Table 4.

Terms and definitions for anti-drug antibody (ADA) assays

Term	Definition
ADA assay	Bioanalytical method used to determine if a sample is qualitatively positive or negative for ADA and that can provide quasi-quantitative information about the amount of ADA (typically reported as an antibody titre). Often considered synonymous with binding ADA assay (assay designed to detect antibodies that bind the biopharmaceutical (BP) regardless of the functional activity of the ADA) and total ADA assay (assay that measures all ADA in the sample, although usually not capable of binding immunoglobulin (Ig)E due to its low levels).
Free ADA assay	ADA assay that measures ADA not bound to the BP. Generally a binding ADA assay can be considered a free ADA assay unless specific ADA–BP dissociating conditions are incorporated into the assay procedure.
<Isotype/subclass x> ADA assay	ADA assay designed to detect ADA of specific isotype(s) or group of isotypes; e.g. IgE ADA assay
Neutralizing antibody assay	Assay used to determine whether ADA in a sample can neutralize some aspect of drug activity. Encompasses bioassay (cell-based or enzymatic) or competitive ligand-binding assay
<Epitope/domain> ADA assay	Assay designed to detect ADA specific for a particular epitope or domain of the BP, e.g. Fab assay
Screening assay	In a tiered testing strategy, the assay used to distinguish potentially positive samples (based on screening cut-point) versus negative samples
Confirmatory assay	An assay conducted on samples found to be potentially positive in the screening assay in a tiered testing strategy to identify false and true positives (based on confirmatory cut-point)
ADA characterization assay	Investigational assay that is designed to obtain additional information on the specificity or type of antibodies present in a sample. Information obtained from these assays may include, but is not limited to, the following: titre, neutralizing antibody assay, isotyping assay (see definitions below and above)
Qualitative assay	Assay that reports test results as positive/negative
Quasi-quantitative Assay	Assay that reports a relative magnitude of ADA present in a sample (e.g. ADA titre)
Titre assay	A quasi-quantitative assay providing titre as the unit of the amount of antibody in a sample. The titre is often defined as the reciprocal of the lowest dilution of a sample generating a signal that is above the assay cut-point. Alternatively, the titre is defined as the reciprocal of the dilution of a sample generating a signal that is equivalent to the assay cut-point, calculated by an interpolation formula provided in an assay-specific bioanalytical method
Relative concentration assay	A quasi-quantitative assay providing sample results reported in relative mass units, determined by comparing the assay signal generated by the sample relative to a signal generated by a diluted positive control sample. Because the positive control generally contains a different mixture of antibodies than the sample, concentrations reported by this result are generally not accurate and should be reported as ‘relative concentrations’ or defined units
Cut-point	An assay signal threshold that distinguishes positive samples from negative samples, as defined in an assay-specific analytical procedure. The cut-point is usually set based on statistical analysis with treatment-naive samples representative of the study population (bioanalytical cut-point) but could be based on a biological (e.g. change in pharmacodynamics marker; biological cut-point) or clinical end-point (e.g. loss of efficacy; clinical cut-point). Cut-points are typically set for each assay in the tiered analysis strategy (screening assay, confirmatory assay, neutralizing assay cut-points)