MTT Viability Assay

[caroline.berns at cup.uni-muenchen.de]

I would like to hear from someone who is using the MTT viability assay, if possible for leukemia cells like HL-60.
My problem is that I don't know how to interprete my results correctly, because even the dead cells show  certain absorbance. My substances are dissolved in DMSO and I add 1µl per 99µl of cell suspension. As a control I add 1µl of pure DMSO and set the absorbance measured for these wells as 100%. 
Though I am trying to keep the number of cells constant each time there are different results for my  'zero-values', i.e. a well with only dead cells, varying from 18 to 24%. 
Is there a chance to determine a sort of blank for this assay? How can I calculate the IC50?
 
I hope someone can help me
Cheers, Caroline

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