Are there any differences between titers based upon p24 ELISA and FACS analysis?

A common method for calculating viral titers is to use an ELISA assay, which measures the amount of the viral capsid protein p24 in the supernatant of the packaging cell line after transfection with the transfer and helper plasmids. Because free, unassociated p24 may be present and because not all viral particles containing p24 are competent to transduce cells, estimations of the number of transducing units based on p24 quantification can overestimate the effective or functional titer. For this reason, when titers are assessed for SMARTvector 2.0 Lentiviral shRNA Particles by p24 ELISA, a reference SMARTvector 2.0 Lentiviral shRNA containing hCMV and TurboGFP with known functional titer is included with each assay. The functional titer of the reference lentiviral vector stock is determined by transduction of HEK293T cells with serial dilutions of virus and using flow cytometry to count GFP-positive cells.