When obtaining patient history from the patient's mother, she thought that some of the vomit was "green tinged."
As previously discussed, bilious vomiting changes your differential diagnosis and an upper GI series would be the next study to further evaluate this patient. Typically, emesis in the setting of hypertrophic pyloric stenosis looks like stomach contents (e.g. formula or breast milk), rather than bilious, since the obstruction is proximal to the ampulla of Vater.
Also, at hospitals without readily available experienced ultrasonographers, the patient may undergo upper GI evaluation to confirm hypertrophic pyloric stenosis prior to surgery or transfer to a dedicated pediatric centre.