How Stem Cell Counting Works

A cell count and a viability percentage answer two different questions. Here's how each is actually measured in a lab.

The Basic Methods

Counting cells in a biologic product is a more exacting task than it sounds, and the method used affects both the accuracy of the number and how much confidence a provider should place in it.

• Hemocytometer (manual count) — a calibrated glass slide with a grid, viewed under a microscope. A technician counts cells by eye within defined grid squares and calculates concentration from the known chamber volume. Inexpensive, but subject to operator variability.
• Automated cell counters (e.g., image-based counters like the Nexcelom Cellometer or Countess-style instruments) — a small sample is imaged and software identifies and counts individual cells, often simultaneously scoring viability via a co-loaded dye. Faster and more reproducible than manual counting.
• Flow cytometry — cells pass single-file through a laser in a fluidic stream, and the instrument counts events while simultaneously measuring fluorescent markers. This allows a single run to report count, viability, and identity/phenotype markers together, which is why it's typically used for regulatory-grade release testing.

What a Count Does — and Doesn't — Establish

A total cell count says how many particles the instrument classified as cells. It does not, by itself, say how many of those cells are alive, and it says nothing about whether they are the cell type the product claims to contain.

This is why a complete characterization panel reports several numbers side by side rather than one: total count, viable count (count × viability %), and identity markers confirming cell type. Any one of these reported alone is a partial picture.

This page is educational reference material for licensed healthcare providers and does not constitute medical, legal, or regulatory advice. HCT/P products are not FDA-approved.