RDW-SD
RDW-SD (Red Cell Distribution Width - Standard Deviation) measures the absolute variation in red blood cell size (width of the RBC volume distribution curve at 20% height).
Category: Complete Blood Count
Also known as: rdw-sd
Reference range: 36.6–50.8 fl (Source: ABIM)
Standard unit: fl
High values may indicate
Elevated RDW-SD may indicate anisocytosis (variation in RBC size), seen in iron deficiency anemia, vitamin B12/folate deficiency, or mixed anemias.
Low values may indicate
Low values are generally not clinically significant.
Frequently asked questions
- What is a normal RDW-SD level?
- A typical reference range for RDW-SD is 36.6–50.8 fl (source: ABIM). Reference ranges vary by laboratory, age, and sex, so always compare against the range on your own lab report.
- What units is RDW-SD measured in?
- RDW-SD is most commonly reported in fl. Labs in different countries may use other units — you can convert between them with BloodId's blood test unit converter.
- Where do RDW-SD reference ranges come from?
- BloodId uses ABIM standard reference ranges and educational information from MedlinePlus (see our methodology). Reference ranges still vary by lab, age, and sex, so always compare against your own report.
- Can I track RDW-SD over time?
- Yes. Upload results from any lab and BloodId charts your RDW-SD over time against its reference range, so you can see whether the trend is rising, falling, or stable.
Related biomarkers
Source: BloodId admin-reviewed