I’m so sorry — I have been where you are, and it’s awful. I don’t know your fur baby’s full story, but the very first cat i adopted when i lived out here was an older cat who’d lived a rough life from the sound of it. I adopted him in early February 2020, and by the end of the month, I’d had to have him euthanized because he had severe kidney disease. His labs had been fine according to the medical director at the adoption agency, however, all the vets agreed that this can happen with older cats who’ve led rough lives; their bloodwork is ok until they find their forever home where they feel safe and loved, and then any underlying disease will rear its ugly head. It’s cruel, but apparently very common when adopting older cats. Cost a lot of money for the xrays, ultrasound, labs, medications, IV fluids, etc. Sorry to ramble— i don’t know if any of this was helpful or not. However, I feel your pain and I am so sorry.