How Magnetism Can Improve Rapid Diagnostic Tests: Pneumococcal Pneumonia Detection

The SARS-CoV-2 virus has generated an unprecedented need for rapid diagnostic tests to enable the efficient detection and mitigation of COVID-19 pandemic. In its first stages, the diagnostic was based on PCR or other techniques which were scarce, expensive and with the additional problem of the need of hiring trained personnel. The availability of the gold standard tests used at that time (i.e., PCR) hindered the response in well-funded health care systems. This situation was even more dire in low- and middle-income countries. Lateral flow assays (LFAs) appeared as the most promising rapid tests thanks to their low cost and ease of use. Traditional ones use gold or latex nanoparticles to detect the presence of the molecule of choice. In such a case, LFAs are limited to qualitative detection and lack the necessary sensitivity for low-concentration analysis (giving rise to frequent fake negative results), especially in complex biological matrices. The use of magnetic nanoparticles as detection labels coupled with magnetic sensors would improve both limitations [1].

Developing magnetic LFAs offers advantages (i.e., magnetic concentration of the analyte) and challenges (i.e., optimization for biomarker detection). Then, the quantification of the nanoparticles can be performed by different approaches thanks to the magnetic properties of the labels, which should be addressed without adding excessive complexity to the method. We have developed in our laboratory LFAs for real-world applications. Two recent examples will be presented in this talk: Detecting antibodies generated by SARS-CoV-2 and quantifying pneumolysin, the protein that indicates pneumococcal pneumonia in urine [2] thanks to a radio-frequency inductive sensor based on a planar coil to both excite and detect the particles was used for that purpose. Using magnetic nanoclusters, we proved their ability to concentrate diluted samples thanks to their magnetic character. Thanks to this simple technique, we considerably improved the detection limit for the inductive sensor to 0.2 ng/mL.