RDW-SD Blood Test: Normal Range & Causes

What Is RDW-SD?

When I review a full blood count with a patient, one value that often raises a quiet eyebrow is the RDW-SD. It measures how varied the sizes of your red blood cells are – think of it as the 'size diversity' index of your erythrocytes. RDW stands for Red Cell Distribution Width, and the SD suffix means it is calculated using the standard deviation method.

This test is a standard part of the complete blood count (CBC) and is reported alongside other indices like MCV and MCH. It tells your doctor whether your red cells are uniform in size (normal) or have a wide range of sizes (anisocytosis).

How Is RDW-SD Different from RDW-CV?

You may also see a value called RDW-CV on your lab report. Both describe red cell size variation but are calculated differently. RDW-SD uses the absolute width of the red cell histogram (in femtolitres), while RDW-CV is a coefficient of variation derived from the MCV and SD.

In my clinical practice, I find RDW-SD more sensitive in certain cases, particularly when red cell fragments or very small cells are present. RDW-CV can be falsely normal in some anemias, so I always check both if a patient has unexplained anemia.

Normal Reference Ranges for RDW-SD

Reference ranges can vary slightly between laboratories, but the values below are typical for adults and children. RDW-SD is generally not affected by gender.

*Always interpret your results using the reference range provided by the lab that analysed your sample.

What Does a High RDW-SD Mean?

An elevated RDW-SD indicates that your red blood cells vary significantly in size. This is often seen in conditions where the bone marrow is producing new, larger cells (reticulocytes) while older, smaller cells are still circulating. Common causes include:

• Iron deficiency anaemia – especially in the early stages, RDW-SD rises before MCV drops.
• B12 or folate deficiency – leads to a mixed population of large and normal-sized cells.
• Recent blood transfusion – transfused cells mix with your own.
• Chronic liver disease – alters red cell membrane.
• Sickle cell disease or thalassaemia – cell fragments and varied sizes.

I often see a high RDW-SD in patients with early iron deficiency who have not yet developed microcytosis. This makes it a very useful early marker.

Is a Low RDW-SD a Concern?

Low RDW-SD is much less common and typically means that the red cells are uniformly small (microcytic) or uniformly large (macrocytic). A low RDW-SD can be seen in:

• Homogeneous microcytic anaemia – for example, long-standing iron deficiency or thalassaemia trait.
• Macrocytic anaemia with uniform macrocytosis – as in certain B12 deficiencies.
• Myelodysplastic syndromes – in rare cases where red cell production is abnormally uniform.

Low RDW-SD is rarely an isolated concern; your doctor will assess it alongside MCV and other indices.

RDW-SD in Pregnancy

During pregnancy, the blood volume expands and iron stores are often depleted. I regularly check RDW-SD in pregnant women to detect early iron deficiency anaemia before haemoglobin drops. A rising RDW-SD in the second trimester can be the first sign that supplementation is needed.

Physiological changes in pregnancy can increase RDW-SD mildly, but any significant rise above 50 fL warrants further investigation.

LOINC Standard Name

The internationally recognised LOINC code for this test is 77606-2, and the standard name is Red Cell Distribution Width - SD.

If you have questions about your RDW-SD result, please discuss it with your healthcare provider – a single number rarely tells the whole story.