The same concentration of biotherapeutic can be used with different biotherapeutics to form complexes with ADA. Signal-to-background ratios obtained in a pooled cynomolgus monkey serum spiked with purified cynomolgus monkey anti-human IgG ADAs at 0 to 50 μg/mL and with 8 biotherapeutics at 0 to 20 μg/mL. The biotherapeutics consisted of 4 ADCs, 3 mAbs, and a one-arm antibody. With all the biotherapeutics, the signal-to-background ratios increased with increasing concentrations of ADA to reach a plateau at biotherapeutic concentrations between 2 and 10 μg/mL. Based on these data, a biotherapeutic concentration of 5 μg/mL in neat serum was determined to be appropriate to form complexes with ADA in the samples containing up to 50 μg/mL of ADA, the highest level tested in our study. ADAs = anti-drug antibodies; ADC = antibody-drug conjugate; mAb = monoclonal antibody.