False positives with MTT occur very often. It can be due to some components in media or even the tested compounds can add to a false positive result (particularly if they are colorful). I use Opti-MEM instead of medium and use several positive/negative controls.
MTT gives result even in presence of 90% PBS nicely, you can check color development in microscope, some time 2 h incubation is enough. If you are doubtful about accuracy you can use some positive controls as well.
It depends on what do you realy want to do : mentain, specific test, what cells..........
Thanks very much for your very helpful answers. I get same color in wells that have a fewer cells than controls. I want to reduce incubation time to 2 h, because i think the number of cells in the test wells is high.
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