Anti-TNF alpha antibodies were among the first approved antibody drugs and now belongs to the best-selling drugs. Today, several companies are developing biosimilars to those drugs which will increase the access of medications and potentially reduce health care costs. There is a great demand for pharmacokinetic assays for anti-TNF-alpha drugs and the bridging assay format is a potential tool, mostly due to its high serum tolerance. This project at Gyros Protein Technologies AB aimed to investigate the properties of the solid phase on the Gyrolab and to utilize this to optimize the bridging assay to be used as a pharmacokinetic assay for a human antibody in the presence of serum. The solid phase was optimized by incorporating three reagents with increasing molecular weight and examining the column profiles generated. Furthermore, the capture reagent was titrated with b-BSA to avoid cross-binding of both arms of the antibody to the capture reagent. Since the background was relatively high, further optimization was done to reduce background and increase the signal to noise ratio. The performance of the optimized bridging assay was compared to alternative PK assay formats. The estimated sensitivity of the bridging assay was 5 ng/ml compared to 250 ng/ml for the indirect antibody assay and 2.5 ng/ml for the bridging assay using an anti-idiotypic antibody as detect. The optimized bridging assay performed well without dilution in buffer and was therefore used for affinity determination of Humira in neat serum. Variable concentrations of TNF-alpha were added to a fix concentration of Humira to compete with the interaction. Calculated KD-values were similar regardless of whether the measurements were performed in neat serum or after dilution in Rexxip buffer.