Expanding the Scope of Cell-Free Diagnostics: Equipment-Free Quantification of Biomarkers in Human Serum

Cell-free expression (CFE) systems have enabled a variety of low-resource diagnostic tools, but these have been largely limited to assays that detect (rather than quantify) a target analyte. Biomarker quantification is challenging in CFE diagnostic systems, largely because the complex makeup of biological samples affects assay output, which precludes the use of traditional calibration methods.

We will present the development of a CFE diagnostic platform that can quantitatively measure biomarkers in complex matrices in an equipment-free fashion. The key innovation is the development of a standardization approach that allows equipment-free quantification in assays consisting of 25-100% human serum. The standardization scheme is designed so that the serum sample to be analyzed is added both to the test reaction and to a set of standard reference reactions. Analogous to a litmus test, the color of the test can be matched to the colors of the reference reactions so that the user can easily determine the sample concentration. We demonstrate this method’s utility, reliability, and robustness with a biomedically relevant zinc sensor. The test reliably quantifies clinically relevant zinc concentrations and is a significant step towards a low-cost, field-deployable test for zinc deficiency, which causes an estimated 100,000 annual childhood deaths. We demonstrate the generalizability of our quantification method by applying it to a sensor that uses an established toehold switch. The developed test is the first CFE assay to provide equipment-free biomarker quantification in a complex matrix, and the platform developed can be readily expanded to quantify other molecules in cell-free expression reactions.