The Future of Cell Analysis Is Stain-Free

New Data Show How a Stain-Free Method Matches Flow Cytometry for Cell Viability Testing

Cell viability results depend heavily on the method used to measure them. This METTLER TOLEDO white paper compares CytoDirect, flow cytometry, and trypan blue staining across five cell models. The data show that stain-free cell viability testing with CytoDirect closely aligns with flow cytometry results, the reference technique for detecting apoptosis and necrosis.

Download the white paper to review the complete comparison and decide whether a stain-free workflow fits your lab.

Flow cytometry remains the reference method for detecting apoptosis and necrosis in cell samples. It requires expensive equipment, a lengthy staining process, and an expert operator to run and interpret results correctly. These requirements limit its practicality for routine laboratory testing, even in labs that trust its accuracy.

Trypan blue exclusion solves the cost and speed problem, and its simplicity keeps it in wide use across general and testing labs. It comes with a known limitation, though: manual counting with a hemocytometer cannot detect early-stage apoptotic cells. That gap affects the accuracy of viability data at the exact moment researchers need it most, before a sample moves further into a study.

Putting Stain-Free Cell Viability to the Test

The white paper tests CytoDirect against both reference methods across five experimental models, each chosen to represent a different real-world scenario. Researchers treated HeLa and Jurkat cells with staurosporine to induce apoptosis, a well-characterized cytotoxic agent used widely in apoptosis research. They also exposed HeLa cells to UV irradiation, a non-chemical stressor that produces more heterogeneous cell responses, and compared fresh versus thawed PBMCs at multiple time points after isolation.

Each model used defined live-to-dead cell ratios of 0:100, 50:50, and 100:0. This design let the study measure how each method performs across low, intermediate, and high viability samples, not only at the extremes where differences are easiest to hide.

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The results show close agreement between CytoDirect and flow cytometry across every model tested:

HeLa cells treated with staurosporine: 87% viability by CytoDirect versus 91% by flow cytometry

Jurkat cells treated with staurosporine: 96% versus 97%

HeLa cells after UV irradiation: 92% versus 93%

PBMCs measured 24 hours after thawing: 89% versus 86%

Trypan blue showed more variation from the reference method across these same samples. It tended to underestimate viability in low-survival samples, such as fully apoptotic Jurkat cells, and to overestimate it during the gradual cell decline seen in the PBMC model.

See the full side-by-side comparison, including accuracy, viability assessment, sample preparation, and cost.

A Faster, Stain-Free Workflow

CytoDirect reaches these results using digital holographic microscopy combined with machine learning, with no dyes and no manual gating step required. The instrument analyzes each sample directly, based on the morphological and biophysical properties of the cells themselves, rather than on a fluorescent marker.

This stain-free approach removes the staining and gating steps that slow down flow cytometry, and it removes the manual counting step that limits trypan blue's sensitivity. Each sample in the study ran in triplicate, with results averaged for reporting, giving the method a level of consistency routine workflows can rely on.

What Is Digital Holographic Microscopy?

Digital Holographic Microscopy, or DHM, captures a hologram of each cell instead of a direct image. CytoDirect combines DHM with machine learning to measure cell concentration and viability without dyes or stains. This approach preserves sample integrity and removes the manual staining steps required by traditional cell counting methods.

Read More about Digital Holographic Microscopy

The CytoDirect Cell Counter is a compact, portable solution for rapid cell counting and viability checks suitable for most laboratories

For labs running routine viability checks, stain-free cell viability testing offers flow-cytometry-level confidence without the cost, the wait, or the need for a dedicated operator. That combination matters most in general and testing labs, where sample volume is high and turnaround time affects the rest of the workflow.

For labs working with primary cells such as PBMCs, the added sensitivity to early apoptotic events gives a clearer picture of sample quality before it affects downstream results. Catching that early signal can change how a sample gets used, rather than discovering the issue after the fact.

Cell viability data is only as reliable as the method behind it. This white paper lays out the full data set behind CytoDirect, with HeLa, Jurkat, and PBMC results included, along with the scatter plots and viability thresholds used to generate each result, so you can see exactly how the method performs against the tools your lab already trusts.