Can diluting biofluids with PBS prior to ultracentrifugation enhance recovery of exosomes?

This depends on the type of biofluid you’re using. Dilution of some fluids with PBS may indeed enhance the recovery of exosomes by altering their viscosity.1 Théry and colleagues recommend diluting plasma, serum, lymph, tumor ascites, urine and bronchoalveolar lavage (BAL) with an equal volume of PBS prior to centrifugation.2 This may not be necessary for other biofluids.

[1] Momen-Heravi F, Balaj L, Alian S, Trachtenberg AJ, et al. Impact of biofluid viscosity on size and sedimentation efficiency of the isolated microvesicles. Front Physiol. 3; 162: (2012). doi: 10.3389/fphys.2012.00162.

[2] Théry C, Amigorena S, Clayton A and Raposo G. Isolation and characterization of exosomes from cell culture supernatants and biological fluids. Curr Protoc Cell Biol. Chapter 3; Unit 3.22: (2006).