Will a cystoscopy show interstitial cystitis or can it still be missed or left undiagnosed?

Most likely: Most likely a cystoscopy will show interstitial cystitis, it is the definitive test for is. Having said that, i'm certain that a urologist will tell you that he/she has had patients who have a negative cystoscopy, and yet have all the symptoms. As with some diagnoses, sometimes we treat based on symptoms only. If you haven't seen a urologist, consider making an appointment with one. Best luck.