Cell Viability Determination by MTT Colorimetric Assay

The following protocol uses the MTT colorimetric assay as a classic literature-established method for assessing cell viability/metabolic activity in cultured mammalian cells. MTT[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] is reduced by metabolically active cells to a colored formazan product; the amount of formazan is quantified spectrophotometrically and provides an indirect measure of metabolically active viable cells. Importantly, MTT reduction reflects cellular oxidoreductase/metabolic activity rather than an absolute direct count of living cells, so changes in cellular metabolism can alter the signal independently of cell number^[1][2][3].

The original MTT method demonstrated that tetrazolium reduction can distinguish living from dead cells and can be applied to proliferation and cytotoxicity experiments. Subsequent mechanistic work established that NAD(P)H-dependent cellular dehydrogenase/oxidoreductase reactions contribute to tetrazolium reduction, while comparative studies have shown that MTT measurements can be influenced by experimental compounds and cellular metabolic state. Consequently, MTT results are most appropriately interpreted as relative metabolic viability under the specified experimental conditions rather than as a direct enumeration of viable cells^[1][2][4].

Reagents and chemicals: Use complete culture medium suitable for the selected mammalian cell line and the compounds used for experimental treatment. MTT serves as the tetrazolium substrate; a solubilizing agent is required following MTT reduction. In mammalian cell protocols, dimethyl sulfoxide (DMSO) or an acidified alcohol solution is typically used for solubilization^[3][5]. Dye or kit: MTT; the final MTT concentration commonly used in mammalian cell protocols is approximately 0.5 mg/mL-for example, incubating cells with 100 µL of 0.5 mg/mL MTT at 37°C for 3 hours^[5]. Equipment and instruments: 96-well culture plates and a spectrophotometric microplate reader capable of measuring absorbance at 570 nm^[1][5].

Preparation Steps

Seed cells into a 96-well plate at a density empirically established to keep the assay response within its useful cell-density range. Cell density should be optimized for the specific cell line rather than assumed to be universal because MTT signal depends on both cell number and metabolic activity; published work specifically recommends regular cell counting and control of culture conditions for reliable MTT measurements^[3][4].

Include untreated/vehicle-treated control wells and cell-free background wells. Cell-free wells allow background absorbance to be measured and subtracted from experimental measurements. When testing compounds, the corresponding vehicle should be present at the same concentration in control wells because treatment components themselves can affect MTT reduction or directly interfere with the assay chemistry^[4][5].

Allow adherent cells to attach under the culture conditions appropriate to the selected cell line before experimental treatment. Apply the experimental treatment for the biologically defined exposure period; because treatment duration is application-specific, no universal treatment concentration or exposure duration is specified here^[3][4].

Operation Steps

At the assay endpoint, remove the treatment medium carefully from adherent cells. Add 100 µL of 0.5 mg/mL MTT solution per well and incubate for 3 h at 37°C, conditions used in a published comparative mammalian-cell viability study^[5].

For suspension cells, one published protocol centrifuged 96-well plates for 5 min at 1,000 × g before carefully removing medium, followed by addition of MTT. This modification minimizes loss of suspension cells during medium removal; the same study nevertheless found that MTT measurements were more challenging in suspension than adherent cells^[5].

After MTT incubation, solubilize the generated formazan completely before measurement. One published protocol added an equal volume of 89% isopropanol, 10% Triton X-100, 0.1 M HCl solubilization solution and dissolved the crystals by pipetting; alternatively, mammalian-cell studies have used DMSO after MTT incubation to dissolve the insoluble formazan^[5][6].

Measure absorbance at 570 nm with a microplate spectrophotometer after the formazan has been completely solubilized. Maintain identical assay timing and processing conditions among treatment and control wells because the amount of formazan produced depends on assay conditions as well as cellular metabolic activity^[3][5].

Data Acquisition and Analysis

Subtract the mean absorbance of cell-free background wells from experimental measurements before normalization. For relative viability experiments, the background-corrected untreated or vehicle control can be defined as 100%, and relative metabolic viability can be expressed as 100 × (background-corrected absorbance of treated cells / background-corrected absorbance of control cells). Published MTT experiments use this control-normalization approach, while comparative methodology studies explicitly incorporate cell-free controls to remove background signal^[5][6].

Use technical replicate wells and independent biological experiments rather than relying on a single well. In one comparative viability study, six technical wells were analyzed per condition and biological replicates were generated from cells of different passages; concentration-response data were analyzed using nonlinear variable-slope curve fitting when GI50 or LC50 values were required^[5].

Interpret a lower normalized MTT signal as reduced metabolic activity relative to the control, but do not automatically equate the magnitude of that decrease with the same magnitude of cell death. MTT can be affected by treatment-induced metabolic changes and direct chemical interference, and comparative studies have demonstrated discrepancies between MTT, direct cell enumeration, SRB, resazurin, and other viability measurements. When conclusions about cell death itself are important, an independent viability/cell-death method provides a useful cross-check^[2][4][5].

Problem: MTT absorbance does not correspond to the apparent number of cells.

Possible Cause: The experimental treatment may alter cellular oxidoreductase/metabolic activity per cell, so MTT-derived formazan is no longer proportional to cell number.

Solution: Establish the cell-number/MTT-response relationship under the relevant experimental conditions and corroborate important cytotoxicity conclusions using an assay based on a different biological endpoint^[3][4][5].

Problem: A test compound produces unexpectedly high apparent viability or otherwise inconsistent MTT results.

Possible Cause: Test compounds can directly reduce MTT or otherwise interfere with formazan generation independently of viable cells; interference has been demonstrated experimentally for glycolysis inhibitors and other reducing compounds^[4].

Solution: Include cell-free wells containing the test compound and MTT to identify direct chemical interference and, when interference is present, use an orthogonal viability method rather than interpreting the affected MTT signal as cell viability^[4][5].

Problem: Replicate wells show high variability or poor linearity.

Possible Cause: Cell density, culture conditions, MTT incubation conditions, and incomplete or inconsistent formazan handling can influence signal generation.

Solution: Optimize the cell-number range before the experiment, maintain standardized MTT incubation conditions, inspect cultures microscopically, and ensure complete and homogeneous solubilization of formazan before absorbance measurement^[3][5].

Problem: Suspension-cell measurements are inconsistent.

Possible Cause: Suspension cells can be lost during medium-removal and processing steps, and comparative studies report greater difficulty using MTT for suspension cultures.

Solution: A published approach centrifuged plates for 5 min at 1,000 × g before carefully removing medium to retain cells; results should still be interpreted cautiously and can be cross-checked with a method better suited to direct assessment of suspension-cell viability^[5].