Consider for:
Post-obstructive diuresis >200mL/hr for 2 hours or 3L over 24 hours
Elevated BUN/Cr (acute renal failure)
Significant hematuria or clot<unk> 
New neurologic cause (e.g. cord compression)
Otherwise consider discharge with catheter placed to leg bag and urology follow up within 1 week
Acute urinary retention
Obstructive causes
Acute cystitis
BPH
Diabetes insipidus
Prostate cancer
TURP syndrome
Blood clot