What is the difference between a p24 and qPCR titer?

A p24 titer is acquired via a p24 Gag antigen ELISA, whereas qPCR detects the amount of lentivirus DNA present in the sample.

Traditional p24 ELISA titrations measure both functional and non-functional lentivirus particles. However, it overestimates the functional titer, as the p24 protein pool includes a variable amount of free p24 and p24 associated with non-functional vector particles. The ratio can vary greatly between each lot so it is inherently inaccurate.

Following transduction, qPCR titration quantifies the number of lentivirus genome copies in a cell population as compared to a cellular reference gene. Absolute quantification via the standard curve method indicates the number of integrated genomes per cell. Thus, although qPCR titers are often lower than p24 titers, qPCR titration yields a more reliable functional titer.