In a chap your age, it's extremely unlikely that there is anything too serious going on here.
Your GP, however, is faced with a patient with a persistent sign that should now be investigated.
I'm assuming that infection has been excluded, and a blood test of your renal function is normal.
It sounds like your GP's actions are reasonable. The procedure is relatively (actually very) safe in someone your age, and will hopefully give you some answers through exclusion. And shut your AME up .
I'd be scared to, but by the time said doc has bored you with the tale of how he had a good time in The Bahamas, the procedure will be over with.
