Assessment of immunoassays for insulin in diagnostic tests for insulinoma

A severe but reversible reduction in insulin sensitivity is observed in patients with insulinoma

BACKGROUND

Hypoglycemic manifestations are highly variable in patients with an insulinoma and largely independent of tumour size and severity of insulin hypersecretion.

OBJECTIVES

We investigated the clinical, biological and tumoral characteristics of insulinomas in a large monocentric series of patients and we evaluated their insulin sensitivity before and after successful pancreatic surgery.

PATIENTS AND METHODS

This was a retrospective analysis of 40 patients treated for an insulinoma between 1982 and 2012 in our academic hospital. Insulin sensitivity and beta cell function were evaluated by a HOMA test outside hypoglycaemic episodes in a large subset of these patients.

RESULTS

The mean age at onset of symptoms was 48.8±20.1 years and the mean age at diagnosis was 50.7±19.9 years. Neuroglycopenic symptoms were observed in 90% of patients. The most effective preoperative imaging technique to localize the tumour was endoscopic ultrasound. Insulin sensitivity was greatly reduced in patients with insulinoma (38.9%±22.3%), while beta cells function was increased (359.0±171.5%), but to a variable extent (range: 110.6-678.6%). After complete resection of the tumour and remission of hypoglycemic episodes, insulin sensitivity increased in all evaluated subjects (72.8±36.7%) and normalized in the majority.

CONCLUSION

Although neuroglycopenic symptoms are present in most patients, diagnosis of insulinoma is often delayed. Endoscopic ultrasound remains the most sensitive preoperative technique to localize the tumour. We also show that in response to chronic hyperinsulinemia, patients with insulinoma develop protective mechanisms responsible for a marked insulin resistance, which is reversible after complete resection of the tumour.

Beta-cell adenomas without hyperinsulinemia with use of highly specific insulin radioimmunoassays: case report and review of literature

OBJECTIVE

To report a case of a proinsulin-secreting islet cell adenoma in which the diagnosis was obscured by an ultraspecific insulin assay.

METHODS

We describe the case of a 46-year-old woman, who presented with fasting hypoglycemia and appropriately low insulin values.

RESULTS

A prolonged supervised fast produced symptomatic hypoglycemia (20 mg/dL) after only 7 hours. During the entire fasting test, highly specific insulin remained at <3 mIU/L, with a median value (and interquartile range) of 0.9 (0.8 to 2.3) mIU/L, when the glucose concentration was <50 mg/dL. The serum C-peptide level remained high normal (mean +/- SD, 2.7 +/- 0.6 ng/mL; normal fasting levels, 0.8 to 3.9), and no evidence of sulfonylurea use was detected in the patient's urine. Circulating proinsulin levels were persistently high (>200 pmol/L in all determinations when hypoglycemia was present; expected value, <5 pmol/L). Magnetic resonance imaging and endoscopic ultrasonography confirmed the presence of a 2.5-cm tumor in the head of the pancreas. A proinsulin-secreting islet cell tumor was diagnosed. Surgical resection of the tumor was successfully accomplished, but diabetes mellitus developed 4 months postoperatively.

CONCLUSION

The diagnosis of a hypoglycemia-producing pancreatic adenoma can be missed when an ultraspecific insulin assay is used. The direct measurement of proinsulin established the diagnosis in this case.

Diagnosis of insulinoma in a patient with hypoglycemia without obvious hyperinsulinemia

BACKGROUND

A 41-year-old Maltese woman with a 12-month history of severe, morning episodes of confusion, blurred vision and sweating was referred to a specialist center for evaluation of fasting hypoglycemia. She was not taking medication and did not report any prior personal or familial history of endocrinopathy or other relevant pathology.

INVESTIGATIONS

Measurement of plasma glucose, insulin, C-peptide, and beta-hydroxybutyrate concentrations during a prolonged supervised fast; sulfonylurea screen; CT, MRI scan and endoscopic ultrasonography of the pancreas; calcium stimulation test; surgical exploration and intra-operative ultrasonography of the pancreas.

DIAGNOSIS

Insulin-secreting lesion (insulinoma) in the tail of the pancreas.

MANAGEMENT

The tumor was resected with cure of symptoms.

A fuller understanding of pancreatic neuroendocrine tumours combined with aggressive management improves outcome

BACKGROUND

Neuroendocrine tumours of the pancreas (PNETs) represent 1-2% of all pancreatic tumours. The terms 'islet cell tumours' and 'carcinoids' of the pancreas should be avoided. The aim of this review is to offer an overview of the history and diagnosis of PNETs followed by a discussion of the available treatment options.

METHODS

A search on PubMed using the keywords 'neuroendocrine', 'pancreas' and 'carcinoid' was performed to identify relevant literature over the last 30 years.

RESULTS

The introduction of a revised classification of neuroendocrine tumours by the World Health Organisation (WHO) in 2000 significantly changed our understanding of and approach to the management of these tumours. Advances in laboratory and radiological techniques have also led to an increased detection of PNETs. Surgery remains the only treatment that offers a chance of cure with increasing number of non-surgical options serving as beneficial adjuncts. The better understanding of the behaviours of PNETs together with improvements in tumour localisation has resulted in a more aggressive management strategy with a concomitant improvement in symptom palliation and a prolongation of survival.

CONCLUSION

Due to their complex nature and the wide range of therapeutic options, the involvement of specialists from all necessary disciplines in a multidisciplinary team setting is vital to provide optimal treatment of this disease.

Diagnosis and Minimally Invasive Resection of an Insulinoma: Report of an Unusual Case and Review of the Literature

Insulinomas are rare endocrine tumors that are usually diagnosed by inappropriate elevations in insulin and C-peptide during hypoglycemia. We report a case of a surgically confirmed insulinoma diagnosed by a mild elevation in proinsulin with suppressed insulin and C-peptide at the time of hypoglycemia during a supervised fast. A supervised fast with serial measurements of plasma glucose, insulin, and C-peptide was performed in a patient with documented hypoglycemia. Proinsulin was measured at the beginning and end of the fast. Tumor localization was accomplished with spiral CT, magnetic resonance imaging, and endoscopic ultrasound. Minimally invasive tumor resection was performed. The presence of an insulinoma was confirmed on the basis of a minimally elevated proinsulin level with a suppressed insulin level at the time of symptomatic hypoglycemia. Tumor resection was performed without complications, resulting in resolution of the hypoglycemia. This case demonstrates the importance of measuring proinsulin as a routine component of the 72-hour fast for detection of an insulinoma. Even mild elevations in circulating proinsulin can be an independent indicator of aberrant insulin secretion during hypoglycemia. Once the diagnosis of insulinoma is made and tumor localization is achieved, minimally invasive tumor resection is a safe and effective treatment modality.

Biochemistry of neuroendocrine tumours

Several circulating or urinary tumour markers can be used for the diagnosis and follow-up of functioning and clinically non-functioning neuroendocrine tumours of the pancreatic islet cells and intestinal tract. Among the specific tumour markers are serotonin and its metabolites--e.g. 5-hydroxyindoleacetic acid (5-HIAA)--in carcinoid tumours and the carcinoid syndrome, insulin and its precursors or breakdown products in insulinoma, and gastrin in gastrinoma. Plasma vasointestinal polypeptide (VIP) determinations have been used in the diagnosis of VIPoma, plasma glucagon for glucagonoma, and serum somatostatin for somatostatinoma. Among the tumour-non-specific markers are: chromogranins, neuron-specific enolase (NSE), alpha-subunits of the glycoprotein hormones, catecholamines, pancreatic polypeptide (PP), ghrelin and adrenomedullin.

Gap analysis of pediatric reference intervals for risk biomarkers of cardiovascular disease and the metabolic syndrome

The childhood obesity epidemic has begun to compromise the health of the pediatric population by promoting premature development of atherosclerosis and the metabolic syndrome (MS), both of which significantly increase the risk of cardiovascular disease (CVD) early in life. As a result, recently, there has been increased recognition of the need to assess and closely monitor children and adolescents for risk factors of CVD and components of the MS. Serum/Plasma biomarkers including total cholesterol, triglycerides, HDL-C, LDL-C, insulin and C-peptide have been used for this purpose for many years. Recently, emerging biomarkers such as apolipoprotein AI, apolipoprotein B, leptin, adiponectin, free fatty acids, and ghrelin have been proposed as tools that provide valuable complementary information to that obtained from traditional biomarkers, if not more powerful predictions of risk. In order for biomarkers to be clinically useful in accurately diagnosing and treating disorders, age-specific reference intervals that account for differences in gender, pubertal stage, and ethnic origin are a necessity. Unfortunately, to date, many critical gaps exist in the reference interval database of most of the biomarkers that have been identified. This review contains a comprehensive gap analysis of the reference intervals for emerging and traditional risk biomarkers of CVD and the MS and discusses the clinical significance and analytical considerations of each biomarker.

Assessment of hyperinsulinaemia at the termination of the prolonged fast

BACKGROUND

Traditional criteria to diagnose hyperinsulinaemic hypoglycaemia are based on insulin measurements by unspecific insulin assays. This study was performed to test whether these traditional criteria can be applied when insulin is measured by specific immunoassays.

METHODS

29 consecutive patients undergoing a prolonged fast were included; 11 patients with insulinoma and 18 healthy individuals. We determined plasma glucose, insulin, C-peptide, proinsulin, and beta-hydroxybutyrate concentrations at the termination of the fast. Insulin was measured by an unspecific radioimmunoassay (RIA) and a specific enzyme-linked immunosorbent assay (ELISA).

RESULTS

In 11 insulinoma patients, insulin concentrations at median plasma glucose concentration of 2.1 (range 1.3-2.5) mmol/l were 170 (76-340) pmol/l measured by RIA and 61 (11-156) pmol/l by ELISA. Insulin concentrations measured by RIA confirmed hyperinsulinaemia (i.e., >36 pmol/l, the proposed cut-off value for traditional insulin assays) in all insulinoma patients, whereas insulin concentrations measured by ELISA were <36 pmol/l in four patients. In three insulinoma patients, insulin concentrations measured by ELISA were <18 pmol/l, a proposed cut-off level to diagnose hyperinsulinaemia for specific insulin assays.

CONCLUSION

When insulin concentrations are measured by specific immunoassays in patients evaluated for fasting hypoglycaemia, traditional reference values cannot be applied.

Dynamic changes in plasma proinsulin/insulin ratio during insulin secretion influence correlation between radioimmunoassay (RIA) and IMX measurements of insulin

Because proinsulin and insulin have different circulatory half-lives, the ratio of proinsulin to insulin in plasma depends on the dynamics of insulin secretion. This variation can potentially influence comparison of IMX assays and radioimmunoassays (RIAs) for [insulin], given that the antibody used in the IMX assay has negligible cross-reactivity with proinsulin compared to the 40% cross-reactivity with proinsulin of the antibody used in the RIA. Simulation of a simple mass balance model for insulin and proinsulin concentrations during an oral glucose tolerance test predicts that the ratio (R) of RIA to IMX insulin measurements of [insulin] should transiently decrease, pass through a minimum, increase past the initial value, pass through a maximum and eventually return to the initial value. Using time course specimens from patients, this pattern of variation in R was observed in the majority (12/16) of cases studied. The variation in R for time course specimens (CV = 26%) was significantly greater than for other specimens (fasting, random or undesignated; P < 0.05). Thus, when comparing IMX and RIA measurements of [insulin], variation in R for samples from differing states of dynamic insulin secretion contains a component that is attributable to dynamic changes in the ratio of [proinsulin]/[insulin] in plasma.

Insulinoma: a review of current management

Insulinomas are a relatively rare tumour which occur predominantly in the pancreas. The majority of the tumours are benign, but have profound effects upon the patient. The diagnosis of insulinoma is often elusive, and the management may involve demanding surgery with a significant morbidity. In this review article, all clinical aspects of insulinomas are examined. Particular emphasis is placed on the myriad modes of presentation, and the methods used to localise the tumour pre-operatively. Medical, as well as surgical treatments are discussed and their role in the management of both malignant and benign tumours. Despite potential difficulties encountered in managing patients with this tumour, a large majority may be either cured or achieve useful palliation.