You can add a standard material (e.g. 100 nm latex) to your serum in low concentration and adjust the viscosity in the table until the standard reads correctly. You can also use the standard for a verification of your system in line with ISO22412:2017 and ASTM 2490-09 (15).

This article can be helpful:Article Whole Blood Viscosity, Plasma Viscosity and Erythrocyte Aggr...

I think people at polyanalytik.com might be able to provide data. They are expert in bioapplications, viscometry and GPC with light scattering

And does the attached article help? Look at Table 2.

There is a different way to consider this. Are you interested in measuring the absolute size or interested in comparing relative sizes? If the latter, then I would just use the diffusion coefficient since it is the fundamental measurement of the technique. For comparative studies with the same serum, you don't need to know viscosity or refractive index (both of which you will need for sizing).

Also, you can put any number you like in for viscosity to make the measurement and correct for it later once you have an appropriate value. This way you can collect data while waiting for a viscometer!

Good points, as usual, John.

Thank you all, your suggestions are really useful! My initial problem was also related to the fact that I was not sure how to treat and take into account the other plasma contents...but now I got few ideas...thank you very much!