Tests to detect the presence or quantity of an antiviral antibody

A method for determining the presence or absence of an anti-drug antibody (ADA) in a sample, the method comprising: contacting the sample with an excess amount of a drug to which the ADA binds to form drug / ADA complexes; contacting the drug / ADA complexes with polyethylene glycol (PEG) to form a precipitate comprising drug / ADA complexes; contacting the precipitate with a solution to dissociate the drug / ADA complexes, thereby forming dissociated ADAs and dissociated drugs; immobilize on a substrate the dissociated ADAs, in conditions to keep the dissociated ADAs and dissociated drugs separate; and determine if the ADA is present or absent in the sample.