From the Guidelines
The work-up for suspected insulinoma should begin with a 72-hour supervised fast, which is the gold standard diagnostic test, as recommended by the most recent guidelines 1. During this fast, blood samples are collected every 4-6 hours for glucose, insulin, C-peptide, and proinsulin levels, with more frequent sampling if the patient becomes symptomatic. The diagnosis is confirmed when hypoglycemia (blood glucose <45 mg/dL) occurs with inappropriately elevated insulin (≥3 μU/mL), elevated C-peptide (≥0.6 ng/mL), and elevated proinsulin levels, as stated in the guidelines 1. Once biochemical diagnosis is established, localization studies should be performed, starting with non-invasive imaging such as contrast-enhanced CT or MRI of the pancreas. If these are negative, endoscopic ultrasound (EUS) is recommended as it has high sensitivity for detecting small pancreatic tumors, as noted in the guidelines 1. For difficult cases, selective arterial calcium stimulation testing may be necessary, which involves injecting calcium into pancreatic arterial branches and measuring hepatic venous insulin gradients to regionalize the tumor, as described in the guidelines 1. Somatostatin receptor scintigraphy (Octreoscan) or Gallium-68 DOTATATE PET/CT can also be helpful for detection of metastatic disease, as mentioned in the guidelines 1. Throughout the diagnostic process, patients should be monitored closely for hypoglycemic episodes, which can be managed acutely with glucose administration. The comprehensive work-up is essential because insulinomas are typically small (less than 2 cm) but are surgically curable in most cases when properly localized, as stated in the guidelines 1.
Some key points to consider in the work-up of insulinoma include:
- The importance of a supervised fast in establishing the diagnosis, as recommended by the guidelines 1
- The use of non-invasive imaging such as CT or MRI to localize the tumor, as noted in the guidelines 1
- The potential need for more invasive testing such as selective arterial calcium stimulation or EUS in difficult cases, as described in the guidelines 1
- The importance of monitoring for hypoglycemic episodes and managing them promptly, as mentioned in the guidelines 1
- The high cure rate for insulinomas that are properly localized and surgically resected, as stated in the guidelines 1.
Overall, the work-up for suspected insulinoma requires a comprehensive and multi-step approach, as outlined in the guidelines 1.
From the Research
Insulinoma Work-up
- The diagnosis of insulinoma is based on demonstrating Whipple's triad during a supervised fast, with the 72-hour fast being the traditional cornerstone for diagnosis 2.
- However, studies have shown that a 48-hour fast can be sufficient for diagnosing insulinoma, with one study finding that the diagnosis can be made within 48 hours in 94.5% of patients 2.
- Another study found that a 48-hour fasting test was successful in diagnosing insulinoma in 95.7% of patients, with no need for fasting beyond 48 hours 3.
- Alternative diagnostic methods have also been explored, such as a 5-hour oral glucose tolerance test, which can provide valuable information for the diagnosis of insulinoma 4.
- A short fasting test, initiated overnight and prolonged up to 24 hours, has also been shown to be a sensitive and reliable method for diagnosing insulinoma, with diagnostic values observed in 91.7% of cases within 24 hours of the beginning of the fast 5.
- The homeostasis model assessment of β-cell function (HOMA-B) has also been proposed as a diagnostic tool for insulinoma, with a study finding that it provides excellent diagnostic efficacy and may represent an alternative to traditional diagnostic criteria 6.