CellTiter-Glo® 3D Cell Viability Assay

How does the CellTiter-Glo® 3D Assay differ from the standard CellTiter-Glo® Assays?

The table below summarizes the key differences between the standard CellTiter-Glo® Assays (for 2D monolayers) and the CellTiter-Glo® 3D Assay, incorporating validated performance data from Promega internal studies.

What are the most common methods for measuring cell viability in 3D cultures?

Several assay platforms can be adapted for 3D viability measurement, each with distinct trade-offs between sensitivity, throughput, spatial resolution and assay time. CellTiter-Glo® 3D Assay performance data are included for direct comparison.

Can I use standard CellTiter-Glo® Assay or CellTiter-Glo® 2.0 for 3D cell cultures?

The standard CellTiter-Glo® Assays are not recommended for compact 3D spheroids due to incomplete lysis. Use the CellTiter-Glo® 3D Cell Viability Assay because it is specifically optimized for 3D models.

How sensitive is the CellTiter-Glo® 3D Assay compared to other viability assays?

ILike other luminescent assays, the CellTiter Glo® 3D Assay has very low background, so it typically produces optimized signal to background ratios in the tens to thousands range. In comparison, fluorescence- or absorbance-based viability assays often generate signals only a few fold above background.

What is the Z′-factor for CellTiter-Glo® 3D Assay in high-throughput 3D screening?

The CellTiter-Glo® 3D Assay achieved a Z′-factor of 0.81 in a 96-well 3D precision experiment using HCT116 colon cancer spheroids (~350μm diameter). Thirty spheroids were treated with 100μM panobinostat (positive control) and 30 with vehicle (negative control). A Z′-factor above 0.5 is the accepted HTS threshold; 0.81 indicates excellent assay window separation, confirming suitability for high-throughput 3D drug screening.

Which 3D culture formats are compatible with CellTiter-Glo® 3D?

CellTiter-Glo® 3D Assay has been validated across three major 3D culture formats with equivalent dose-response results in all three: hanging-drop cultures (InSphero GravityPLUS™ platform), ultra-low attachment (ULA) round-bottom plates, and Matrigel® 3D cultures. All formats used HCT116 colon cancer cells grown for 4 days and treated with compounds for 48 hours before assay.

Are there protocol differences between CellTiter-Glo® 3D Assay and the standard assay?

Yes—three modifications are required for 3D cultures: (1) add reagent at 1:1 volume ratio with culture medium (same as standard), but mix more vigorously for 5 minutes rather than 2 minutes; (2) incubate for 25–60 minutes rather than 10 minutes to allow the enhanced lysis buffer time to fully penetrate the 3D structure; (3) equilibrate both the plate and reagent to room temperature for 30 minutes before addition. Total assay time from reagent addition to read is 30 minutes for standard spheroid sizes.

What alternatives exist to CellTiter-Glo® 3D Assay for 3D viability measurement?

Alternatives include: fluorescent live/dead staining (e.g., Calcein-AM/EthD-1) for spatial viability maps requiring imaging; resazurin-based assays (alamarBlue®) as nondestructive options with lower sensitivity in 3D and a 3-hour protocol; and ATPlite™ 1Step (PerkinElmer), which uses standard lysis chemistry that provides inferior penetration and ATP recovery compared to the CellTiter-Glo® 3D Assay in compact spheroids. For kinetic monitoring in 3D, the RealTime-Glo™ MT Assay is a non-lytic option that requires experimental validation in each 3D model.

How do CellTiter-Glo® 3D Assay results compare to 2D viability data for the same compound?

IC₅₀ values from 3D spheroids are typically higher than those from 2D monolayers for the same compound, reflecting the more physiologically relevant drug microenvironment in 3D. Quiescent cells in the spheroid interior are less susceptible to antiproliferative agents, and diffusion barriers limit drug penetration to inner cell layers. This makes 3D IC₅₀ values more predictive of in vivo activity, and the difference is a biologically meaningful feature rather than an assay artifact.

Why do drug responses differ between 2D and 3D models?

3D tumor spheroids replicate in vivo tumor microenvironments more closely than monolayers, including oxygen and nutrient gradients, cell-cell and cell-matrix interactions, and the emergence of hypoxic cores in larger spheroids. These biological features produce drug responses that often differ from 2D data: IC₅₀ values are typically higher (lower apparent potency) in 3D, and some compound classes that appear active in 2D show reduced activity in 3D when the target requires dividing cells or relies on efficient tissue penetration. This is considered a feature of 3D models—the data are more predictive of in vivo compound activity.

What plates and readout settings are recommended?

Solid white opaque plates are best for reading. Grow spheroids in clear U-bottom/low-attachment plates if needed, then transfer lysates to opaque solid white plates for reading. Use a luminometer with top-read and no emission filter to maximize signal and minimize cross-talk. Most standard luminescence settings are suitable and the signal is stable so you can test to find what gives you the best signal above the background

How do I optimize lysis and shaking for 3D cultures?

Ensure vigorous but spill-free shaking (start around 450 rpm) and extend mixing/incubation until signal plateaus. For large or dense 3D samples, perform ATP recovery experiments as described in Technical Manual #TM412.

Can the CellTiter-Glo® 3D Assay be used with tissues or non-3D samples (pellets, suspensions, tissue material)?

This assay can be used with cell pellets and suspension cells. Researchers outside of Promega have tried it on small tissue pieces, but these applications are not fully validated. Large spheroids, tissues and collagen or other complex matrices require case-by-case optimization and may not be suitable. Perform ATP recovery experiments as described in Technical Manual #TM412.

What are the limitations of the CellTiter-Glo® 3D Assay?

Understanding these limitations supports appropriate experimental design:

• The reagent penetration and ATP recovery decrease as microtissues become large, but for small to moderate size spheroids (~350µm or less), penetration and recovery are excellent. Very large or highly compact spheroids (>350μm) may require extended incubation and additional vortexing.
• Endpoint/destructive—cell lysis prevents kinetic monitoring. For continuous real-time viability in 3D, the RealTime-Glo® MT Assay requires separate validation for each 3D model.
• No spatial viability information—the assay reports total viable ATP from the full spheroid but cannot reveal necrotic core vs. viable rim distribution. Complement with live/dead fluorescence imaging when spatial context is needed for mechanistic interpretation.
• Matrix interference—Matrigel® and other hydrogel matrices may impede reagent penetration at high concentrations. Run matrix-only controls to characterize background ATP contribution and confirm equivalent lysis efficiency vs. non-matrix formats.
• Luciferase inhibitor compounds—some library compounds directly inhibit luciferase activity, generating false-positive cytotoxicity signals. Screen at the highest tested concentration with a luciferase counter-assay or ATP spike if compound interference is suspected.