We describe progress towards developing a low-cost, simple, and compact imaging system for Digital Bead Assays (DBA) for use in Point-of-Care (POC) diagnostic systems. DBA — such as digital ELISA using single molecule arrays (Simoa) — have emerged as a key advance for the sensitive detection of proteins down to attomolar concentrations, i.e., single-digit numbers of proteins in a droplet of blood or another clinical sample. These assays have enabled unique clinical research and diagnostic measurements, e.g., the measurement in blood of protein biomarkers of neurological conditions, enabling “blood tests for the brain” that can detect Alzheimer’s disease 16 years before dementia symptoms arise. For DBA to have its maximum impact on society, it must be available in low-cost, compact equipment that can be used by anyone around the world. For this goal to become a reality, low-cost and simple imaging systems are needed. In this paper, we will describe a concept for a low-cost, simple, and compact DBA imager. We will describe evaluation of low-cost optics — such as cell-phone optics — and cameras, and how image analysis methods can be used to generate useful data from the lower resolution images provided by these systems.

We report methods that improve the quantification of digital bead assays (DBA)─such as the digital enzyme-linked immunosorbent assay (ELISA)─that have found widespread use for high sensitivity measurement of proteins in clinical research and diagnostics. In digital ELISA, proteins are captured on beads, labeled with enzymes, individual beads are interrogated for activity from one or more enzymes, and the average number of enzymes per bead (AEB) is determined based on Poisson statistics. The widespread use of digital ELISA has revealed limitations to the original approaches to quantification that can lead to inaccurate AEB. Here, we have addressed the inaccuracy in AEB due to deviations from Poisson distribution in a digital ELISA for Aβ-40 by changing the AEB calculation from a fixed threshold between digital counting and average normalized intensity to a smooth, continuous combination of digital counting and intensity. We addressed issues with determining the average product fluorescence intensity from single enzymes on beads by allowing outlier, high intensity arrays to be removed from average intensities, and by permitting the use of a wider range of arrays. These approaches improved the accuracy of a digital ELISA for tau protein that was affected by aggregated detection antibodies. We increased the dynamic range of a digital ELISA for IL-17A from AEB ∼25 to ∼130 by combining long and short exposure images at the product emission wavelength to create virtual images. The methods reported will significantly improve the accuracy and robustness of DBA based on imaging─such as single molecule arrays (Simoa)─and flow detection.

We report the development of digital enzyme-linked immunosorbent assays (ELISAs) based on single molecule arrays (Simoa) with improved sensitivities over conventional digital ELISA, enabling detection of proteins at sub-attomolar concentrations. The improvements in sensitivity were based on using fewer beads to capture the target proteins (≤5000 vs.∼500 000 beads) that increased the ratio of molecules to beads, and increasing the fraction of beads that were analyzed (bead read efficiency) from ∼5% to ∼50%. Bead read efficiency was increased by: a) improving the loading of beads into arrays of microwells by combining capillary and magnetic forces in a method called magnetic-meniscus sweeping (MMS); b) using a centrifugal washer to minimize bead loss during the assay; and, c) improved optics and image analysis to enable the analysis of more microwells. Using this approach, we developed an assay for IL-17A with a limit of detection (LOD) of 0.7 aM, 437-fold more sensitive than standard digital ELISA. A digital ELISA with improved sensitivity was used to measure IL-17A in 100 serum and plasma samples with 100% detectability, compared to 51% for standard digital ELISA. Low numbers of capture beads yielded improved LODs for IL-12p70 (0.092 aM), p24 (9.1 aM), and interferon alpha (45.9 aM). IL-4 and PSA showed no improvements in sensitivity using fewer beads, primarily due to low antibody loading on beads and increased non-specific binding, respectively. The results were consistent with a kinetic model of binding that showed that combining capture antibodies with high on-rates with high antibodies per bead yields the greatest improvement in sensitivity.

The concentration of cytokines in blood are often at pictogram per mL and sub-picogram per mL. Recent advances in the sensitivity of immunoassays, such as single molecule arrays (Simoa), have allowed the detection of many of these cytokines that were previously undetectable in blood. While the limits of detection (LOD) of these new technologies are in the femtogram per mL range, the concentrations of cytokines in many healthy individuals can still not be detected. In fact, the quantitative measurement of several important cytokines are <50% in healthy individuals. We will report on advances to improve the sensitivity of the Simoa technology into the attogram per mL range that improve the quantitative measurement of a broad range of cytokines.