You will have definitely much higher RNA yield by using precipitation methods like ExoSpin (Cell Guidance Systems). This because the exosome yield is much higher when using precipitation methods rather than ultracentrifugation. The comparative analysis has been performed here:

https://www.nature.com/articles/srep07639

I'm using the precipitation method but RNA yield is very low about 17-25 ng/ul and 260/230 ratio is highly reduced as well !

and as i have no idea what is the reliable concentration of RNA that should be obtained from exosomes ?!!! I've guessed that isolation method may affect the RNA yield

In terms of purity, In my experience, ultracentrifuging followed by flotation on a density graident (i.e. sucrose or OptiPrep) is much better than using precipitation methods, which typically yield a lot of "junk" (debris, proteins, etc).

In fact, Drs. Thery, Lotvall (renowned experts in EVs), and their co-authors say that "Separation of EVs from other particulate material can be guaranteed only by floatation (=upward displacement). However, for some other separations, sedimentation (=downward displacement) may be more appropriate." (Lotvall et al. 2014). 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4275645/

Lobb et al. also published a great paper comparing methods of EV isolation int he Journal of Extracellular Vesicles, he official journal of the International Society for Extracellular Vesicles. 

https://www.ncbi.nlm.nih.gov/pubmed/26194179

Regardless of the method used to isolate EVs, at a bare minimum EM or AFM and Western blots (alternatives to WB include FACS and mass spec) should be done (i.e. for CD63, TSG101, etc for exosomes) to confirm purported EVs. Nanotracking is also nice as well. Minimal experimental requirements are detailed in the previously mentioned paper by Lotvall et al. 

While Lane et al. show that the precipitation methods do have a higher yield of EVs, they also do not show EM/AFM of their EVs or Western blots.  Without seeing these images, it is impossible to know if their methods isolated pure EVs or EVs with additional "junk". 

Hope this helps!

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4275645/

https://www.ncbi.nlm.nih.gov/pubmed/26194179

https://www.nature.com/articles/srep07639

Dear Rachel,

Thank you .. this is very helpful 

Of course i'm intending to carry out  western blot , but now i'm so curious about how much RNA should i get from both ultracentrifuge and precipitation methods