Why a nanoparticle count is not a dose

What a particle count measures

The standard instruments for counting in this size range detect individual particles by the way they scatter or block light, or by the way they move under Brownian motion, and infer a size distribution and a concentration from that behaviour. They are useful instruments and they measure what they measure well.

What they cannot do is tell you the nature of the particle. A protein aggregate, a lipoprotein particle, a piece of debris and a vesicle all scatter light. Instruments differ in their lower size limit and in how they handle heterogeneous samples, and the same sample measured on two platforms can yield different numbers. That is a known feature of the measurement, not a scandal, but it does mean a bare number carries less information than it appears to.

The three gaps between count and effect

Between a number on a specification sheet and a change in a person's skin sit three unmeasured quantities.

Gap one: how many particles are vesicles

Addressed by purity measures. The most widely used is the ratio of particles to total protein: a preparation carrying a large protein load per particle contains a lot of material that is not vesicle. Marker profiling adds more, particularly when negative markers are included to demonstrate what was depleted. A count reported without any purity measure leaves this gap entirely open.

Gap two: how much active cargo each vesicle carries

Vesicles from the same preparation are not identical. Cargo distribution across a population is uneven, and the fraction of vesicles carrying any given molecule can be small. Since the identity of the active principle in aesthetic preparations is generally not established, this gap cannot even be posed precisely for most products. It is the reason particle count and potency are not interconvertible.

Gap three: how much reaches a target cell

Determined by route, by tissue barriers, by clearance and by the delivery efficiency discussed in how vesicles are taken up by cells. Applying material to a skin surface and measuring what arrives at a fibroblast are very different propositions, and the second is rarely attempted outside research settings.

A higher particle count indicates a more potent preparation.

Proposed mechanism

More particles deliver more cargo, producing a larger biological effect.

What has been shown

Particle concentration can be measured reproducibly within a platform. No published conversion exists between particle count and biological potency for aesthetic preparations, because the active principle is generally unidentified. Potency assays measuring a defined biological readout are the standard approach in fields where this problem has been solved, and are rarely reported for products in this market.

Highest level reached

Not shown

Main confounders

Instrument platform differences. Non-vesicular particles counted alongside vesicles. Uneven cargo distribution across the vesicle population. Aggregation, which reduces count while increasing material.

GradeNOT SUPPORTED

What would change thisA validated potency assay reporting a defined biological readout per unit of product, with demonstrated dose response, and published batch data showing that readout tracks with particle count. Until such an assay exists, count and potency are separate quantities.

Why bigger numbers are easy to produce

Several routine choices increase a reported count without increasing the amount of anything biologically relevant.

• Measure lower. Lowering the detection threshold brings smaller objects into the count, including non-vesicular material.
• Concentrate more. Reducing volume raises concentration. This is not a change in total content, and if the number is quoted per millilitre rather than per vial, it can double while the vial contains the same material in half the liquid.
• Purify less. A cruder preparation retains more particles of all kinds. Purity and count often move in opposite directions.
• Choose a favourable platform. Platforms differ systematically in the size range they see well.

None of these is fraudulent. Each is a legitimate methodological choice. Together they mean that comparing counts between suppliers, measured on unstated platforms with unstated thresholds, is not a comparison of anything.

The potency assay, and its absence

In fields where biological products are regulated as medicines, potency is established by an assay: a defined biological readout, validated, with an acceptance range, run on each batch. That is how a manufacturer demonstrates that batch two does the same thing as batch one. It is demanding, expensive and unavoidable when a product is licensed.

The absence of such an assay in most of this market is the single most informative fact about it. It means batch equivalence is being asserted on the basis of physical similarity rather than biological similarity. A supplier that has developed a potency assay has done something substantial and will usually say so in detail. We set out what a full specification looks like in what a real specification sheet shows.

What we would accept as a dose statement

A meaningful dose statement would name the active principle, state the quantity of it per unit of product, state the route, and reference a study in which that quantity by that route produced a measured effect. Every part of that sentence is doing work. In the absence of any of it, the number on the box tells you what was counted, not what will happen.

This is not a demand for perfection. Plenty of legitimate products in medicine began life with incomplete characterisation. It is a demand that the number be described accurately. A particle count is a particle count. Calling it a dose is a category error, and once you have seen it, you will see it on nearly every specification sheet in the sector.

Questions readers ask

What does a particle count actually tell you?

It tells you how many light-scattering objects within a size range were detected in a measured volume on a particular instrument. It does not tell you what those objects are or what they will do.

Is a higher count better?

Not on its own. Counts can be raised by lowering the detection threshold, concentrating the sample, or purifying less. Without a purity measure alongside, a higher count may indicate a cruder preparation.

What is a particle to protein ratio?

A purity indicator comparing the number of particles to the total protein present. A low ratio suggests a large amount of non-vesicular protein accompanying the particles. It is one of the simplest useful numbers a supplier can publish.

What is a potency assay?

A validated test measuring a defined biological effect per unit of product, run on each batch with an acceptance range. It is how batch equivalence is established for biological medicines, and it is rarely reported in this market.

Can counts be compared between suppliers?

Not reliably, unless the platform, detection threshold and dilution are stated and match. Different instruments see different parts of the size range, and the same sample can yield different numbers on each.