I had something similar when the Anesthetist was concerned about my ECG and I had to wear a monitor for 48 hours. On another occasion my GP's surgery only did a urine 'Dip Test' rather than a 'Culture' one which also served to postphone my operation. On a third occasion they were not happy with the blood oxyen level probably because I was breathing very shallowly at the time. I got my GP to put that clothes peg like thing on my finger having just previously taken in some deep gulps of air and then it was OK So there are 3 reasons for further checks and delay and there will doubtless be others, not all of which may delay your operation. Not a lot of point speculating or worrying. It could be something quite simple. You will just have to attend the appointment.