Single cell microfluidic impedance cytometry has been used to identify cells at high speed, on the basis of their dielectric properties. However, there is no electrical analogue to a fluorescent label, meaning that it is not possible to identify subpopulations of cells. We demonstrate discrimination and enumeration of antigenically defined cell subpopulations using an alternating current (AC) impedance labeling method. Small antibody conjugated beads are mixed with cells and bind to the target population, changing the electrical properties of the target subset of cells. The principle of the technique is demonstrated by identifying and enumerating the CD4 T-lymphocyte subpopulation in human whole blood. The technique represents a simple method for detecting a subpopulation of cells within a heterogeneous mix. The impedance-based antibody identification method could form the basis of simple low-cost point of care diagnostic technologies.
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