If you have ever looked at a blood test report, you know how confusing the rows of abbreviations and numbers can be. One value that often catches the eye is RDW, short for red blood cell distribution width. When the RDW result is flagged as high, it is natural to wonder what it means, whether it signals a serious problem, and what you should do next. This guide explains RDW in plain language, describes why it can be high, and explains how doctors usually interpret it alongside other results. It is general educational information only and is not a substitute for medical advice. If any result on your report worries you, or if you have symptoms that concern you, talk to a clinician who can review your full history and results.
Red blood cells carry oxygen around the body, and they are not all exactly the same size. In a healthy sample, most red blood cells fall within a fairly narrow size range. RDW is a measurement of how much variation there is in the size of your red blood cells. A low RDW means the cells are fairly uniform in size. A high RDW means there is more variation than usual, with some cells noticeably larger or smaller than others. The value is usually reported as a percentage, and laboratories set their own reference ranges, so what counts as high can differ slightly from one lab to another. That is one reason a single flagged value should never be read in isolation.
What RDW Actually Measures
To understand RDW, it helps to picture what a blood sample looks like under analysis. A laboratory analyser counts thousands of red blood cells and measures the size of each one. It then calculates how spread out those sizes are. If most cells cluster closely around the average size, the distribution is narrow and the RDW is lower. If the sizes are spread widely, the distribution is broad and the RDW is higher. RDW does not measure how many red blood cells you have, how much haemoglobin they contain, or how well they carry oxygen on their own. Those jobs belong to other values on the same report, such as the red blood cell count, haemoglobin, and mean corpuscular volume, often shortened to MCV.
MCV and RDW are frequently read together because they complement each other. MCV tells you the average size of your red blood cells, while RDW tells you how much the individual cells vary around that average. Two people can have the same average cell size but very different RDW values. For example, one person might have cells that are all close to the average, while another has a mix of very small and very large cells that happen to average out to the same number. That difference is exactly what RDW captures, and it is why clinicians look at the pattern across several values rather than treating any one number as a diagnosis by itself.
Why RDW Is Read Alongside MCV
When RDW is high and MCV is low, the pattern suggests a mixture that includes many smaller-than-usual cells. When RDW is high and MCV is high, the mixture includes many larger-than-usual cells. When RDW is high and MCV is normal, the sample may contain both smaller and larger cells balancing each other out. Each pattern points a clinician toward different follow-up questions, such as diet, recent illness, medication, or the need for further blood tests. None of these patterns, on its own, identifies a single condition. They are clues that help a clinician decide what to investigate next, not conclusions.
What Counts As A Normal Range
Reference ranges vary by laboratory, age, and sometimes sex, so there is no single universal number that applies to everyone. In general, many laboratories report an RDW reference range roughly in the low teens as a percentage, but you should always compare your result with the range printed on your own report. A value just above the range is different from a value far above it, and a value that has risen gradually over several reports may be viewed differently from a one-off result. If your report does not show a range, or if you are unsure how to read it, ask the clinician or laboratory that ordered the test to explain it in context.
Common Reasons RDW Can Be High
A high RDW is a non-specific finding, which means it can occur in many different situations and does not point to one particular cause. One of the most commonly discussed contributors is a shortage of nutrients needed to make healthy red blood cells, such as iron, vitamin B12, or folate. When the body lacks one of these nutrients, red blood cell production can become uneven, producing cells of varying sizes. This is one reason clinicians often check iron studies or vitamin levels when RDW is high, especially if haemoglobin is also low.
Another broad group of causes relates to conditions that affect red blood cell production or lifespan. Some long-term health conditions, ongoing inflammation, and certain inherited blood conditions can all be associated with greater variation in cell size. Recent blood loss, recovery from an illness, or a recent blood transfusion can also temporarily change the mix of cell sizes in circulation, because newer and older cells may differ. Even the natural process of red blood cells ageing and being replaced means that some variation is always present. The key point is that a high RDW is a starting point for questions, not an answer in itself.
Nutrient Shortages And Red Cell Production
Red blood cells are produced continuously in the bone marrow, and that process depends on a steady supply of building materials. Iron is needed to make haemoglobin, the substance that carries oxygen. Vitamin B12 and folate are needed for cells to divide and mature properly. When any of these is in short supply, the marrow may release cells that are smaller or larger than usual, or a mixture of both, which raises RDW. Diet, absorption problems in the gut, increased demand during pregnancy, and some medications can all affect nutrient levels. A clinician who suspects a nutrient shortage will usually confirm it with dedicated tests rather than relying on RDW alone.
Other Conditions Linked To A High RDW
Beyond nutrient shortages, a high RDW has been observed alongside a wide range of health conditions, including chronic inflammation, liver or kidney conditions, and disorders of the bone marrow. Researchers have also noted associations between higher RDW and poorer outcomes in some serious illnesses, but an association in research does not mean RDW causes those illnesses or predicts them for any individual. For an individual patient, the more useful question is usually simpler: is there an explanation in this person’s history, diet, medications, or other test results, and does anything need follow-up? That is a question for the treating clinician, who can see the whole picture.
Symptoms And When To Seek Care
A high RDW by itself often causes no symptoms at all. Many people only learn about it because it appears on a routine blood test. When symptoms are present, they are usually related to whatever underlying issue may be contributing to the result, such as tiredness, weakness, pale skin, shortness of breath, dizziness, or a rapid heartbeat. These symptoms are also non-specific and can have many causes, so they do not confirm any particular condition either. The safest approach is to treat symptoms and test results as two separate pieces of information to discuss with a clinician, rather than trying to connect them on your own.
You should seek medical advice promptly if you notice concerning symptoms such as persistent tiredness, breathlessness, chest discomfort, fainting, unusual bruising or bleeding, or a rapid or irregular heartbeat. You should also contact a clinician if a high RDW result is new, much higher than before, or appears alongside low haemoglobin or other flagged values. Bring your full report to the appointment, including the laboratory reference ranges and any previous results you have, because trends over time are often more informative than a single reading. Do not try to diagnose yourself from one value, and do not start or stop any supplement or medication based on an RDW result alone.
What Usually Happens After A High RDW Result
When a clinician sees a high RDW, the next step depends on the rest of the report and the patient’s history. Often the first step is simply to review the other blood values, ask about diet, symptoms, medications, and recent illness, and decide whether any follow-up test is needed. Common follow-up tests may include a repeat blood count, iron studies, or vitamin B12 and folate levels. In some cases, no further testing is needed at all, and the value is simply watched over time. In other cases, the clinician may refer the patient for more specialised assessment. The pathway varies widely, which is why two people with the same RDW value can receive quite different advice.
It is also worth knowing that RDW can change over time. A value that is high during an illness or while recovering from blood loss may return to the usual range once the underlying situation resolves. On the other hand, a value that stays high or keeps rising may prompt further investigation. Keeping a record of your results, with dates, makes it easier for you and your clinician to spot trends. If you change laboratories, remember that reference ranges may differ, so compare each result with the range on its own report rather than with a range from a different lab.
Questions Worth Asking Your Clinician
If you want to make the most of an appointment about a high RDW result, it helps to arrive with a few questions. You might ask what your RDW value is compared with the laboratory range, whether your other blood values are normal, whether any follow-up test is recommended, and whether anything in your diet, medications, or health history could explain the result. You could also ask whether the result needs monitoring and, if so, when it should be rechecked. Writing down the answers helps you avoid misremembering numbers later, and it gives you a clear record to compare with future results.
Common Misunderstandings About RDW
One common misunderstanding is that a high RDW means a serious disease is present. In reality, RDW is a descriptive measurement of cell size variation, not a disease test. It can be raised in mild, temporary situations as well as in more serious ones, and the result alone cannot distinguish between them. Another misunderstanding is that lowering RDW should be a goal in itself. RDW is not something to treat directly; if an underlying cause is found and addressed, RDW may normalise as a side effect, but chasing the number without understanding the cause is not helpful.
A third misunderstanding is that a normal RDW rules out all blood problems. It does not. Some conditions affect red blood cells without increasing size variation, so a normal RDW does not guarantee that every other value is normal. That is why blood reports contain many values, and why clinicians interpret them as a set. Reading a single line of a report and drawing a conclusion from it, in either direction, is a common source of unnecessary worry. The full report, read by someone who knows your history, is the reliable unit of interpretation.
| Pattern On The Report | What It Describes | What It Usually Prompts |
|---|---|---|
| High RDW, low MCV | Mixture including many smaller cells | Review of iron status and other causes of small cells |
| High RDW, high MCV | Mixture including many larger cells | Review of vitamin B12, folate, and related factors |
| High RDW, normal MCV | Both smaller and larger cells present | Look for mixed causes or recent changes in cell production |
| High RDW, normal haemoglobin | Size variation without low blood count | Often monitoring, with follow-up if the trend changes |
Frequently Asked Questions
Is a high RDW dangerous by itself?
Not necessarily. RDW describes variation in red blood cell size, and it can be raised in many situations, including temporary ones. Whether it matters depends on the size of the increase, the other values on the report, and your health history. A clinician can judge that context; the number alone cannot.
Can diet alone bring RDW down?
A balanced diet supports healthy red blood cell production, but diet is only one of many factors that can influence RDW. If a nutrient shortage is confirmed, a clinician may discuss dietary changes or supplements. Do not start supplements based on an RDW result alone, because excess intake of some nutrients carries its own risks.
How often should RDW be rechecked?
There is no universal schedule. Some clinicians recheck after a few weeks or months, others simply watch it at the next routine test. The timing depends on why it was high and whether any follow-up test or treatment is under way. Ask your clinician what interval makes sense for you.
Does RDW mean the same thing in children?
The concept is the same, but reference ranges differ by age, and children’s blood values change as they grow. Paediatric results should always be interpreted with age-appropriate ranges and by a clinician familiar with children’s health.
Conclusion
A high RDW level means that your red blood cells vary more in size than usual. It is a useful clue, but it is not a diagnosis, and it does not by itself identify a single cause. Nutrient shortages, recent illness or blood loss, ongoing inflammation, and a range of other conditions can all be associated with a higher value, and many people with a mildly raised RDW need nothing more than monitoring. The most sensible next step is to review the result with a clinician, look at the full blood report and your health history together, and follow whatever follow-up, if any, is recommended. If you have concerning symptoms, or if a result on your report worries you, do not wait for the next routine appointment to raise it — contact a clinician and ask for the result to be explained in context. For more plain-language guides, see our guide to everyday terms explained simply and our explainer on common abbreviations.
