Lab on paper: assay of beta-lactam pharmaceuticals by redox titration

Field tests to detect substandard antibiotics are sorely needed in many low and middle-income countries (LMICs). We created a field screening test for amoxicillin capsules and ampicillin tablets based on USP method <425>. To measure the amount of antibiotic, the user dissolves the contents of the capsule in water, carries out a simple chemical degradation, and adds drops of the sample to a paper test card. The test card performs a titration and displays a series of blue dots for visual comparison to standard images. Pharmaceutical samples were collected in western Kenya and analyzed using a blinded methodology to validate the test card's quantitative performance against HPLC assay. Within the quantification range of the test card, the absolute assay error for dosage forms was 4.4% for amoxicillin and 5.3% for ampicillin, and inter-operator precision was 2%. Users correctly categorized 94% of amoxicillin (n = 84) and 88% of ampicillin (n = 56) samples as either containing less than or greater than 90% of the expected amount of antibiotic, which is the lower regulatory limit. The test card detected several lot numbers of a substandard amoxicillin product, confirmed by HPLC analysis to contain only 40–60% of the stated amoxicillin content. IR, PXRD, and gravimetric analysis revealed that talc made up the remainder of the adulterated medications.