Hypertrophic pyloric stenosis: volumetric measurement of nasogastric aspirate to determine the imaging modality The authors postulated that volumetric measurement of residual gastric aspirate in neonates and infants with nonbilious projectile vomiting could enable differentiation between patients with hypertrophic pyloric stenosis (HPS) and those with gastroesophageal reflux (GER) and help to determine whether ultrasound (US) or fluoroscopy of the upper gastrointestinal tract would best confirm the diagnosis. In the 38 patients (all but two of whom had been fasting for 3-4 hours), 10 mL or more of nasogastric aspirate was considered indicative of obstruction. HPS occurred in 91.7% of patients with 10 mL of aspirate or more, whereas GER occurred in 85.7% of patients with less than 10 mL. The differences between the two groups were statistically significant. Solely on the basis of residual volume (greater than or equal to 10 mL), the cause of vomiting could be differentiated, prior to standard radiologic studies, 89.4% of the time. It is concluded that patients with projectile vomiting who have 10 mL or more of residual aspirate in the stomach should undergo US for confirmation of HPS; those with less than 10 mL should undergo fluoroscopy for confirmation of GER. 