Protein LoBind® microcentrifuge tubes were used to handle and contain samples containing proteins and reagents. Lateral flow immunoassays were conducted in nonbinding flat-bottom microplates (Grenier). Unless otherwise stated, all chemical reagents and proteins were purchased from Sigma Aldrich and did not undergo further purification before use. Concentrated buffer solutions, such as phosphate buffered saline (PBS, Gibco) and borate buffer (Thermo Fisher), were diluted to desired concentrations prior to use. Fab(Her) fragments (FabHER) were produced via the antibody digestion of Herceptin antibody (University College London Hospital) following a protocol by Szijj et al.. Nylon syringe filters (0.45 μm; Nalgene) were used to filter solutions containing semi-skimmed milk (Marvel) as blocking agent prior to assay use.