Multicolor flow cytometer assays are routinely used in clinical laboratories for immunophenotyping, monitoring disease and treatment, and determining prognostic factors. However, existing methods for quantitative measurements have not yet produced satisfactory results independent of flow cytometers used. This chapter details a procedure for quantifying surface and intracellular protein biomarkers by calibrating the output of a multicolor flow cytometer in units of antibodies bound per cell (ABC). The procedure includes the following critical steps: (a) quality control (QC) and performance characterization of the multicolor flow cytometer, (b) fluorescence calibration using hard dyed microspheres assigned with fluorescence intensity values in equivalent number of reference fluorophores (ERF), (c) compensation for correction of fluorescence spillover, and (d) application of a biological reference standard for translating the ERF scale to the ABC scale. The chapter also points out current efforts for implementing quantification of biomarkers in a manner which is independent of instrument platforms and reagent differences.
Keywords: Antibodies bound per cell; CD4+ lymphocytes; Compensation; Equivalent number of reference fluorophores (ERF); Fluorescence calibration; Instrument quality control; Instrument sensitivity; Multicolor flow cytometry; Pulsed light-emitting diode (LED) light source.