Where I work we test cats 4 months and older. Positives are retested 1-2 months later.

If they test negative for FELV we go ahead and vaccinate. We don't use the FIV vaccination where I work because it causes a cat that has been vaccinated to test positive for the disease since the test (SNAP combo) doesn't know the difference between the actual disease and the vaccine immune response. (BTW this is not the ONLY reason our hospital doesn't use that vaccine)

That's about all I know about it, although if I get a chance I'll ask the head vet next time I'm at work because now I'm curious. lol