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Antibody titration
Antibody titration in cytometry refers to a procedure used to find out the optimal amount of antibody that should be used in a particular multicolor experiment.

Antibodies used for their specificity towards certain epitopes can also bind to different epitopes for which they lack specificity, hence the phenomenon is called nonspecific binding. This phenomenom is undesired during the experiment, because it increases background signal.

During antibody titration, fluorescence intensities of positive and negative populations of known quantities are compared for different concentrations of antibodies. The concentration with the highest and most stable stain index (global maximum on the curve) is usually chosen.

The concentrations recommended by a distributor are tested to pass the standard tests. However, the conditions in particular experiment can vary to a great extend, therefore it is desirable to optimize the concentration of the antibody by titration following the same settings as the actual experiment will be performed.

Isotype controls should be used at the same concentrations.

Stain index
Stain index in cytometry is a measure, that can be used to quantify the difference in the fluorescence intensity of positive and negative populations. It can be calculated as follows : $$SI = \frac{MFI_{pos}-MFI_{neg}}{2*SD_{neg}}$$, where MFI stands for mean fluorescence intensity and SD for standard deviation.