What does '+ve' or '-ve' mean in a medical context?

+ve means positive-ve means negative

Some publishers use these terms instead of just + and - to make clear that the + indicates that a certain parameter is positive or negative, to avoid confusing - with a dash and + with and.

For example, writing p53-ve clearly indicates that p53 is negative. What positive or negative actually means depends on the context, e.g. it can mean that p53 is not mutated in a group of patients. See this paper for an example where p53-ve is used in that way.