Flow cytometry and FACS are often used interchangeably, but they’re not the same. FACS is a subset of flow cytometry. Knowing the difference helps you ask for the right instrument time and design experiments that match what your facility offers.
Flow cytometry: the umbrella term
Flow cytometry analyzes cells in suspension as they pass single-file through a focused laser beam. As each cell crosses the laser, the instrument measures:
- Forward scatter (FSC): a proxy for cell size
- Side scatter (SSC): a proxy for granularity/internal complexity
- Fluorescence in multiple channels from labeled antibodies, dyes, or fluorescent proteins
Modern instruments measure 10–30+ parameters per cell at speeds of tens of thousands of cells per second. Output is a multi-dimensional dataset analyzed with FlowJo or FCS Express.
FACS: flow cytometry that sorts
FACS — Fluorescence-Activated Cell Sorting — uses flow cytometry hardware plus a sorting mechanism. After analysis, individual droplets containing single cells are charged based on user-defined gates and deflected into different collection tubes. The result is a physically separated, viable population of cells you can use for downstream culture, sequencing, or transplantation.
Side-by-side comparison
| Feature | Analytical flow cytometry | FACS |
|---|---|---|
| Output | Data only | Data + sorted cells |
| Cell viability | Cells discarded | Sorted cells remain viable |
| Speed | Higher (10K–30K cells/sec) | Slower (1K–10K for sorting) |
| Cost per sample | Lower | Higher |
| Operator skill | Self-service possible | Often facility staff only |
| Biosafety | Standard | Containment for unfixed cells |
When you need flow cytometry
- Immunophenotyping — defining cell populations by marker expression
- Cell cycle analysis with DNA dyes
- Apoptosis assays (Annexin V, propidium iodide)
- Quantifying transfection efficiency
- Calcium flux, phosphoflow, intracellular cytokine staining
When you need FACS
- Isolating live cells for downstream culture or transplant
- Single-cell sorting for scRNA-seq, monoclonal generation, or rare cell capture
- Sorting on multiple parameters simultaneously
- Recovering rare populations from heterogeneous samples
Practical workflow tips
- Compensation. Multi-color experiments require compensation for spectral overlap. Use single-stained controls.
- Viability staining. Always include a live/dead stain — dead cells autofluoresce and bind antibodies non-specifically.
- FMO controls. Fluorescence-Minus-One controls help set proper gates for each marker.
- Sample prep. Use single-cell suspensions filtered through 70 µm mesh; clumps clog sorters.
- Spectral flow cytometry. Newer instruments use the full emission spectrum, allowing far more parameters with less compensation hassle.
If you only need to characterize populations, an analytical flow cytometer is faster, cheaper, and easier to access. If you need to recover populations for downstream work, FACS is essential.
