Unfortunately, a fecal won't necessarily turn up parasties, especially if they're protozoan (giardia, coccidia, toxoplasmosis, t. foetus, others). With rescue kitties, it's just best to assume they have something.
Here's info on t. foetus:
http://www.fabcats.org/breeders/info...ichomonas.html
Even a cat with something as "obvious" as round worm won't necessarily have a fecal sample test positive. One of our recently rescued kitties threw up a round worm. We took in a fecal, and it tested negative. But we knew he had them!
Basically it's best just to treat them and not worry about "negative" fecals if there's an issue with unresolved diarrhea/soft stools. Unfortunately t. foetus is generally resistant to the standard treatments (which include Panacur (fenbendazole) or Flagyl (metronadizole)).
Of course, it could be a food sensitivity. Some cats have a problem with various proteins or grains. When our kitty's diarrhea didn't resolve after he'd been properly treated for parasites over several months, we switched him to an anti-allergan diet of the Hill's z/d. It's not meant for kittens, which would be a concern.... but that's something to discuss with your vet. It uses a hydrolyzed chicken protein, so even if chicken is a problem, it shouldn't trigger a reaction.
Something simple to try is fasting your kitty for 24 hours, then feeding her just plain boiled chicken and rice for a few days. If her stool firms up, you
know it's a food sensitivity and not a parasite. If it doesn't - then you're where you are right now, because the problem could be the chicken, the rice, or a parasite.
But we treated for parasites with Profender (tropical) every three weeks (three times), and three courses of treatment with metro. Then we fasted him, put him on chicken and rice - and as our kitty's problem was diarrhea, we were really pleased to see soft stools after just a few days.
As to the blood work.... we do it annually anyway. We think it's good to have the baseline measurement with which to compare in the future... You won't find the reason for soft stool in blood work (usually). But it's a good way to stay on top of your cat's overall health.
