Since Luna (who turned 14 three days ago) was dealing with an UTI two weeks ago, I got some of these urine test strips to find out if the infection was gone. Unfortunately, the strips also showed that her bilirubin level is notably increased which means that something's wrong with her liver. More precisely, the biliary duct must be blocked by something which is definitely NOT good. Just spoke to my vet about the options we have. We could do some bloodwork (and I guess I will sooner or later), but that would be torture because it cannot be done without anaesthesia, which is not without risk at her age. Besides, the bloodwork would only tell that something's wrong with her liver, but won't tell what exactly it is. The only way to find out would be a biopsy, and I don't think I will put her through this. Not at her age! Especially since there wouldn't be much we could do for her if it was cancer.
So, my vet advised to treat her with Hepar comp to support her liver, and I will start doing that next week. BTW, there are no signs of jaundice yet (and no other signs of illness), so obviously the bilirubin level in her blood is not that high - not yet, at least.