Immuno-PCR with digital readout.

Immuno-PCR (IPCR) combines the versatile ELISA antigen detection with ultrasensitive PCR signal amplification, thereby enabling the highly sensitive detection of a broad range of targets with a typically very large dynamic detection range. The quantification of the antigen is usually achieved by real-time PCR, which provides a correlation between the target concentration and amplified DNA marker. We here report on the implementation of digital droplet PCR as a means for direct quantification of DNA copies to enable the highly sensitive detection of protein biomarkers. To this end, two alternative approaches, based on either magnetic microbead-based IPCR or a microplate-release IPCR were tested. The latter format worked well and revealed an extraordinary high robustness and sensitivity. While rtIPCR already fulfills typical immunoassay acceptance criteria, ddIPCR enables improved accuracy and precision of the assay because signal response and analyte concentrations are directly correlated. The utility of the novel ddIPCR technology is demonstrated at the example of two cytokines, interleukin 2 and interleukin 6 (IL2, IL6, respectively), with an overall average CV% of 5.0 (IL2) and 7.4 (IL6).