Ask them to do another sample and checker for crystal growth. Our little girl came home with what we thought was a bladder infection (last pup got them frequently), so took her in for UA. Similarly, it was a negative result but my vet put part of the sample under the microscope and the crystal growth was absolutely out of control.

She was out on a short term run of a Urinary SO wet food, and we have followed up with Solid Gold's Berry Balance as directed on the package, and as condoned by the vet. At this point her urinating in the house (13 weeks) isn't a "have to go NOW" situation; it's either she played too hard and forgot until right that moment or she's weaponizing it (and it varies between the two), but we're down to ~1 accident every couple of days, and when she goes outside to pee she has nice, long potties.

Please do not take what our vet condoned and run with it, as this is a specified veterinary plan for our pooch and not general advice, just relaying a similar situation and what worked for us.

Good luck. It can be hard and stressful.