This is basic statistics learned by every doctor in medical school.
Or that they have learned, passed the test, and forgotten. There was quite an interesting read on exactly this subject on the BBC website this week, Do doctors understand test results?:
In one session, almost half the group of 160 gynaecologists responded that the woman's chance of having cancer was nine in 10. Only 21% said that the figure was one in 10 - which is the correct answer. That's a worse result than if the doctors had been answering at random.