A 70-year-old man is reviewed in the pre-operative assessment clinic prior to a planned inguinal hernia repair. He is generally fit and well with no significant past medical history. His routine blood tests show: Hb 125 g/L, WBC 7.0 x10^9/L, Platelets 120 x10^9/L, Na+ 138 mmol/L, K+ 4.0 mmol/L, Urea 6.0 mmol/L, Creatinine 90 µmol/L. Which of these results is most likely to require further investigation or specific management before routine elective surgery?