Unit 3: Cell Viability Assay

I. Orientation — Quantifying viable cellular activity

Cell viability assays estimate how many cells remain alive or functionally active after treatment. The MTT assay is a colorimetric method based on the ability of metabolically active cells to reduce a yellow tetrazolium salt to purple formazan. Because the amount of formazan generally increases with cellular metabolic activity, absorbance provides an indirect estimate of viable cell number under controlled conditions.

• Governing principle: Living cells contain reducing enzymes and cofactors, including NAD(P)H, that can convert MTT into formazan; severely damaged or dead cells have greatly reduced conversion capacity.
• Measurement basis: Purple formazan is quantified by measuring optical absorbance, commonly near 570 nm, with a reference wavelength often used to correct background.
• Operational definition of viability: In MTT experiments, viability usually means metabolic activity relative to an untreated or vehicle-treated control, not an absolute count of living cells.
• Core assumption: Within a validated experimental range, absorbance is proportional to the number of metabolically active cells.
• Important qualification: A treatment can alter cellular metabolism without immediately killing cells; therefore, MTT results indicate metabolic viability rather than proving membrane integrity or long-term reproductive survival.
• Experimental convention: Results are commonly reported as percentage viability, percentage inhibition, or concentration producing 50% inhibition, designated IC₅₀.
• Required comparison: Every treatment value must be interpreted against appropriate controls, including reagent blanks and untreated or vehicle-treated cells.

II. MTT assay for evaluation of cell viability — Colorimetric metabolic assay

The MTT assay is used to compare the metabolic activity of cultured cells after exposure to a drug, toxicant, environmental condition, or other experimental treatment. Cells are incubated with MTT, the resulting formazan is dissolved, and absorbance is measured spectrophotometrically.

A. MTT assay for evaluation of cell viability

The assay consists of adding MTT to cultured cells, allowing viable cells to generate formazan, dissolving the formazan crystals, and measuring the color intensity.

• Assay reagent: MTT is the tetrazolium compound 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. It is typically prepared as a sterile stock solution and protected from prolonged light exposure.
• Color change: MTT is yellow or pale yellow in solution, whereas reduced formazan appears as dark purple crystals, usually within viable cells.
• Biological reaction: Cellular oxidoreductase activity transfers reducing equivalents to MTT, producing insoluble formazan. The overall reaction is often represented as:

MTT (yellow tetrazolium) + reducing equivalents
→ formazan (purple) + oxidized cellular cofactors

• Meaning of absorbance: Higher absorbance generally indicates greater metabolic activity and, when the assay is linear, a greater number of viable cells.
• Not a direct cell counter: The result is influenced by both cell number and the metabolic state of each cell. Two wells containing the same number of cells may produce different signals if one population is stressed or metabolically stimulated.

B. Experimental setup and cell preparation

Reliable MTT data depend on uniform cell seeding, suitable treatment conditions, and a validated density range.

• Cell type: Adherent mammalian cells are commonly seeded in 96-well plates, although suspension cells can also be assayed if they remain evenly distributed during handling.
• Seeding density: Cells should be plated at a density that permits measurable growth without reaching complete confluence before measurement. Excessively dense cultures can produce signal saturation.
• Replicates: Technical replicates, such as three to six wells per condition, help estimate pipetting and well-to-well variation; independent biological experiments are needed to assess reproducibility.
• Attachment period: Adherent cells are generally allowed to attach before treatment. The duration depends on the cell line and experimental design.
• Treatment design: Test concentrations, exposure times, solvent concentration, and treatment volume must be kept consistent across wells.
• Plate arrangement: Randomizing treatments across the plate and avoiding exclusive use of edge wells can reduce positional and evaporation effects.
  • Edge control: Outer wells may contain sterile medium or buffer when evaporation is a concern.
  • Uniform handling: Multichannel pipetting and consistent mixing reduce differences caused by dispensing order.

C. Reagent addition and formazan formation

MTT is added after the treatment period, and the cells are incubated long enough for an appropriate amount of formazan to form.

• MTT concentration: The working concentration must be optimized for the cell line and exposure time; an overly concentrated reagent can increase background or stress cells.
• Incubation time: A common development period is approximately 1–4 hours, but the correct time is determined empirically from the linear range of the assay.
• Reaction appearance: Purple crystals should form in wells containing viable cells. Very pale wells may reflect few cells, toxicity, insufficient incubation, or an invalid reagent.
• Light protection: MTT solutions should be handled with limited light exposure because tetrazolium reagents can be light-sensitive.
• Avoiding disturbance: During formazan formation, excessive plate movement can redistribute cells or crystals and increase variability.
• Endpoint consistency: All wells should receive comparable incubation times and be processed in the same sequence because formazan formation continues during the reaction period.

D. Formazan solubilization and absorbance measurement

Because formazan is usually insoluble in aqueous culture medium, it must be dissolved before absorbance is measured.

• Solubilizing agents: Dimethyl sulfoxide, or DMSO, is widely used; isopropanol-based solutions and detergent-containing acidic solutions are also used in some protocols.
• Solubilization requirement: The solvent must dissolve crystals completely. Undissolved particles scatter light and produce unreliable absorbance values.
• Mixing step: Plates are commonly shaken after solvent addition to produce a uniform purple solution.
• Measurement wavelength: Absorbance is commonly read at approximately 570 nm; a reference wavelength such as 630–690 nm may correct nonspecific optical background.
• Instrumental quantity: Absorbance, or optical density, is dimensionless and reflects the fraction of incident light absorbed by the solution.
• Blank correction: Reagent-only wells correct for color contributed by MTT, solvent, medium, or plate material:

A_corrected = A_sample − A_blank

Here, A_corrected is the corrected absorbance, A_sample is the measured absorbance of a cell-containing well, and A_blank is the absorbance of a well containing assay reagents but no cells.

• Reading consistency: The same wavelength, plate orientation, shaking conditions, and time between solubilization and reading should be used for all wells.

E. Controls and data calculation

Controls distinguish cellular metabolism from reagent color, solvent effects, and nonspecific chemical reduction.

• Reagent blank: Contains medium, MTT, and solubilizing agent but no cells; it estimates noncellular background.
• Vehicle control: Contains cells and the solvent used for the test compound, such as a matched DMSO concentration; it establishes the appropriate baseline for treated wells.
• Untreated control: Contains cells without test compound or vehicle when a solvent is unnecessary.
• Positive cytotoxicity control: A treatment known to reduce viability confirms that the assay can detect cellular injury.
• Cell-free treatment control: Test compound plus MTT without cells can reveal direct chemical reduction of MTT or intrinsic color from the compound.
• Normalized viability: Treatment values are commonly expressed relative to vehicle-control absorbance:

Viability (%) =
[(A_treated − A_blank) / (A_control − A_blank)] × 100

A_treated is the absorbance of a treated well, A_control is the absorbance of the vehicle or untreated control, and A_blank is the reagent-blank absorbance.

• Cytotoxicity or inhibition: When defined relative to the same control, percent inhibition can be calculated as:

Inhibition (%) = 100 − Viability (%)

• Worked example: If a treated well has absorbance 0.42, the control is 0.80, and the blank is 0.05, then viability is (0.42 − 0.05)/(0.80 − 0.05) × 100 = 49.3%, and inhibition is approximately 50.7%.

F. Interpretation, applications, and limitations

MTT is valuable for screening and comparing treatments, but the result must be interpreted within the assay’s biological and technical limits.

• Dose-response analysis: A series of concentrations can reveal increasing inhibition as dose rises. IC₅₀ is estimated by fitting a concentration-response curve, preferably using nonlinear regression rather than selecting the closest measured point.
• Time-course analysis: Measuring several exposure times distinguishes rapid toxicity from delayed effects, such as reduced proliferation or progressive metabolic decline.
• Cell proliferation studies: Increasing absorbance over time may reflect cell growth, but only if cell density remains within the assay’s linear range.
• Drug screening: MTT can compare relative cytotoxicity across compounds in multiwell plates and is suitable for preliminary concentration screening.
• Linearity requirement: A standard curve using known cell numbers can test whether absorbance is proportional to cell number under the chosen seeding density and incubation conditions.
• Metabolic confounding: A compound may inhibit mitochondrial or cytosolic reductases without killing the cell, causing low MTT signal despite temporary survival.
• False elevation: Some treatments stimulate cellular reducing activity or chemically reduce MTT, producing high absorbance that does not represent increased cell number.
• Optical interference: Colored or turbid compounds can absorb or scatter light at the measurement wavelength. Cell-free treatment controls help identify this problem.
• Crystal and pipetting error: Incomplete solubilization, uneven cell distribution, bubbles, evaporation, and inconsistent incubation times can distort absorbance.
• Limited biological conclusion: MTT does not by itself establish apoptosis, necrosis, membrane integrity, clonogenic survival, or mechanism of death; complementary assays may be required.
• Assay optimization: Cell density, MTT concentration, incubation time, solvent volume, wavelength, and treatment duration should be validated for each cell line and experimental system.
• Result presentation: Report normalized mean viability with an appropriate measure of variation, identify the number of independent experiments, and state the controls and calculation method used.