TFF is typically performed to formulate biologic therapeutics to high protein concentrations and includes ultrafiltration (UF) as well as diafiltration (DF) steps. During the process, protein concentrations are typically measured by at-line analytics to determine if target protein concentrations are reached to initiate the next processing step. Strongly increasing viscosities at target protein concentrations greater than 100 g/L pose however challenges to the sample measurement process with the risk of inaccurate concentration determination.