Seeing RDW-SD blood test high on a complete blood count (CBC) can be confusing, especially when the rest of your blood results appear normal. RDW-SD is an index that looks at the variation in the size of your red blood cells. A high result does not diagnose a condition by itself, but it can provide an important clue when interpreted alongside markers such as MCV, haemoglobin, RBC count, and iron or vitamin levels. In many cases, an elevated RDW-SD is associated with nutritional deficiencies or changes in red blood cell production. However, it can also occur with blood loss, chronic disease, or other conditions.
Understanding what the number means and what to do next can help put an abnormal result into perspective.
What Is RDW-SD on a Blood Test?
If you are wondering what is RDW-SD in blood test results, RDW-SD stands for red cell distribution width-standard deviation. It measures how much the volume of your red blood cells varies from one cell to another. Healthy red blood cells tend to be relatively similar in size. When there is a greater difference between the smallest and largest cells, the RDW-SD increases. This variation in red blood cell distribution width is known as anisocytosis.
RDW-SD is reported in femtoliters (fL), a unit of volume. Unlike RDW-CV, it is an absolute measurement of the distribution of red blood cell volumes rather than a percentage. It is commonly reported as part of a CBC and is generally interpreted together with other red blood cell indices.
RDW-SD vs RDW-CV: What's the Difference?
The RDW-SD vs RDW-CV difference mainly comes down to how variation in red blood cell size is calculated and reported
Because laboratories may use different analysers and reference intervals, your own report's range should be used when deciding whether a result is high.
RDW-SD Normal Range
There is no single universal RDW-SD normal range that applies to every laboratory. Reference intervals can differ according to the analyser, laboratory method, and population used to establish the range. For this reason, the reference range printed beside your result is the most appropriate one to use.
A commonly encountered adult reference interval for RDW-SD is approximately 39–46 fL, although some laboratories may use a different range.
A result slightly outside the reference interval does not automatically mean that something is wrong. Reference ranges are statistical guides, and healthcare professionals interpret abnormal results in the context of symptoms, medical history, and other laboratory findings.
What Causes a High RDW-SD?
A RDW-SD blood test high means your red blood cells differ more in size than expected. The possible causes vary, and the most likely explanation often becomes clearer when RDW-SD is compared with MCV, haemoglobin, and other CBC findings.
Nutritional Deficiencies (Most Common)
Several nutritional deficiencies can affect how red blood cells are produced and may increase RDW-SD:
- Iron deficiency: One of the most common causes of an elevated RDW. Iron deficiency can lead to the production of smaller red blood cells, creating greater variation in cell size. Causes can include inadequate dietary intake, reduced absorption, or ongoing blood loss.
- Vitamin B12 deficiency: Vitamin B12 is essential for normal red blood cell formation. Deficiency can interfere with cell development and may eventually lead to larger-than-normal red blood cells and increased RDW.
- Folate deficiency: Folate is important for healthy red blood cell production. A deficiency can result in abnormal cell development and increased variation in RBC size.
When nutritional deficiencies are suspected, RDW-SD should be interpreted alongside MCV, haemoglobin, ferritin, iron studies, vitamin B12, and folate levels where appropriate.
Mixed or Recovering Blood Conditions
RDW-SD can rise when the bloodstream contains populations of red blood cells of noticeably different ages or sizes. For example, after blood loss, the bone marrow increases production of new red blood cells. These younger cells, called reticulocytes, are larger than mature red blood cells. A mixture of older and newer cells can therefore increase the variation in RBC size.
A similar pattern may occur while someone is recovering from a nutritional deficiency after treatment has started. Newly produced cells may have a different size from the older cells already circulating in the blood. This is one reason an RDW-SD blood test high result should not automatically be interpreted as evidence of an ongoing deficiency. The timing of the test and previous blood results matter.
Chronic and Other Conditions
An elevated RDW can also be associated with chronic or systemic conditions. These may include liver disease, kidney disease, diabetes, cardiovascular disease, chronic inflammatory conditions, and some cancers. RDW may also be abnormal in certain inherited blood disorders. However, RDW-SD alone cannot identify which condition is responsible. Other CBC findings and, when appropriate, additional tests are needed to establish the cause.
High RDW-SD With Normal MCV: What It Means
A RDW-SD blood test high with a normal MCV can seem contradictory, but the two measurements assess different aspects of red blood cell size.
RDW-SD Measures Variation in Cell Size
RDW-SD reflects how widely red blood cell volumes are distributed. A high result means there is more variation between individual cells, even if the overall average remains within the expected range.
MCV Measures Average Cell Size
MCV measures the average volume of your red blood cells. Because it is an average, it can remain normal even when the blood contains a mixture of smaller and larger cells.
Why Both Results Can Occur Together
For example, early iron deficiency may produce a mixture of normal-sized and smaller red blood cells before the average MCV falls below the reference range. Similarly, developing vitamin B12 or folate deficiency, recent blood loss, or recovery from anaemia can create mixed populations of cells. This pattern does not establish a diagnosis. A healthcare professional may consider haemoglobin, MCV, MCH, RBC count, reticulocytes, ferritin and other iron studies, and vitamin B12 or folate levels when clinically appropriate.
Symptoms That May Accompany a RDW-SD Blood Test High
RDW-SD itself does not cause symptoms. Any symptoms you experience are more likely to be related to the underlying condition affecting red blood cells. If an elevated RDW occurs alongside anaemia, possible symptoms can include:
- Fatigue or unusual weakness
- Shortness of breath, particularly with activity
- Dizziness or light-headedness
- Headaches
- Pale skin
- Cold hands or feet
- A rapid or irregular heartbeat in some cases
Some people with an abnormal RDW have no symptoms at all, particularly when the underlying change is mild or developing gradually.
Next Steps If Your RDW-SD Blood Test High
If your RDW-SD blood test high result appears on a CBC, avoid trying to diagnose the cause from that number alone. The next step is to look at the overall pattern. A healthcare professional may review:
What Does a High RDW-SD Really Mean?
An RDW-SD blood test high result means that your red blood cells have a greater variation in volume than the laboratory considers typical. It is a clue, not a diagnosis. Nutritional deficiencies, blood loss, recovery from anaemia, and chronic or systemic conditions can all contribute to an elevated result. The most useful interpretation comes from considering RDW-SD alongside MCV, haemoglobin, and other CBC findings.
If you have received an abnormal CBC, MySimpleReport can help you understand your laboratory results and organise your health information so you can have a more informed conversation with your healthcare professional.
Frequently Asked Questions
Is a high RDW-SD always a sign of anemia?
No. A high RDW-SD can occur with anaemia, but it does not prove that anaemia is present. Haemoglobin and haematocrit need to be considered to determine whether anaemia exists.
Can dehydration cause high RDW-SD?
Dehydration can affect some blood measurements, but an isolated high RDW-SD should not automatically be attributed to dehydration. If the result is unexpected, it is better to interpret it alongside the complete CBC and clinical context.
Can RDW-SD change over time?
Yes. RDW-SD can change as red blood cell production changes. For example, correcting a nutritional deficiency or recovering after blood loss can temporarily produce a mixture of older and newer red blood cells.
Can RDW-SD be high during pregnancy?
Changes in blood counts are common during pregnancy, but an RDW-SD blood test high still needs to be interpreted in context. Iron deficiency is particularly important to consider during pregnancy, so a healthcare professional may review haemoglobin and iron studies when appropriate.