The MTT assay is a widely used colorimetric method for quantifying cell viability, proliferation, and cytotoxicity in vitro. Based on the reduction of the yellow tetrazolium salt MTT to purple formazan crystals by metabolically active cells, this technique provides a simple, sensitive, and cost-effective approach for drug screening and biomedical research. This article presents a comprehensive overview of the MTT assay principle, sample types, testing workflow, applications, and frequently asked questions.
Technical Overview
The MTT assay, first introduced by Tim Mosmann in 1983, is one of the most established colorimetric methods for evaluating cell metabolic activity and viability. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) is a water-soluble yellow tetrazolium dye that penetrates viable cells and is reduced by mitochondrial succinate dehydrogenase and other NAD(P)H-dependent cellular oxidoreductases to form insoluble purple formazan crystals. Since these enzymes are only functional in metabolically active cells, the amount of formazan produced is directly proportional to the number of living cells present in the culture.
Following incubation with MTT, the formazan crystals are solubilized using a detergent such as DMSO or acidified isopropanol, producing a homogeneous purple solution. The absorbance of this solution is then measured spectrophotometrically at a wavelength of approximately 570 nm, with a reference wavelength typically set between 630 nm and 690 nm to correct for optical interference. By comparing absorbance values from treated samples against untreated controls, researchers can quantitatively assess cell viability, calculate growth inhibition rates, and determine half-maximal inhibitory concentration (IC50) values for pharmacological compounds.
The MTT assay offers several advantages, including high sensitivity, good linearity, ease of use, compatibility with standard 96-well microplate readers, and no requirement for radioactive materials. However, it is an endpoint assay, meaning the cells cannot be reused for downstream analysis, and results may be influenced by compounds that directly alter cellular metabolism.
Testing Samples
Adherent Cell Lines
Adherent cell lines such as HeLa, HEK293, HepG2, MCF-7, A549, and fibroblasts are the most common samples analyzed using the MTT method. These cells attach to the bottom of culture plates, allowing easy medium exchange during washing steps. Typical seeding densities range from 3,000 to 10,000 cells per well in 96-well plates, depending on cell size, growth rate, and the intended incubation period.
Suspension Cell Lines
Suspension cells, including Jurkat, K562, THP-1, and various lymphoma lines, can also be evaluated by MTT assay. Because these cells do not attach to the culture surface, careful aspiration or centrifugation is required when removing the culture medium prior to formazan solubilization to avoid cell loss and inaccurate readings.
Primary Cells and Clinical Specimens
Primary cells isolated from animal or human tissues, including peripheral blood mononuclear cells (PBMCs), hepatocytes, and tumor-derived cells, may be tested to assess ex vivo drug sensitivity. These samples often require optimized seeding densities and shorter assay windows due to limited proliferation capacity.
Drug-Treated and Stimulated Samples
Cells exposed to candidate pharmaceuticals, natural product extracts, nanoparticles, environmental toxins, or biological stimuli such as cytokines constitute a major category of MTT test samples. Parallel sets of treated and vehicle-control wells enable robust quantification of treatment effects on cell survival.
Testing Items
The MTT assay supports a broad range of quantitative cell biology measurements. Core testing items include:
- Cell viability determination: quantification of the proportion of metabolically active cells in a population.
- Cell proliferation analysis: measurement of growth rates over defined time intervals.
- Cytotoxicity evaluation: assessment of compound-induced toxicity across a concentration gradient.
- IC50 and EC50 calculation: determination of the half-maximal inhibitory or effective concentration of test agents.
- Growth factor and serum activity testing: evaluation of supplements that support or inhibit proliferation.
- Biocompatibility screening: testing of medical device extracts, biomaterials, and implant-related leachables.
- Radiation and photodynamic therapy sensitivity: quantification of treatment-induced loss of viability.
- Anticancer drug combination studies: analysis of synergistic or antagonistic drug interactions.
Testing Methods
The standard MTT assay follows a well-defined workflow optimized for 96-well microplates. All procedures should be performed under aseptic conditions to prevent microbial contamination, and MTT solutions must be protected from light.
Reagents and Materials
- MTT stock solution: typically prepared at 5 mg/mL in sterile PBS and filtered through a 0.22-micron membrane.
- Solubilization solution: DMSO, acidified isopropanol (0.04 M HCl), or detergent-based solutions such as SDS in diluted HCl.
- Complete culture medium, PBS, and trypsin-EDTA for cell handling.
- 96-well flat-bottom culture plates, multichannel pipette, and a microplate reader equipped with a 570 nm filter.
Step-by-Step Protocol
- Seed cells into a 96-well plate at an optimized density (commonly 5,000 to 10,000 cells per well in 100 microliters of medium), including blank wells containing medium only for background correction.
- Incubate the plate at 37 degrees Celsius in a humidified atmosphere with 5% CO2 until adherent cells attach, usually 12 to 24 hours.
- Add test compounds at serial dilutions to designated wells, keeping a vehicle control group and, optionally, a positive cytotoxicity control.
- Incubate for the desired treatment period, typically 24, 48, or 72 hours.
- Add 10 to 20 microliters of MTT stock solution to each well to reach a final concentration of approximately 0.5 mg/mL.
- Incubate for 2 to 4 hours at 37 degrees Celsius until visible purple formazan crystals form at the well bottom.
- Carefully remove the culture medium without disturbing the crystals, particularly important for suspension cultures.
- Add 100 to 150 microliters of DMSO to each well and gently agitate on an orbital shaker until all crystals are fully dissolved.
- Measure absorbance at 570 nm using a reference wavelength of 630 to 690 nm within one hour of solubilization.
Data Analysis and Quality Control
Absorbance values are corrected by subtracting the blank well background. Cell viability is expressed as a percentage relative to the untreated control using the formula: viability (%) = (absorbance of treated wells minus blank) divided by (absorbance of control wells minus blank) multiplied by 100. Dose-response curves are plotted and fitted using nonlinear regression to derive IC50 values. Key quality control measures include maintaining cells within logarithmic growth phase, confirming that absorbance falls within the linear detection range, filling peripheral wells with sterile PBS to minimize edge evaporation effects, and running technical replicates in triplicate or greater for statistical reliability.
Applications
The MTT assay is extensively applied across pharmaceutical development, academic research, and industrial biocompatibility testing. Major application areas include:
- Anticancer drug discovery: high-throughput screening of synthetic compounds, natural products, and herbal extracts for tumor cell growth inhibition.
- Pharmacological toxicology: safety evaluation of drug candidates and estimation of therapeutic windows before animal studies.
- Nanomaterial safety assessment: measurement of nanoparticle cytotoxic effects on target cell lines.
- Medical device biocompatibility: testing of material extracts in accordance with ISO 10993-5 guidelines for cytotoxicity.
- Immunology research: functional assessment of lymphocyte proliferation in response to mitogens and antigens.
- Food and cosmetic science: evaluation of bioactive ingredient efficacy and safety on skin and intestinal cell models.
- Radiotherapy and photodynamic therapy: quantification of treatment-induced cytotoxicity in cancer cell panels.
- Genetic and molecular studies: functional validation of gene knockdown or overexpression effects on cell survival.
FAQ
At what wavelength should the MTT assay be read?
The primary measurement wavelength is 570 nm, where formazan absorbance peaks. A reference wavelength between 630 nm and 690 nm is recommended to correct for background scattering, fingerprints, and well imperfections. If a 570 nm filter is unavailable, wavelengths between 540 nm and 600 nm can be used with acceptable sensitivity.
What is the difference between MTT, MTS, CCK-8, and WST-1 assays?
MTT produces insoluble formazan that requires a solubilization step, making it an endpoint assay. In contrast, MTS, WST-1, and CCK-8 (WST-8-based) generate water-soluble formazan products, eliminating the dissolution step and allowing continuous absorbance monitoring. These newer tetrazolium salts are generally less toxic to cells, but MTT remains the most economical and historically validated option.
Why is DMSO used in the MTT assay?
Formazan crystals produced by MTT reduction are insoluble in aqueous culture medium. DMSO efficiently dissolves these crystals into a uniform purple solution suitable for accurate spectrophotometric measurement. Acidified isopropanol or SDS-based solutions are alternative solubilization agents that also help precipitate protein debris.
How long should cells be incubated with MTT reagent?
Standard incubation is 2 to 4 hours at 37 degrees Celsius. Highly metabolically active cells may require only 1 to 2 hours, while slow-growing or primary cells may need up to 4 hours. Incubation times should be consistent across all wells within an experiment.
Why did my formazan crystals fail to dissolve completely?
Incomplete dissolution is usually caused by insufficient DMSO volume, inadequate mixing, or excessively high cell density. Prolonged shaking on an orbital plate shaker, warming the plate slightly, or repeating pipetting typically resolves the issue. Persistent cloudiness suggests contamination or protein precipitation, requiring the use of acidified solvent.
How is the IC50 value calculated from MTT data?
Plot the logarithm of compound concentration against the percentage of cell viability, then fit a sigmoidal dose-response curve using four-parameter nonlinear regression in software such as GraphPad Prism or Origin. The concentration corresponding to 50% viability relative to the control is reported as the IC50.
What factors can interfere with MTT assay accuracy?
Reducing agents such as ascorbic acid, dithiothreitol, and certain antioxidants can directly reduce MTT independent of cell metabolism. Colored or fluorescent compounds may absorb near 570 nm, and some drugs alter mitochondrial enzyme activity without changing cell number. Including compound-only blank wells and confirming results with orthogonal methods such as cell counting or ATP-based luminescence assays is strongly recommended.
Can the MTT assay distinguish cytostatic from cytotoxic effects?
MTT measures metabolic activity, which reflects both cell number and per-cell metabolism. A decreased signal indicates either fewer cells or reduced metabolic output. To distinguish growth inhibition from cell death, combine MTT results with complementary assays such as trypan blue exclusion, annexin V staining, or clonogenic survival analysis.