Why should I consider k-factor when establishing a protocol for ultracentrifugation?

Because one of the most important factors in determining sedimentation efficiency of nanovesicles in a differential centrifugation protocol is the k-factor of the rotor.1 In essence, the k-factor indicates the relative pelleting efficiency of a specific rotor at maximum rotation speed.2 The most efficient rotors have the lowest k-factor value, operate at a relatively high centrifugal force, and have a low sedimentation path length.1

In addition, k-factor can be used to predict time (in hours) required for sedimentation of exosomes using different rotors. Finally, when reporting the results of your research, including the k-factor for the rotor you used can help improve the accuracy, reproducibility and adaptation of new exosome isolation protocols.1,2

[1] Momen-Heravi F, Balaj L, et al. Current methods for the isolation of extracellular vesicles. Biol Chem. 394;1253-62: (2013). doi: 10.1515/hsz-2013-0141.

[2] Cvjetkovic A, Lötvall J, Lässer C. The influence of rotor type and centrifugation time on the yield and purity of extracellular vesicles. J Extracell Vesicles. 3; 23111: (2014). doi: org/10.3402/jev.v3.23111.