Functionalization of Antibody-Coated Beads

Anti-pLDH and anti-HRP2 beads were prepared as reported previously.^{19 (link),24 (link)} Briefly, anti-pLDH (Vista 19g7) or anti-HRP2 (Abcam ab9203) was biotinylated with EZ-Link NHS–PEG4–Biotin, No-Weigh Format (Thermo no. 21329). Unreacted NHS–PEG4–Biotin was removed using 7K MWCO Zebra spin desalting columns (Thermo no. 89882). Next, 5 mg of Dynabeads MyOne streptavidin T1 was washed three times with PBS and incubated with 500 μL of 0.4 mg/mL biotinylated antibody in PBS for 30 minutes. The beads were then washed before incubating in excess D-biotin in PBS for 30 minutes. Finally, the beads were washed and resuspended in 500 μL of PBS with 0.01% Tween-20. Stock solutions of antibody-functionalized beads were transported to Zambia in ambient conditions and stored at 4°C on arrival.