What Is a Flow Cytometer?

What a flow cytometer is

A flow cytometer is a laboratory instrument that measures the physical and chemical properties of individual cells (or other small particles) as they flow, one at a time, through a focused beam of laser light. Rather than looking at a sample as a whole under a microscope, a flow cytometer streams thousands of cells past a fixed measurement point every second and records, for each one, how much light it scatters and how much fluorescent light it emits. Because every cell is measured individually and immediately assigned to a data point, a single run can characterize hundreds of thousands of cells in a population within a few minutes, producing quantitative data on cell size, internal complexity, and the presence or amount of specific molecules on or inside each cell.

The name describes exactly what the instrument does: it analyzes cells (cyto-) as they flow past a detector, rather than cells sitting still. That single design choice — measuring cells in continuous single-file motion instead of as a static image — is what makes flow cytometry a fundamentally different kind of measurement from microscopy, and it’s the reason the instrument exists as its own equipment category in a life-science lab.

How it works, in outline

A cell suspension is injected into a fast-moving stream of sheath fluid. Because the sheath stream is much wider than the sample stream, hydrodynamic focusing narrows the sample to a single-file line of cells passing through the same point one at a time — this is what lets the instrument interrogate one cell at a time instead of a clump. As each cell crosses one or more laser beams, two things are measured optically:

• Light scatter — forward scatter (light bent slightly off-axis) correlates roughly with cell size, and side scatter (light deflected at a wider angle) correlates roughly with internal granularity or complexity. Scatter alone, with no stain at all, already separates broad cell types (e.g. lymphocytes from granulocytes in blood) by size and internal structure.
• Fluorescence — if the cells have been labeled with fluorescently tagged antibodies or dyes, each fluorophore emits light at a characteristic wavelength when excited by the laser. Optical filters and dichroic mirrors route each wavelength band to its own detector (a photomultiplier tube or, on newer instruments, an avalanche photodiode), so the instrument can report, per cell, how brightly it’s stained for each marker in the panel.

The result is a multiparameter dataset — typically visualized as dot plots or histograms — where every dot is one cell, plotted by whichever two measured parameters (scatter or fluorescence channels) the researcher chooses to compare. CASRAI’s Flow Cytometry: Principles, Panel Design, and Gating Workflow guide covers that gating and plotting process — how a researcher goes from raw per-cell measurements to identified, counted cell populations — in full detail; this page focuses on the instrument itself.

What flow cytometers are used for

The instrument’s core capability — quantifying multiple markers on large numbers of individual cells, fast — supports several distinct applications:

• Immunophenotyping — identifying and counting cell subsets by the combination of surface or intracellular markers they carry, most commonly using antibodies against CD antigens (e.g. distinguishing CD4+ helper T cells from CD8+ cytotoxic T cells, or characterizing a leukemia/lymphoma sample by its marker profile). This is the single most common clinical and research use of flow cytometry.
• Cell sorting — some flow cytometers can also physically separate and collect specific cell populations in real time as they’re measured, rather than just recording data about them. This capability, and how it differs from an analyzer-only instrument, is covered in the disambiguation section below.
• Cell cycle and proliferation analysis — DNA-binding dyes let a flow cytometer quantify how much DNA each cell contains, which distinguishes cells in different phases of the cell cycle.
• Apoptosis and viability assays — dye combinations such as Annexin V/PI distinguish live, early-apoptotic, late-apoptotic, and dead cells by flow cytometry; see CASRAI’s Annexin V/PI apoptosis assay guide for the specifics of one such assay.
• Basic counting and enumeration — with or without fluorescent labels, a flow cytometer can enumerate cells and, using calibrated bead standards, convert relative counts into absolute cell concentrations.

Who uses one

Flow cytometers are core equipment in immunology and cancer research labs, clinical hematology and pathology labs (immunophenotyping is a standard diagnostic tool for leukemias and lymphomas, and CD4 T-cell counting by flow cytometry is used in HIV monitoring), vaccine and immune-monitoring trials, stem cell and developmental biology labs, and microbiology labs studying cell populations in culture. Because a research-grade instrument with multiple lasers and detectors represents a substantial capital cost, flow cytometers are frequently housed in shared core facilities rather than in every individual lab — a researcher typically books instrument time, and core staff often assist with panel design and instrument setup, rather than each lab owning and maintaining its own cytometer.

Flow cytometer vs. related instruments and terms

The bare term “flow cytometer” is often used loosely to cover instruments and techniques that are related but not identical. Four distinctions are worth being precise about:

Flow cytometer vs. cell sorter (FACS). Every cell sorter is a flow cytometer, but not every flow cytometer can sort. An analyzer-only instrument measures and records data about each cell as it passes the laser, then the cell goes to waste. A cell sorter adds the ability to physically separate specific populations for downstream use — commonly by charging droplets containing cells of interest and deflecting them electrostatically into separate collection tubes. “FACS” (fluorescence-activated cell sorting) specifically refers to that sorting capability, not to flow cytometry analysis in general, though the terms are frequently used loosely as if interchangeable. If you need to physically recover a purified population of live cells to grow or use further, you need a sorter, not just an analyzer; CASRAI’s Cell Sorting by Flow Cytometry guide covers the sorting-specific parameters (nozzle size, sheath pressure, sort mode, purity, and post-sort viability).

Flow cytometer vs. microscope. A microscope produces a spatial image of a sample and is well suited to seeing where something is within or around a cell, or to examining a small number of cells in detail. A conventional flow cytometer produces no image at all — it trades spatial and morphological detail for throughput and quantification, measuring thousands of cells per second and reporting numeric values per cell rather than a picture. Imaging flow cytometry is a distinct instrument class that captures an actual image of every cell while still running them through a flow cytometer’s fluidics at meaningful throughput, combining some of both approaches; see CASRAI’s Imaging Flow Cytometry vs. Conventional Flow Cytometry comparison for how the two trade off.

Flow cytometer vs. Coulter counter / hemocytometer. A Coulter counter and a manual hemocytometer both count and size cells (by electrical impedance or by eye under a grid, respectively), but neither measures fluorescence or identifies cell subtypes by marker expression — they answer “how many cells, roughly what size,” not “how many of which specific cell type.” A flow cytometer can do basic counting too, but its distinguishing capability is the fluorescence-based, multiparameter phenotyping those simpler instruments can’t do. See CASRAI’s Hemocytometer Cell Counting and Viability guide for the simpler manual method.

Flow cytometer vs. plate reader. A plate reader measures a bulk, pooled fluorescence or absorbance signal from an entire well of cells or solution at once — it reports an average across the population, with no per-cell resolution. A flow cytometer measures each cell individually, so it can report what fraction of a population is positive for a marker, not just the average signal across all of them. For fluorescence-based assays that don’t require single-cell resolution, a plate reader is the simpler, higher-throughput, lower-cost tool; see CASRAI’s What Is a Plate Reader? guide for that comparison in more depth.

Where to go next on this site

This page covers what a flow cytometer is and how it fits into a lab’s workflow. CASRAI has deeper coverage of specific flow cytometry techniques and decisions once you’re past the “what is it” stage:

• Gating logic, dot plots, and panel design: Flow Cytometry: Principles, Panel Design, and Gating Workflow.
• Preparing a blood-derived sample for flow cytometry: PBMC Preparation and Staining for Flow Cytometry.
• Daily instrument setup and QC beads for a multicolor panel: Multicolor Flow Cytometry: Daily Instrument Setup and QC Beads.
• Setting compensation and using FMO controls to read a multicolor panel correctly: Flow Cytometry Compensation and FMO Controls in Flow Cytometry.
• Physically separating populations by cell sorting: Cell Sorting by Flow Cytometry.

Practical relevance for research administration and lab management

A flow cytometer is a significant capital instrument, and that has real consequences beyond the bench. Multi-laser, multi-detector systems are expensive enough that many institutions fund them as shared infrastructure rather than as a single lab’s purchase — in the US, this is exactly the kind of instrument NIH’s Shared Instrumentation Grant (S10) program exists to fund, since it pays for a single piece of shared, high-cost equipment used by a defined group of already-funded investigators rather than for a research project itself; see CASRAI’s NIH Shared Instrumentation Grant (S10) entry. Once acquired, a flow cytometer typically lives in a core facility with its own scheduling, cost-recovery, and staff-assist model, which means research administrators and lab managers deal with it as a recurring access and budgeting question (instrument time, reagent cost, core facility fees charged to a grant) as much as a one-time procurement decision. Panel design and antibody costs also scale with the number of colors used, which is a real budget line for any lab running immunophenotyping work regularly.

Frequently asked questions

Is a flow cytometer the same thing as a FACS machine?

Not exactly. “FACS” specifically means fluorescence-activated cell sorting — a flow cytometer with the added ability to physically sort cells. Every FACS instrument is a flow cytometer, but plenty of flow cytometers are analyzer-only and have no sorting capability, so “FACS machine” is really a subtype, not a synonym for the whole category.

Does a flow cytometer produce an image of the cells?

A conventional flow cytometer does not — it reports numeric scatter and fluorescence values per cell, with no picture. Imaging flow cytometry is a separate instrument class that adds actual per-cell images while keeping flow cytometry’s throughput; see CASRAI’s comparison of the two approaches for the tradeoffs.

How many cells can a flow cytometer measure at once?

Modern flow cytometers commonly run at rates from several hundred to several thousand cells per second, letting a single run characterize a population of hundreds of thousands of cells within a few minutes. Actual throughput depends on the instrument, the sample’s cell concentration, and the acquisition settings used.

Do I need a flow cytometer, or would a plate reader work for my assay?

If the assay only needs an average signal across a whole population — how much total fluorescence a well produces — a plate reader is simpler and cheaper. If the question is what fraction of the population is positive for a marker, or which specific cell subsets are present, that requires the per-cell resolution only a flow cytometer provides.