Answer: When he's a surgeon.
In Grasp a Nettle, Jenny 'Not Dickens' Wren twits Eduard about being a doctor. He gets mildly huffy:
"My dear girl...I'm a surgeon. You, a nurse, should know the difference."
"Well, of course I do," she was a little impatient. "I didn't know you were so fussy about a little thing like that."
The Professor made a small choking sound. "There is a considerable difference ---" he began..to be cut short by her:
"Oh, don't be so stuffy!"
The only response to that is a swoop and a kiss...
He sort of contradicts himself a few pages later:
"One should drink plenty in this heat. Doctor's advice."
"But you're not a doctor - you're a surgeon, you said so."
"Ah yes - so I did. I'm a modest man; I tend to hide my light under a bushel. I do happen to be a doctor of medicine..."
Can't we all just get along?