Well as a survivor of Hodgkin's Disease a type of blood cancer you will be re-assured to know that even this could not be detected with a blood test - had to be a biopsy of a lump in my neck that was in 1987 & again in 1991.
To my knowledge cancers are still diagnosed by means of a biopsy so if you rule out worst case scenario's everything gets better.
Do not know your age or reasons for the tests but common things as you get older are diabetes, high colestrol / lipid levels / high billirubens. All very treatable.
We all fear the unknown but start to worry when you have the facts - then get your head round them and cope with what life throws at you.
That my philosophy on life - just had the one heart attack 2 years ago but managed to have 4 stents fitted prior to that. Still here to tell the tale and working at 65 !
Sound logical advice... but I think it would still be hard to not be worried... :lol
Like Rick, our Vet takes a sample of blood from the ferrets, sends out to the waiting room, unless the nurse is there to do it, and there are more things to check on the ferret, and in 20 mins, the basic results are back.. When Suz had Pneumonia at the beginning of the year, and the hospital needed to know which strain it was, in order to use the right antibiotics, I think it took them less than half an hour as well although time frames when you are in a hospital are all warped...
I guess it depends on what they are looking for though, and if they need to culture it to see what is growing, then things take longer.
My Dad had Hodgkin's disease, cancer of the Lymph system... unfortunately, he was one of the early ones, and the treatment was just as dangerous as the cancer, so it was the treatment that got him in the end. Luckily, he was a "Straight up" man, and saw the irony in the doctors saying that they had learnt a lot from him, and he would be helping a lot of people live in the future... but as macabethiel says, those sort of results take a lot longer to come back, and are not found in a blood test.
I hope your visit to the vet.. er I mean doctor goes well today.:doh