Negative then positive FeLV test?

I am bringing home my new babies from the SPCA on Friday, and can't shake the anxiety I have. Since I lost a cat a few months ago to feline leukemia, I'm paranoid about most everything. These kittens have tested negative on their felv test, but how common are false negatives?

There are four litter mates at the shelter and they've all tested negative.

I think if they've ALL tested negative, you're fine. If 3 litter mates tested positive, and 1 tested negative, that'd be more of a "false negative" situation. I wouldn't worry about it. If you're super worried about it, you can take them to the vet and have them perform the ELISA test with serum. They spin the blood to concentrate it, before testing it. But honestly, with 4 litter mates testing negative, I wouldn't worry about it.