A 88-year-old man with moderate Alzheimer's disease is admitted with acute confusion. He has a urinary catheter in situ. His 4AT is 9. His temperature is 37.4°C. Urine dipstick shows leucocytes ++ and nitrites +. CRP is 12. All other investigations are normal. The junior doctor diagnoses a UTI and starts antibiotics. What is the MOST important consideration before attributing his delirium to the catheter urine findings?