XTT Assay
Quick Facts
| Feature | Information |
|---|---|
| Category | Cell Viability / Cytotoxicity Assay |
| Principle | Tetrazolium Reduction |
| Detection Method | Colorimetric |
| Sample Type | Adherent and Suspension Cells |
| Quantitative | Yes |
| Typical Assay Time | 2-6 hours |
| Typical Output | Absorbance proportional to metabolically active cells |
| Readout | Soluble Formazan Dye |
Overview
The XTT Assay is a colorimetric assay used to evaluate cell viability, proliferation, and cytotoxicity by measuring cellular metabolic activity. Viable cells reduce the tetrazolium salt XTT into a water-soluble orange formazan dye, allowing direct spectrophotometric measurement without an additional solubilization step.
Compared with MTT, the XTT assay is less labor-intensive and is commonly used for routine viability screening and high-throughput drug testing.
Biological Principle
Metabolically active cells contain mitochondrial and cytosolic oxidoreductase enzymes capable of reducing XTT to a colored formazan product.
The quantity of soluble formazan produced reflects the metabolic activity of viable cells.
Because the assay depends on cellular metabolism, it provides an indirect measure of viable cell number.
Principle of Detection
XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) is reduced by viable cells in the presence of an electron-coupling reagent such as phenazine methosulfate (PMS) or phenazine ethosulfate (PES).
The resulting water-soluble orange formazan dye accumulates in the culture medium and is measured directly by absorbance.
Workflow
Cells
↓
Add XTT Reagent + Electron-Coupling Reagent
↓
Incubation
↓
Formation of Soluble Formazan
↓
Measure Absorbance
↓
Calculate Relative Cell Viability
What Does It Measure?
| Measurement | Interpretation |
|---|---|
| High absorbance | Greater metabolic activity |
| Low absorbance | Reduced metabolic activity |
| Increasing signal | Cell proliferation |
| Decreasing signal | Cytotoxicity or metabolic inhibition |
Applications
The XTT assay is commonly used for:
- Cell viability studies
- Drug cytotoxicity screening
- Cell proliferation analysis
- Anticancer drug evaluation
- Biomaterial biocompatibility testing
- Toxicology studies
- High-throughput screening
Interpretation of Results
| Observation | Biological Interpretation |
|---|---|
| High absorbance | High viable cell activity |
| Low absorbance | Reduced viability or metabolism |
| Progressive increase | Cell growth |
| Progressive decrease | Cytotoxic or antiproliferative effect |
Advantages
- Water-soluble reaction product
- No crystal dissolution step
- Simple and rapid workflow
- Suitable for high-throughput screening
- Lower handling variability than MTT
- Compatible with automated plate readers
Limitations
- Measures metabolic activity rather than direct cell number
- Requires an electron-coupling reagent
- Some compounds may chemically reduce XTT, causing false-positive results
- Colored compounds or media may interfere with absorbance measurements
- Different cell types produce different metabolic signals
Comparison with Similar Assays
| Assay | Primary Measurement |
|---|---|
| XTT | Cellular metabolic activity (soluble formazan) |
| MTT | Cellular metabolic activity (insoluble formazan) |
| MTS | Cellular metabolic activity (soluble formazan) |
| WST-1 | Cellular metabolic activity (soluble formazan) |
| CCK-8 | Cellular metabolic activity (water-soluble formazan) |
| Trypan Blue | Plasma membrane integrity |
| ATP Assays | Intracellular ATP concentration |
Common Misinterpretations
- High absorbance does not necessarily indicate increased cell number.
- Reduced signal may reflect decreased metabolism rather than cell death.
- XTT cannot distinguish apoptosis from necrosis.
- Metabolic activity varies among different cell types and experimental conditions.
Key Takeaways
- The XTT assay is a colorimetric tetrazolium assay used to assess cell viability, proliferation, and cytotoxicity.
- Viable cells reduce XTT into a water-soluble orange formazan dye, eliminating the need for a solubilization step.
- The assay is simple, reproducible, and suitable for high-throughput applications.
- Results reflect cellular metabolic activity, which serves as an indirect indicator of viable cell number.
- Complementary assays should be used when distinguishing specific mechanisms of cell death is required.
References
- Scudiero DA et al. Evaluation of a soluble tetrazolium/formazan assay for cell growth and drug sensitivity. Cancer Research (1988).
- Riss TL et al. Cell Viability Assays. Assay Guidance Manual.
- Freshney RI. Culture of Animal Cells: A Manual of Basic Technique and Specialized Applications.
- Roche Applied Science. XTT Cell Proliferation Kit Instructions.
- ATCC. Cell Proliferation and Viability Assays.