Additional thoughts on Merlin.
Pre renal azotemia is often associated with mild azotemia - slight elevation of the BUN and normal or near normal creatinine. Azotemic patients with evidence of adequate urinary concentration (specific gravity >1.040 in cats) have pre-renal azotemia.
A list of pre-renal causes: consumption of a high protein diet, gastrointestinal hemorrhage, treatment with corticosteroids (as it creates a catabolic state), dehydration, hypovolemia and decreased cardiac output.
Side note on chronic renal failure in cats: most will have unconcentrated urine with a specific gravity of 1.008-1.012. Some dogs can have minimal concentration up to 1.025 and some cats can concentrate up to 0.035
Important note: since the specific gravity sometimes alters a diagnosis of pre-renal versus renal, it is important that this is not an old sample that has 'dried' up and become concentrated. I have seen cases misdiagnosed as pre-renal because of urine that was 'concentrated' by evaporation.
How is Merlin doing by the way?