference range printed on your specific laboratory report rather than using a general online figure as your sole benchmark.

A normal RDW is a reassuring finding but it does not rule out all health conditions. Some conditions, such as aplastic anaemia and certain types of anaemia of chronic disease, can present with a normal RDW alongside a low haemoglobin or low red blood cell count. This is why RDW must always be interpreted alongside the full CBC panel and your clinical symptoms.

Additionally, early-stage nutritional deficiencies may not yet have progressed enough to cause measurable changes in RDW. If you have persistent symptoms of fatigue, weakness, or breathlessness despite a normal RDW, speak to your GP about additional investigations.

Yes. While the standard normal range of 11.5% to 14.5% is widely used, individual laboratories may set slightly different reference ranges based on the analyser model they use, the population they serve, and internal calibration standards. It is therefore essential to compare your RDW result against the reference range printed on your own laboratory report rather than relying solely on general figures.

If you have results from two different laboratories and the values differ slightly, this is usually due to methodological differences rather than a genuine change in your health. Discuss any differences with your GP for context.

The standard reference range of 11.5% to 14.5% generally applies across most age groups including children and older adults. However, newborns and very young infants may have slightly different expected ranges as their red blood cell composition is still transitioning from foetal haemoglobin to adult haemoglobin in the first months of life.

Older adults may also show slightly higher RDW values as a result of accumulated nutritional changes, chronic conditions, and the natural ageing process of the bone marrow. Your doctor will consider age-related context when reviewing your results.

A high RDW, defined as a value above 14.5%, means that your red blood cells vary significantly in size. This variation, known as anisocytosis, occurs when the body is producing red blood cells of inconsistent sizes due to an underlying problem with nutrient availability, bone marrow function, or red cell destruction and replacement.

The most common causes of a high RDW include iron deficiency anaemia, vitamin B12 or folate deficiency, mixed nutritional deficiency, liver disease, haemolytic anaemia, and recent blood transfusion. A high RDW does not itself confirm a diagnosis but is a strong signal that further investigation is warranted.

No. While anaemia is the most commonly associated condition, an elevated RDW can also occur in the absence of anaemia. Research has linked high RDW values to cardiovascular disease risk, chronic kidney disease, liver disease, inflammatory conditions such as rheumatoid arthritis, and certain malignancies, even when haemoglobin levels appear normal.

In these cases, the elevated RDW may reflect systemic inflammation or bone marrow stress rather than outright anaemia. Your doctor will review your full blood count and clinical picture to determine what the elevated value means in your specific case.

A high RDW combined with a low MCV is the classic pattern of iron deficiency anaemia. The low MCV indicates that red blood cells are smaller than average, while the high RDW reveals that their sizes vary widely, with some cells being very small and others closer to normal. This mixed sizing pattern occurs because iron-depleted cells produced recently are smaller than older cells produced when iron levels were still adequate.

This combination typically prompts a serum ferritin test, full iron studies, and potentially an investigation for a source of blood loss such as gastrointestinal bleeding or heavy menstrual periods.

A high RDW alongside a high MCV strongly suggests vitamin B12 or folate deficiency anaemia. Both nutrients are essential for proper DNA synthesis during red blood cell production. Without them, the bone marrow produces abnormally large red blood cells (macrocytes) that are also highly variable in size, driving both MCV and RDW upward.

This pattern is particularly common in strict vegans, older adults with pernicious anaemia, patients with Crohn's disease or coeliac disease impairing absorption, and those taking medications such as metformin or methotrexate which interfere with B12 or folate metabolism.

When RDW is elevated but MCV falls within the normal range, it often indicates an early or mixed nutritional deficiency where both iron deficiency and B12 or folate deficiency are occurring simultaneously. Because iron deficiency tends to lower MCV while B12 deficiency raises it, the opposing effects can cancel out, producing a deceptively normal average cell size. The elevated RDW is often the only visible clue that something is wrong.

Other causes of this pattern include early-stage haemolytic anaemia, sickle cell trait, and early post-haemorrhagic states where new cells of varying sizes enter the circulation before the profile has fully shifted.

Chronic psychological stress on its own does not directly raise RDW. However, lifestyle factors that impair nutrition and red blood cell production can contribute indirectly. A diet consistently low in iron, vitamin B12, and folate will over time produce abnormal red blood cell size variation and elevate RDW.

Excessive alcohol consumption is a recognised cause of elevated RDW as it impairs folate absorption and direct bone marrow function. Smoking has also been associated with mildly elevated RDW values in research studies. Maintaining a balanced diet, limiting alcohol, and avoiding smoking all support healthier red blood cell production.

RDW often takes longer to normalise than haemoglobin or other blood count markers because it reflects the full range of cell sizes in circulation, including older cells that were produced during the deficiency period. Red blood cells live for approximately 90 to 120 days, so the full turnover of the red blood cell population takes around three to four months.

For iron deficiency anaemia treated with oral iron supplementation, most patients see haemoglobin improve within 4 to 8 weeks, but RDW may remain elevated for up to 3 months as the older smaller cells are gradually replaced. For B12 deficiency treated with injections, improvement in RDW typically begins within 4 to 6 weeks of treatment starting.

A low RDW, generally considered to be a value below 11.5%, means that your red blood cells are unusually uniform in size. While a degree of uniformity is expected in healthy blood, a very low RDW can occasionally reflect an underlying condition where the bone marrow is consistently producing abnormally uniform cells of the wrong size.

The most clinically significant cause of a low RDW is thalassaemia trait, where uniformly small red blood cells are produced due to a genetic defect in haemoglobin synthesis. A low RDW is rarely alarming on its own but should always be reviewed in the context of your MCV, haemoglobin, and clinical symptoms.

A low RDW on its own is rarely a cause for urgent concern. Unlike a high RDW which is associated with a broader range of conditions, a low RDW without any other blood count abnormalities typically does not require treatment.

However, if your low RDW is accompanied by a low MCV and low haemoglobin, it may point to thalassaemia or anaemia of chronic disease, both of which require appropriate medical management. Your doctor will review the full picture and advise on any further testing needed, such as haemoglobin electrophoresis to confirm or exclude thalassaemia.

Both thalassaemia trait and iron deficiency anaemia produce small red blood cells and a low MCV. This overlap used to make differentiation challenging from the CBC alone. However, the RDW provides a critical distinguishing clue: thalassaemia produces uniformly small cells (low or normal RDW), while iron deficiency produces cells that vary widely in size (high RDW).

This distinction, often called the Mentzer Index when a calculation is applied, allows clinicians to triage patients appropriately. Those with a low RDW and low MCV are directed toward haemoglobin electrophoresis and genetic counselling, while those with a high RDW and low MCV are typically investigated for iron deficiency and blood loss sources first.

While medications more commonly cause elevated RDW, some treatments that suppress bone marrow activity or reduce overall red blood cell production can result in uniformly smaller or fewer cells and a low RDW. Chemotherapy agents and immunosuppressants are among the drug classes most associated with changes to red blood cell morphology and distribution.

If you are on long-term medication and your RDW is low alongside other abnormal CBC values, inform your prescribing doctor so they can assess whether your current treatment may be contributing to the finding.

The RDW blood test itself is not a painful procedure. It requires a standard venepuncture, where a small needle is inserted into a vein in your arm, typically the median cubital vein in the inner elbow, to collect a blood sample of approximately 3 to 5 millilitres. Most patients experience only a brief, mild sharp sensation when the needle is inserted.

If you are anxious about needles or have had previous episodes of fainting during blood draws, tell the phlebotomist before the procedure. Lying down rather than sitting can significantly reduce the risk of a vasovagal reaction, and applying a topical anaesthetic cream beforehand (available from pharmacies) can reduce needle sensation for those who are particularly sensitive.

RDW results form part of a CBC which is processed by an automated haematology analyser. Most NHS laboratories process routine CBC samples within 24 to 48 hours of receipt. Results are then reviewed by the requesting clinician before being communicated to the patient, which typically means you receive your results 2 to 5 working days after your blood draw.

Private diagnostic services often offer faster turnaround times, with some providing results within the same day or the following morning when samples are received by midday. Online results portals on private testing platforms typically notify you by email when your results are ready to view.

The risks associated with an RDW blood draw are minimal and are the same as those for any routine venepuncture. They include mild bruising or soreness at the puncture site, occasional lightheadedness or fainting in susceptible individuals, rare formation of a small haematoma under the skin, and a very small risk of infection at the site.

These risks are significantly reduced by using sterile single-use needles, proper site sterilisation with antiseptic swabs, applying gentle pressure after the draw, and staying hydrated before your appointment. Patients with bleeding disorders or those on anticoagulant medications should inform their phlebotomist before the procedure.

In the laboratory, your blood sample is loaded into an automated haematology analyser. The machine passes the sample through a flow cell where each individual red blood cell is measured for its volume using electrical impedance or light scattering technology. Thousands of cells are measured within seconds.

The analyser then calculates the statistical distribution of all measured red blood cell volumes. The RDW-CV is calculated as the standard deviation of red blood cell volumes divided by the mean corpuscular volume, then multiplied by 100 to express the result as a percentage. This mathematical measure captures how spread out the distribution of cell sizes is around the average.

If your RDW result is above 14.5%, the most important first step is to speak to your GP rather than attempting to self-diagnose or self-treat. Your doctor will review your full CBC alongside your symptoms, medical history, and any relevant risk factors to determine what is causing the elevation.

Depending on the pattern of results, your GP may request additional blood tests such as serum ferritin, vitamin B12, folate, iron studies, liver function tests, or a reticulocyte count. Treatment will then be directed at the underlying cause rather than the RDW value itself. Do not start iron or B12 supplements without medical guidance as taking the wrong supplement can mask the actual diagnosis.

Yes, when an elevated RDW is caused by a nutritional deficiency, improving dietary intake of the deficient nutrient alongside medical supplementation can normalise RDW over time. For iron deficiency, increasing dietary iron through red meat, leafy green vegetables, lentils, and fortified foods, alongside prescribed ferrous sulfate tablets, supports red blood cell recovery.

For B12 deficiency, dietary sources include meat, fish, eggs, and dairy. Vegans typically require B12 supplementation. For folate deficiency, dark green vegetables, beans, and citrus fruits are valuable dietary sources. However, dietary changes alone are rarely sufficient to correct established deficiencies quickly. Medical supplementation prescribed by your GP remains essential.

The frequency of repeat testing depends on the underlying condition being treated and its severity. For iron deficiency anaemia treated with oral supplements, most GPs recommend a repeat CBC after 4 to 6 weeks to assess haemoglobin response, with a further check at 3 months to confirm full recovery and normalisation of the red blood cell profile including RDW.

For B12 deficiency treated with intramuscular injections, a repeat CBC is typically checked 8 to 12 weeks after treatment begins. For chronic conditions such as kidney disease or chronic inflammatory disease causing anaemia, blood count monitoring frequency will be guided by the specialist managing those underlying conditions.

The RDW value itself does not directly cause symptoms. It is the underlying condition driving the elevated RDW that affects daily life. For example, if iron deficiency anaemia is the cause, you may experience fatigue, reduced exercise tolerance, difficulty concentrating, and breathlessness, all of which can impact work and daily activities until haemoglobin levels are restored.

Once the underlying cause is identified and treated appropriately, most patients see a significant improvement in energy levels and overall wellbeing within 4 to 8 weeks, with the RDW itself gradually normalising over the following 2 to 3 months as healthy red blood cells replace the abnormal ones.

You should seek urgent medical attention if you experience severe breathlessness at rest, chest pain, rapid or irregular heartbeat, extreme fatigue preventing normal function, fainting, very pale or yellow skin, dark tarry stools (which may indicate internal bleeding), or blood in your urine or vomit. These symptoms alongside an abnormal blood count may indicate a serious underlying condition requiring immediate evaluation.

For non-urgent but concerning results, contact your GP within a few days of receiving your report. Do not wait weeks to follow up on an abnormal RDW result if you have any symptoms. If your GP is unavailable, NHS 111 can advise on the urgency of your situation and direct you to appropriate care.