= MTT assay
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The \b[MTT assay] measures formation of colored <formazan> when metabolically active <biological cells> reduce the <tetrazolium salt> <MTT>. The initially yellow reagent produces purple crystals; those crystals are solubilized before optical measurement with a multiwell <spectrophotometer> or <microplate reader>. The Roche instructions describe this chemical conversion and readout explicitly.<Cell Proliferation Kit I MTT instructions by Roche>[§3.1, printed p. 9 and Figure 4; printed p. 10, solubilization and optical measurement]{ref}
The measured endpoint is a metabolic color response. Under a calibrated linear model it can track viable-cell number, but per-cell activity also matters. <Tim R. Mosmann>'s original 1983 paper noted that the signal depends on cellular activation as well as the presence of living cells.<Rapid colorimetric assay for cellular growth and survival: application to proliferation and cytotoxicity assays by T. Mosmann>[Abstract, activation dependence and multiwell readout]{ref}
For an original normalized comparison, suppose treated wells give <absorbance> $A_t=0.62$, matched untreated controls $A_c=1.02$ and the reagent blank $A_b=0.02$. The blank-corrected signal ratio is
$$
V=\frac{A_t-A_b}{A_c-A_b}=0.60.
$$
This means $60\%$ of the control's measured MTT signal. It does not establish that $40\%$ of cells died. In $A-A_b=qN$, fewer viable cells $N$ and lower response per cell $q$ can produce the same result.
A calibration series with independently known cell numbers tests the useful response range. Cell-free wells containing the tested material help identify direct color, light scattering or reagent interactions; a vehicle control separates treatment-solvent effects. Complete and comparable solubilization matters because the readout concerns the colored solution rather than an arbitrary suspension of crystals.
For a <cytotoxicity> study, complementing this assay with a cell count or membrane-integrity endpoint can distinguish some ambiguities. Incubation history, cell state and observation time belong in the interpretation. MTT color formation, <cell proliferation> and <cell death> are related experimental questions, not interchangeable definitions.
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