Assessment of colorimetric amplification methods in a paper-based immunoassay for diagnosis of malaria.

Colorimetric detection methods that produce results readable by eye are important for diagnostic tests in resource-limited settings. In this work, we have compared three main types of colorimetric methods - enzymatic reactions, silver deposition catalyzed by gold nanoparticles, and polymerization-based amplification - in a paper-based immunoassay for detection of Plasmodium falciparum histidine-rich protein 2, a biomarker of malarial infection. We kept the binding events in the immunoassay constant in order to isolate the effect of the detection method on the outcome of the test. We have highlighted that the optimal readout time in a test can vary significantly - ranging from immediately after addition of a visualization agent to 25 minutes after addition of a visualization agent - depending on the colorimetric method being used, and accurate time keeping is essential to prevent false positives in methods where substantial color develops over time in negative tests. We have also shown that the choice of a colorimetric method impacts the calculated limit-of-detection, the ease of visual perception of the readout, and the total cost of the assay, and therefore directly impacts the feasibility and the ease-of-use of a test in field settings.