Endocrine Abstracts

Background: The gold standard investigation for suspected spontaneous hypoglycaemia is the supervised 72 h fast. This aims to ‘capture’ a hypoglycaemic episode, to confirm Whipple’s triad, and to measure simultaneous insulin and C-peptide levels. These should confirm or refute endogenous hyperinsulinaemia.

75% patients with confirmed insulinoma actually develop hypoglycaemia within 24 h of fasting. However, some confirmed cases require significantly longer. Conversely, in many individuals in whom clinical suspicion is low, it remains uncertain how long is long enough to rule out spontaneous hypoglycaemia. We propose that ketone (β-hydroxybutyrate (BOHB)) testing may significantly shorten the test required in many such cases.

Method: Following new guidance in 2009, we introduced BOHB testing to our 72 h supervised fast protocol in 2010. We present a retrospective study of case notes and lab records of BOHB results in all patients undergoing a fast at our institution since this time.

Results: 43 patients underwent a fast. Three were excluded because ketone testing had not been performed, and one because they self-discharged prior to 72 h. 39 patients (male:female 1:3.8) were analysed. Two had positive fasts and were later confirmed to have insulinoma. Neither of these patients demonstrated a rise in ketones over the 2.7 mmol/l threshold (maximum recorded: 1.9 and 0.2 mmol/l). 74% of the remainder demonstrated a rise in blood ketones to over 2.7 mmol/l during the fast: median 50.5 h.

Conclusions: A rise in BOHB levels during the 72 h fast confirms that the patient has complied with the fast. However, we propose that a rise in BOHB >2.7 mmol/l is an excellent surrogate marker for hypoinsulinaemia and hence could rule out pathological hyperinsulinaemia (or IGF mediated hypoglycaemia). We therefore propose that a rise in BOHB >2.7 mmol/l indicates a ‘negative’ fast allowing this unpleasant and expensive investigation to be terminated early in 74% patients.