The interviewer may be testing your integrity to find out whether, in your zeal to ingratiate yourself with him, you reveal confidential information from your previous position. The other possibility is that he is fishing for information. Can you blame a guy for trying? Yes, actually, you can. The problem for you is that you may not be able to establish the true motive for the question, no matter what the interviewer says. You might want to keep that in mind. One thing is for sure, you are not telling the interviewer what diagnostic method your previous company sold to ABC Corporation unless it is already published public knowledge. Being direct in your answer will show that you understand this as a question about your integrity, whereas deflecting or using plausible deniability ('well, I wasn't that close to the project') does not send a direct message to the interviewer.