{c}

\b[MTT] is the <tetrazolium salt> 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide used as a color-forming reagent. It contains a substituted tetrazolium cation and a bromide <counterion>. The manufacturer's documentation names the reagent and describes its <reduction> to a purple <formazan> product.<Cell Proliferation Kit I MTT instructions by Roche>[Printed p. 3, reagent identity; Section 3.1 and Figure 4, printed p. 9]{ref}

The reagent is not the same thing as an <MTT assay>. The assay combines cellular or chemical reduction, product handling, optical measurement and an interpretation model. In the manufacturer example, product crystals are solubilized before a multiwell optical readout. An <MTT formazan solubilization check> addresses product recovery and dissolution rather than proving how many cells were present.

An original response model makes the distinction quantitative. Let each of $N$ cells produce product at constant rate $v$ during observation time $t$, with no other source. Product amount is $n_F=Nvt$ and, after complete dissolution,
$$
A-A_0=\epsilon\ell\frac{Nvt}{V}.
$$
At fixed optical conditions, $N=10000$ cells with per-cell rate $v_0$ and $N=5000$ cells with rate $2v_0$ give the same modeled signal. A signal doubling could result from more cells, a higher rate per cell or both. This is a chosen proportional-response example, not a universal MTT calibration.

The primary 1983 method used tetrazolium reduction and a multiwell scanning <spectrophotometer>, while explicitly noting dependence on cellular activation.<Rapid colorimetric assay for cellular growth and survival: application to proliferation and cytotoxicity assays by T. Mosmann>[Abstract]{ref} Later primary work showed that direct chemical reduction can generate formazan without cells.<Interference of plant extracts, phytoestrogens and antioxidants with the MTT tetrazolium assay by Regina Bruggisser and others>[Abstract, cell-free tests]{ref}

A useful interpretation therefore specifies reagent identity, cell conditions, observation interval, product dissolution, <background subtraction> and the optical observable. If the goal is <cell viability> or cell number, an appropriate independent comparison should test whether the signal model remains valid in the treatment conditions. The chemical color reaction alone does not supply that conclusion.


 MTT (source code)