Fasting for 72 hours ..for the diagnosis?

Well..I recently admitted a patient for exactly  the same....72 hour fasting test.

A 43 yo male with 4 year history of many ER and walk in clinic visits with symptoms of sweating ..feeling of hunger and many episodes of syncope where he suddenly looses consciousness.Twice this happened while driving ..but he was also drunk at that time(so was arrested.....happened twice in jail!). Regains consciousness in minutes. Eating something just before the episode prevents it. Twice there was documented glucose levels of 45 & 50 during these episodes. His fasting glucose in clinic was 78 with a Insulin level of 78(high), and a C peptide level of 2.3 (normal).He also has a 5 year h/o abdominal pain.
The differential in this guy is 1.Insulinoma 2.Factitious hyperinsulinemia 3.Autoimmune hyperinsulinemia 4. Nesidioblastosis (islet cell hypertrophy)

The test entails admitting patient after a overnight fast ..to the hospital. Get baseline glucose, Insulin level, proinsulin level, and C peptide. Keep the pt NPO( nil per oral) with a maintenance iv fluid without glucose...or can be allowed to drink water. Monitor glucose every 4-6 hours. We are looking for hypoglycemia (glucose <45 mg/dl) or hypoglycemic symptoms. Do the same blood tests  at that point and stop the test.(for this...start checking glucose every hour once it reaches 60 mg/dl). If the above two criteria are not met...continue test until 72 hours. The idea ..is pts with Insulinoma will become hypoglycemic, and their C peptide, Insulin and proinsulin levels will be high when he has hypoglycemia. This is explained in this article from 2007. 72 hour fasting seems like..cruel..So researchers looked at options with lesser duration..and they have shown that a 48 hour fast would be enough to identify almost all patients with endogenous hyperinsulinemia(Clin Endo & Met 2000). Our patient has been on the fast for 20 hors now...without any hypoglycmeia or symptoms...

A quick look at evaluation of hypoglycemia(in absence of anti diabetic meds):
1. Whipples triad has to be satisfied( hypoglycemia, neuroglycopaenic symptoms, and prompt relief of symptoms with the administration of glucose).
 2.Then do a 72 hr formal fasting. A positive test is a glucose <50, Insulin > 3microIU/ml & C peptide >200 pmol/L. This confirms endogenous hyperinsulinemia. ( also get a drug screen for sulphonylureas...as they can mimic the same).
3. Once confirmed..then start the localisation process--- CT angiography, trans esophageal US can help. Or an experienced surgeon can go in..and find the insulinoma. If not possible...a Selective arterial calcium stimulation with hepatic venous sampling(SACST) ..where a catheter thru femoral vein lies in hepatic vein..and a 2nd catheter thru femoral artery goes into splenic/mesentric/gastroduodenal aa. and infuse Ca.This stimulates release of Insulin as picked up by the catheter in hepatic vein. ...this has a sensitivity of> 90%