Need advice for blood in urine

Hi all,

Our cat, after a wellness checkup, has been discovered to have blood in the urine. This was determined by a dip test which showed the highest count possible for a dip test. We have no clue how long this issue has been evident.

So far we've had...

1. Bladder and kidney scans - nothing found.
2. Bloods sent for test - nothing found.
3. UTI test - nothing found, no infections.
4. A four week course of supplements for the bladder wall - no change.
5. Anti inflammatories - no change

Vet is now suggesting idiopathic cystitis and has us on another round of anti inflammatories as well as introducing wet food into the diet (has been on only dry food to date).

Cat is indoor, 13yrs, no other cats. Also has been on Atopica for a long while now but carefully managed doses (half) to balance against itching and barbering without saturation.

Does anyone have any advice they can offer? Vet has suggested another scan if nothing changes in a few weeks but at close to a grand per scan, we're not convinced thats going to show anything different to the last in early December.