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In a national study of 8,175 adults aged 45 or older, a higher RDW was strongly linked to a higher risk of death. This was also true for people without anaemia.[1]
On this page you can read what RDW is, what the guidelines say about a value that is too low or too high, what affects your value and which values are best to look at alongside it.
Also known as: red cell distribution width, erythrocyte distribution width, RDW-SD. At the lab: RDW.
RDW stands for red cell distribution width. The test measures how much those cells differ in size and volume. Red blood cells are made in your bone marrow. They contain haemoglobin, an iron-containing protein that carries oxygen from your lungs to every cell in your body.[2]
Normally your red blood cells are roughly the same size. The lab plots their size on a graph. If the cells lie close together, your RDW is low. If they are spread further apart, your RDW is high.[2]
RDW is often part of the full blood count. It usually appears as a percentage on your result and is compared with a reference range. That range can differ from lab to lab.[2]
Differences in the size of your red blood cells can affect how well they carry oxygen around your body. That is why RDW helps a doctor look for the cause of anaemia.[2]
A doctor may order the test if you have symptoms that fit anaemia, such as shortness of breath, weakness or tiredness, headaches, dizziness or a heartbeat that is not regular. RDW, together with other tests, can also help with an inherited blood disorder.[2]
Research in adults aged 45 or older found RDW to be a strong predictor of the risk of death in the general population. This link also held in people without anaemia.[1]
A normal RDW does not tell the whole story either. Your red blood cells can still be larger or smaller than usual, and you can still have anaemia.[2]
Labplusarts, the lab BioProfile works with, does not give a number as a limit for RDW. The lab calls it a qualitative test. It calls a value within the limits normal.[3]
| Value | What it can mean | Who it is for |
|---|---|---|
| 11% to 15%[1] | The reference range in this study. Even within this range, a higher RDW was linked to a higher risk of death. | Adults aged 45 or over without a deficiency of iron, folic acid or vitamin B12 |
| Every 1% higher[1] | A 22% higher risk of death | Adults aged 45 or over in the general population |
| Highest fifth of values[1] | Risk ratio of 2.1 for death, compared with the lowest fifth | Adults aged 45 or over in the general population |
Source: Patel and colleagues, Archives of Internal Medicine. A national study of 8,175 adults aged 45 or older. The findings apply to groups, not to one person.[1]
A low RDW means your red blood cells are roughly the same size. It is not a sign of anaemia and usually no cause for concern.[2]
A high RDW means your red blood cells vary in size more than normal. This can point to a long-term liver condition, a heart or kidney condition, a blood sugar disorder or a malignant condition, especially of the large bowel. Your doctor will then also look at your other blood values.[2]
BioProfile is working on its own zones for each value, from risk to optimal. A doctor reviews those zones before they appear in your result.
If you eat little iron and other minerals, a doctor may have your RDW measured. Together with other blood values, RDW can help detect iron deficiency.[2]
Heavy blood loss from a wound or an operation is a reason to measure RDW.[2]
If an inherited blood disorder runs in your family, or you have a long-term illness, for example of the bowel, a doctor may have your RDW measured.[2]
You do not need to prepare for the test in any special way. The reference range can differ from lab to lab, though.[2]
RDW is included in all five packages, from BioProfile 31 to BioProfile 150.[4]
The BioProfile editorial team wrote this page on 3 October 2026 from the sources below, which we read on 3 October 2026. Every number on this page appears in one of those sources, with that source's number next to it. An expert has yet to review this page; once they have, the expert's name and the date of the review will appear at the top. This page explains and does not diagnose. Talk to your GP about a value outside the range.
