Transport Phenomena at Test and Control Lines in Lateral Flow Immunoassays Reveal a Path to Expanding Their Dynamic Range
This study presents theoretical underpinnings for how the dynamic range of a lateral flow immunoassay (LFIA) may be expanded by real-time imaging. The dynamic range of a sandwich LFIA is limited by the ‘hook effect’, according to which, test line signal intensities reduce with increasing analyte concentration beyond a threshold analyte concentration. Rey et al. (Anal Chem, 2017, 89(9)) have shown experimentally that the hook effect in sandwich LFIAs may be mitigated by real-time imaging of test and control line, but theoretical understanding of the transport phenomena that govern this phenomenon is lacking. In fact, transport phenomena at the control line of an LFIA have never been modelled. In this paper, we use a transport-reaction model to understand how the kinetics of signal generation at the test and control lines of an LFIA relate to analyte concentration. Using this model, we developed a method for determination of analyte concentration accurately over a much larger range than the traditional end-point detection method. The model was validated using a commercially available lateral flow assay (home pregnancy test) on which real time imaging was conducted using a time-lapse app on a smartphone; there was a strong agreement between the predictions of our model and experiments results. The newly developed readout method increased the dynamic range for the detection of human chorionic gonadotropin (hCG) to 3 orders of magnitude (compared to ~1.5 orders of magnitude achieved by traditional end-point detection), without any modification to the test strip.