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A new classification plot for the C-peptide suppression test
Christiane Saddig1, Ralf Bender, Achim A R Starke
1Department of Metabolic Diseases and Nutrition, Heinrich-Heine-University of Duesseldorf, Duesseldorf, Germany.
Context And Objective:
To evaluate the C-peptide suppression test as a screening test in patients with symptoms of hypoglycemia as compared to the standard fasting test.
Design:
Retrospective discriminant analysis of data from C-peptide suppression tests.
Setting:
Clinical study.
Patients:
Patients with insulinomas and patients without insulinomas but having symptoms compatible with hypoglycemia.
Interventions:
The results from C-peptide suppression tests of 26 patients with insulinomas and 100 patients without insulinomas were compared.
Main Outcome Measures:
A classification plot which introduces two discriminant parameters for the C-peptide suppression test: the ratio of [blood glucose]/[C-peptide] at the lowest C-peptide concentration and mean glycemia during insulin infusion.
Results:
In patients with insulinomas, minimal serum C-peptide levels were higher (1.81+/- 0.87 ng/mL; median 1.83 ng/mL; maximal suppression 37 +/- 24% of basal C-peptide levels) as compared to patients without insulinoma (0.40 +/- 0.15 ng/mL; median 0.30 ng/mL; maximal suppression of 75 +/- 9%; P<0.001). Mean glycemia during the test was lower in patients with insulinomas (30.8 +/- 3.3 vs. 47.5 +/- 8.3 mg/dL; P<0.001) as was the [blood glucose]/[C-peptide] ratio (21.9 +/- 14.6 vs. 139.2 +/- 43.8; P<0.001). Discriminant analysis revealed a specificity of 96% to rule out the diagnosis of 'insulinoma' at a 1% probability threshold with a sensitivity of 100%.
Conclusions:
We developed a new classification plot for the C-peptide suppression test in order to accurately identify those patients whose symptoms of hypoglycemia are not due to endogenous hyperinsulinemia/insulinomas. Thus, the need for fasting tests and hospitalization costs can be reduced.