If my 5-hiaa 24 hour ursine test comes back positive does that 100% mean there is a carcinoid tumour somewhere?

Carcinoid: It is a good test for carcinoid if the tumor is secreting serotonin frequently or constantly. About half do not or will intermittantly secrete it. Factors that give you a false positive: foods may give mildly elevated levels of 5-hiaa, especially bananas, chocolate, kiwi fruit, pineapples, plums, tomatoes and walnuts. These should be avoided for 2 days before and during the urine collection.

Possibly: But nothing is certain. There may be other explanations, but the best thing you can do is to continue listening to the doctor who has ordered these tests. They obviously know what they are doing and you are fortunate to have an excellent physician caring for you.