Pre Mate Blood Tests - Are They Anygood?

Just looking for some opinions really. I tried the whole Mate-Tel thing on Isla's last season and tbh it didn't work, although I do think I didn't get enough saliva in the microscope. Although I think we are at serioius risk of Isla eating the microscope if I do try and get more saliva!

Anyway, in preparation for the hopeful litter next year my night time reading is currently The Book of The Bitch. It mentioned pre mate blood tests and I'm just interested to see if anyone has used them and if they're anygood?

Isla is one of those dogs who likes to keep you on your toes and her seasons are no different, so any help is much appreciated!