Stain index is a measure of resolution sensitivity (the ability to resolve a dim positive signal from background). It depends upon the difference between positive and background peak medians (D) and the spread of the background peak (W). The stain index is a metric that captures both of these factors.
Stain index = D/W
= positive peak (median) - negative peak (median)/negative peak (2 x robust std devn)
unstained population (negative); stained population (positive)
(source: BD Biosciences 'Multicolor Fluorochrome Guide')
Cryopreserved human peripheral blood mononuclear cells were single-stained with anti-human CD8 (clone RPA-T8) conjugated to various fluorochromes and acquired with different flow cytometers. The stain index of gated viable lymphocytes was calculated and is presented below for each of DartLab's flow cytometers: