Understanding MSC Exosome Dosing: Particle Count vs. Protein Concentration
When clinicians and researchers compare mesenchymal stem cell (MSC) exosome preparations, one of the most common points of confusion is dosing. Unlike a small-molecule drug measured in milligrams, extracellular vesicles are quantified in several different ways — and those numbers are not interchangeable. Understanding what each measurement represents is essential when evaluating research-grade material.
Why “dose” is not a single number
An exosome preparation can be described by the number of particles it contains, by its total protein content, or by both. Each metric answers a different question, and a preparation that looks concentrated by one measure may look modest by another. For research and comparison purposes, it is important to know which unit is being reported.
Particle count (measured by NTA)
Nanoparticle Tracking Analysis (NTA) estimates the number of particles per milliliter and their size distribution. A typical report describes billions of particles in a given volume, along with a mean and mode diameter in the expected exosome range (roughly 30–150 nm). Particle count is the metric most often used to describe how concentrated a preparation is.
Protein concentration
Total protein (often measured by BCA or a similar assay) reflects the amount of protein in the sample. Protein can come from the vesicles themselves and from co-isolated non-vesicular material. Because of this, a high protein reading does not automatically mean a high number of exosomes — it can also indicate impurities.
The particle-to-protein ratio
This is where the two measurements become most useful together. The particle-to-protein ratio (particles per microgram of protein) is widely used in the literature as an indicator of purity. A higher ratio generally suggests that more of the protein present is associated with vesicles rather than contaminants. When comparing preparations, this ratio is often more informative than either number alone.
What to look for on documentation
- Particle concentration and size distribution (NTA)
- Total protein concentration and the assay used
- A stated particle-to-protein purity ratio where available
- Identity markers confirming the vesicles are exosome-derived
You can review how these parameters are characterized in our Exosome Research Library, and licensed practitioners can request per-lot documentation through the Physician Portal.
Research-use-only. This article is provided for educational purposes and is not intended to diagnose, treat, cure, or prevent any disease.