My cat had one before had the crystals in his urine so he had
something done to release his urine at the vet.

Then recently he had the crystals, he peed on the little silver
platter that made me fully realize he had a problem before so it was
off to the vet. You could really see the crystals when it dried, more
to less from successive pees.

Anyway he took all his antibiotics as perscibed then not more than a
few days later he is doing his pee performance on strategically
arranged grocery bags. But this kind of makes me worry, his urine on
one bag has blood in it. He will get care asap but is there anything
known (common) that this likely is?

thanks for advice hints suggestions