Evidence checked, 25 October 2026. General information, not individual medical advice.
What does RDW mean on a blood test?
RDW describes how much your red blood cells differ in size. A high result means there is more variation than the laboratory expects. It does not tell you the cause on its own. A low result means the cells are more similar in size and is not usually a concern by itself. The useful next step is to read RDW alongside haemoglobin, MCV, symptoms and previous results. [1,2]
Finding an unfamiliar abbreviation on your results can turn a routine check into an evening of searching. You do not need to diagnose yourself from it. A proper review explains what the result changes, whether anything needs checking and when to follow up. That is more useful than a list of every illness ever associated with the marker.
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What does RDW measure, and how is it different from MCV?
Red blood cells carry oxygen using haemoglobin. RDW is a measure of variation in their size. MCV, or mean cell volume, describes their average size. The two measures answer different questions. A population of cells can have an average size within the reference range while still containing an unusually wide mixture of smaller and larger cells. [1,2]
Your report may show RDW-CV as a percentage or RDW-SD in femtolitres. They are not interchangeable numbers. Use the units and the reference interval printed on that specific report. Comparing a value from one laboratory with an internet range from another can create a problem that is not really there.
Haemoglobin also matters. Anaemia is not diagnosed from RDW alone. A clinician considers whether haemoglobin is low, what the other blood-count measures show and what is happening clinically. A normal RDW does not exclude anaemia or every cause of symptoms. [1]
Why might RDW be high?
A raised RDW can help a clinician investigate anaemia. Iron deficiency is one possibility, but there are others, including vitamin deficiencies and conditions that affect red-cell production or survival. Different explanations can coexist. Recent treatment, bleeding or transfusion may also change the picture a clinician needs to interpret. [1,2]
It is tempting to read the first online association as the answer. That is not how this test works. Research can find an association between a marker and an illness across a population without making that marker a useful stand-alone diagnostic test for an individual. A raised RDW is not, by itself, a diagnosis of cancer, heart disease or any other named condition. [1,2]
The right question is: does the pattern suggest a cause that needs checking in this person? Your history, haemoglobin and other results help answer that. An isolated flag should lead to a proportionate explanation, not automatic treatment or an unrestricted panel of tests.
Which other results help make sense of it?
Bring the full report rather than a cropped RDW screenshot. The following details often make the discussion more useful:
- Haemoglobin and whether it has changed since your previous test.
- MCV and the laboratory's reference range.
- The rest of the full blood count, including white cells and platelets.
- Any iron studies, ferritin, vitamin B12 or folate already measured.
- The date of the test and any recent treatment, illness or bleeding.
A clinician decides which of these needs further attention. Do not assume that every item needs repeating. For example, evidence of iron deficiency calls for a conversation about why iron is low, not simply how to make a blood-count flag disappear. Test selection should follow the clinical question. [1,2]
If your results have been taken across several services, put them in date order with units visible. A trend can be more informative than one value, although it still needs interpretation. Tell the doctor if you have started supplements since the sample was taken.
Should I take iron or vitamins for a high RDW?
Not on RDW alone. Treatment depends on the cause. Taking a supplement because it appears in an online explanation can make it harder to understand the original problem and may not address what is actually happening. Your doctor may recommend targeted tests, treatment, a repeat blood count or no immediate intervention, depending on the wider picture. [1,2]
Share what you already take, including over-the-counter iron, multivitamins and prescribed medicines. Do not stop a prescribed treatment without advice. If a deficiency is confirmed, ask what caused it, what treatment is appropriate and which result or symptom will be used to judge progress.
A good plan has an end point. You should know whether follow-up is needed and who will check it. Repeating a test without a question can produce another report without giving you an answer.
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When does the result need prompt medical attention?
Urgency depends on symptoms and the whole blood count, not a universal RDW cut-off. Contact the service that ordered the test if it has labelled the result urgent or asked you to arrange a review. Do not wait for a routine appointment if you have severe breathlessness, chest pain, collapse or major ongoing bleeding. Those symptoms need urgent help, irrespective of RDW. [3]
Less dramatic but persistent symptoms still deserve a conversation. Tell the clinician about fatigue, reduced exercise tolerance, dizziness, changes in bleeding or other symptoms that prompted the test. A number within range should not be used to dismiss symptoms that remain unexplained.
What should a private blood-results review include?
The value is in the interpretation and a clear next step, rather than the largest possible marker count. At The Wellness, the published blood-testing options, as of October 2026, include Longevity+ at £1,395, Advanced Health at £650 and Core Wellness at £350. These are package prices, not a quote for an isolated RDW test or a guarantee that a particular package is appropriate for your concern. Ask the team to confirm what you actually need and the complete fee. [4]
If you already have results, say that when enquiring. Send the full report through the route the team recommends and explain what you want to understand. You may need a consultation rather than another broad screening panel. An appointment should leave you knowing what the result means for you, what remains uncertain and what happens next.
What can I prepare before the appointment?
Write down when symptoms began, any recent blood loss or illness, the medicines and supplements you take, and your main question. Bring previous reports if you have them. Ask:
- Is this an isolated RDW change or part of a wider pattern?
- Is there evidence of anaemia or a specific deficiency?
- Would another test change the plan?
- Do I need treatment, a repeat sample or a review of the cause?
- Who will follow up, and when?
You do not have to arrive with a diagnosis. The point of the review is to replace a worrying abbreviation with an explanation you can use.
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