When in the digital range (where all compartments contain either 0 or 1 target molecule) it is possible to multiplex qPCR assays without concern for competition or cross reactivity, as each target-containing reaction will proceed with the target binding to its primers/probe specifically, whereas no reaction will occur in compartments without targets. Having each molecule in a separate reaction compartment allows both high and low abundance targets to be counted in the same experiment without concern for “swamping out” the low abundance target (since each compartment has at most 1 target, independent of its concentration in the average sample volume). When more than one target is counted (e.g., in a duplex assay format), ratios of the counts for one target relative to another (e.g., mutant allele vs. wild type allele) enable “absolute ratios” to be quantified, using one of the targets as an internal normalizing reference (e.g., how many amplifiable genome equivalents were loaded) that has gone through the identical experiment as the other targets assayed.
In addition, since dPCR is performed as an endpoint reaction (PCR is run to completion before measuring fluorescence), having true single target molecules in isolation allows multiplexing based on probe intensity [10]. By adding the target-specific fluorescent assay at a limiting concentration, a compartment with that target molecule will be PCR-positive, but with a limited brightness at PCR endpoint. To count a second target type, a different target-specific probe with the same “color” is added at a higher concentration. A compartment with the second target will have a brighter signal at PCR endpoint than a compartment with the first target, enabling separate counts for each target. Combinations of both different color probes and different concentration probes can be used to multiplex at higher levels (Figure 2).