MRCP2-3871

A 55-year-old man from Ghana received a live-related renal transplant four weeks ago for chronic kidney disease caused by hypertensive nephropathy. He is currently taking ciclosporin, mycophenolate mofetil, prednisolone, cotrimoxazole, ranitidine, amlodipine and atenolol. He experienced no complications after the surgery and his creatinine levels returned to normal a few days later. He is asymptomatic, but his serum creatinine levels were 155 µmol/l two days ago. What is the definitive test that will confirm acute rejection as the issue?