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The Diagnosis of Fasting Hypoglycemia Due to an Islet-Cell Tumor Obscured by a Highly Specific Insulin Assay

Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-5576

Address all correspondence and requests for reprints to: Christopher D. Saudek, M.D., Johns Hopkins University School of Medicine, 600 North Wolfe Street, Osler 576, Baltimore, Maryland 21287-5576. E-mail: csaudek{at}jhu.edu.

Abstract

The work-up of fasting hypoglycemia may be difficult but is crucially important because a wrong diagnosis can lead to either unnecessary pancreatectomy or a missed pancreatic tumor. We describe a patient with severe fasting hypoglycemia [22–32 mg/dl (1.2–1.8 mmol/liter) after 6–10 h of fasting] in which the diagnosis of a secretory islet-cell tumor was obscured, rather than facilitated, by use of a new, highly specific serum insulin assay. Insulin measured by the specific assay suppressed normally during fasting hypoglycemia [undetectable at < 2.0–3.8 µIU/ml (26.4 pmol/liter)], whereas insulin measured by older, less specific assays was diagnostically elevated [34, 73 µIU/ml (236.1, 507.0 pmol/liter)]. Serum proinsulin and C-peptide levels were abnormal, and further work-up revealed an islet-cell tumor that secreted predominantly proinsulin. The tumor was surgically removed, relieving the fasting hypoglycemia. We conclude that insulin levels as measured by new, highly specific insulin assays may obscure the diagnosis of a functional, proinsulin-secreting islet-cell tumor. Because proinsulin cross-reacts with insulin in older insulin assays, C-peptide or proinsulin should be measured to rule out a proinsulin-secreting islet-cell tumor. Normative values for new insulin assays must be established during prolonged fasting.

Footnotes

This work was supported by NIH/NCRR Grant M01-RR00052 to the Johns Hopkins University School of Medicine General Clinical Research Center.

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