Immunoassays are
important for the detection of proteins to enable
disease identification and monitor treatment, but many immunoassays
suffer from sensitivity limitations. The development of digital assays
has enabled highly sensitive biomarker detection and quantification,
but the necessary devices typically require precisely controlled volumes
to reduce biases in concentration estimates from compartment size
variation. These constraints have led to systems that are often expensive,
cumbersome, and challenging to operate, confining many digital assays
to centralized laboratories. To overcome these limitations, we have
developed a simplified digital immunoassay performed in polydisperse
droplets that are prepared without any specialized equipment. This
polydisperse digital droplet immunoassay (ddIA) uses proximity ligation
to remove the need for wash steps and simplifies the system to a single
reagent addition step. Using interleukin-8 (IL-8) as an example analyte,
we demonstrated the concept with samples in buffer and diluted whole
blood with limits of detection of 0.793 pM and 1.54 pM, respectively.
The development of a one-pot, washless assay greatly improves usability
compared to traditional immunoassays or digital-based systems that
rely heavily on wash steps and can be run with common and readily
available laboratory equipment such as a heater and simple fluorescent
microscope. We also developed a stochastic model with physically meaningful
parameters that can be utilized to optimize the assay and enable quantification
without standard curves, after initial characterization of the parameters.
Our polydisperse ddIA assay serves as an example of sensitive, lower-cost,
and simpler immunoassays suitable for both laboratory and point-of-care
applications.