Diagnosis of Vipoma Confirmation of secretory diarrhea Serum vasoactive intestinal peptide (VIP) levels Endoscopic ultrasonography, positron emission tomography (PET), or scintigraphy can localize Diagnosis of vipoma requires demonstration of secretory diarrhea (stool osmolality is close to plasma osmolality, and twice the sum of sodium and potassium concentration in the stool accounts for all measured stool osmolality).