THE EFFECT OF PHLEBOTOMY-INDUCED HEMOLYSIS ON INSULIN LEVEL DETERMINATION

C-peptide level concomitant with hypoglycemia gives better performances than insulin for the diagnosis of endogenous hyperinsulinism: a single-center study of 159 fasting trials

INTRODUCTION

Diagnosis of endogenous hyperinsulinism relies on the occurrence of a hypoglycemia, concomitant with inadequate high insulin and C-peptide levels. However, diagnostic cutoffs are not consensual among the different learned societies. The objective of this work was to propose optimized cutoffs for these three parameters for the diagnosis of endogenous hyperinsulinism.

METHODS

All the patients having performed a fasting trial in Cochin Hospital Endocrinology Department between February 2012 and August 2022 were included. The results of glycemia, insulin and C-peptide levels during fasting trial were collected and analyzed.

RESULTS

One hundred and fifty-nine patients were included: 26 with endogenous hyperinsulinism and 133 without endogenous hyperinsulinism. ROC analysis of glycemia nadir during fasting trial identified the value of 2.3 mmol/L as the optimal cutoff, ensuring a sensitivity of 100% associated with a specificity of 81%. ROC analysis of insulin and C-peptide levels concomitant with hypoglycemia <2.3 mmol/L showed very good diagnostic performances of both parameters with respective cutoffs of 3.1 mUI/L (=21.5 pmol/L; sensitivity = 96%; specificity = 92%) and 0.30 nmol/L (sensitivity = 96%; specificity = 100%). Insulin to glycemia ratio as well as C-peptide to glycemia ratio (in pmol/mmol) at the time of glycemia nadir did not show better diagnostic performances than C-peptide alone.

CONCLUSION

A C-peptide level 0.3 nmol/L concomitant with a hypoglycemia <2.3 mmol/L appears as the best criterion to make the diagnosis of endogenous hyperinsulinism. Insulin level can be underestimated on hemolyzed blood samples, frequently observed in fasting trial, and thus shows lower diagnostic performances.

Insulin murder and the case of Colin Norris

Although insulin is an essential medicine and a life-saving drug, it has also been incriminated in many poisoning deaths; accidental, suicidal and some with malicious intent. Overdosing with insulin precipitates a life-threatening state of hypoglycemia and if untreated leads to coma, irreversible brain damage and death. Normally, the pancreatic β-cells secrete equimolar amounts of insulin and C-peptide into the portal venous blood, although under physiological conditions the plasma concentration ratio (insulin/C-peptide) is less than unity, because insulin is more susceptible to hepatic first-pass metabolism. A high ratio of insulin/C-peptide in plasma from a poisoned patient is compelling evidence that pharmaceutical insulin was administered, which does not contain C-peptide. The analysis of insulin and C-peptide was traditionally done by immunoassay methods (RIA and/or ELISA), although high resolution LC-MS/MS is more suitable for forensic purposes and permits the identification of insulin analogues. Use of insulin as a murder weapon is exemplified by the case of Colin Norris, a male nurse found guilty of murdering four elderly patients and the attempted murder of a fifth by injecting them with insulin. However, the prosecution evidence against Norris was mainly circumstantial and hearsay. Toxicological evidence against Norris consisted of a high insulin/C-peptide concentration ratio in plasma from one of the victims. This analysis was done by an immunoassay method at a clinical laboratory and not a forensic laboratory. Analytical procedures, including chain-of-custody routines, are more stringent at forensic laboratories. Since his conviction, some of the medical evidence against Norris has been called into question, especially the prevalence of spontaneous attacks of hypoglycemia in elderly and frail patients with co-morbidities.

Multiplexed quantification of insulin and C-peptide by LC-MS/MS without the use of antibodies

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Simultaneous quantification of insulin and C-peptide without antibodies.

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Proteolysis with Glu-C permits sensitive and precise measurements.

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Calibration with certified reference material provides traceability.

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Relatively large bias when compared with a commercially available immunoassay.

Introduction

The measurement of insulin and C-peptide provides a valuable tool for the clinical evaluation of hypoglycemia. In research, these biomarkers are used together to better understand hyperinsulinemia, hepatic insulin clearance, and beta cell function. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is an attractive approach for the analysis of insulin and C-peptide because the platform is specific, can avoid certain limitations of immunoassays, and can be multiplexed. Previously described LC-MS/MS methods for the simultaneous quantification of insulin and C-peptide measure the intact analytes and most have relied on immunoaffinity enrichment. These approaches can be limited in terms of sensitivity and interference from auto-antibodies, respectively. We have developed a novel method that does not require antibodies and uses proteolytic digestion to yield readily ionizable proteotypic peptides that enables the sensitive, specific, and simultaneous quantitation of insulin and C-peptide.

Methods

Serum samples were precipitated with acetonitrile. Analytes were enriched using solid phase extraction and then digested with endoproteinase Glu-C. Surrogate peptides for insulin and C-peptide were analyzed using targeted LC-MS/MS.

Results

Inter-day imprecision was below 20 %CV and linearity was observed down to the lower limit of quantitation for both analytes (insulin = 0.09 ng/mL, C-peptide = 0.06 ng/mL). Comparison to a commercially available insulin immunoassay (Beckman Coulter UniCel DxI 600 Access) revealed a 30% bias between methods.

Conclusion

A novel LC-MS/MS method for the simultaneous analysis of insulin and C-peptide using Glu-C digestion was developed and evaluated. A detailed standard operating procedure is provided to help facilitate implementation in other laboratories.

Digenic Variants in the FGF21 Signaling Pathway Associated with Severe Insulin Resistance and Pseudoacromegaly

Insulin-mediated pseudoacromegaly (IMPA) is a rare disease of unknown etiology. Here we report a 12-year-old female with acanthosis nigricans, hirsutism, and acromegalic features characteristic of IMPA. The subject was noted to have normal growth hormone secretion, with extremely elevated insulin levels. Studies were undertaken to determine a potential genetic etiology for IMPA. The proband and her family members underwent whole exome sequencing. Functional studies were undertaken to validate the pathogenicity of candidate variant alleles. Whole exome sequencing identified monoallelic, predicted deleterious variants in genes that mediate fibroblast growth factor 21 (FGF21) signaling, FGFR1 and KLB, which were inherited in trans from each parent. FGF21 has multiple metabolic functions but no known role in human insulin resistance syndromes. Analysis of the function of the FGFR1 and KLB variants in vitro showed greatly attenuated ERK phosphorylation in response to FGF21, but not FGF2, suggesting that these variants act synergistically to inhibit endocrine FGF21 signaling but not canonical FGF2 signaling. Therefore, digenic variants in FGFR1 and KLB provide a potential explanation for the subject's severe insulin resistance and may represent a novel category of insulin resistance syndromes related to FGF21.

Bacitracin attenuates haemolysis-induced insulin degradation during insulin sensitivity testing: Repurposing an old drug for use in metabolic research

Haemolysis of serially collected insulin serum samples frequently causes falsely-low measured concentrations because of the release of intracellular insulin degrading enzyme (IDE). We investigated if bacitracin, an in vitro IDE inhibitor, could prevent haemolysis-induced insulin degradation during insulin sensitivity testing. Blood samples were collected from adults undergoing serial sampling for insulin sensitivity. A dose-finding study measured insulin from experimentally haemolysed samples containing five bacitracin concentrations (0-2.5 g/L) and from non-experimentally haemolysed samples. To confirm the utility of bacitracin in the clinical setting, we compared insulin in samples collected with and without 1 g/L bacitracin from a frequently sampled intravenous glucose tolerance test (FSIVGTT), where haemolysis often occurs accidentally. In the dose-finding study, bacitracin 0.25, 1 and 2.5 g/L all maximally prevented insulin degradation in experimentally haemolysed samples. Among FSIVGTT unintentionally haemolysed samples, insulin concentrations from bacitracin-containing samples were significantly higher than from those without bacitracin (P < .01), and not different from non-haemolysed samples obtained simultaneously from a second intravenous catheter (P = .07). Bacitracin did not significantly alter insulin concentrations in non-haemolysed samples. Bacitracin attenuates haemolysis-associated insulin degradation in clinical samples, enabling a more accurate assessment of insulin sensitivity and glucose homeostasis.

Early effects of roflumilast on insulin sensitivity in adults with prediabetes and overweight/obesity involve age-associated fat mass loss - results of an exploratory study

PURPOSE

Roflumilast (Daliresp, Daxas) is a FDA-approved phosphodiesterase 4 (PDE4) inhibitor for the treatment of moderate-to-severe chronic obstructive pulmonary disease. In mice and in limited human studies, this oral medication can cause weight loss and improve insulin sensitivity. We set out to determine the mechanism of its effect on insulin sensitivity.

PATIENTS AND METHODS

Eight adults with overweight/obesity and prediabetes received roflumilast for 6 weeks. Before and after roflumilast, subjects underwent tests of insulin sensitivity, mixed meal test, body composition, markers of inflammation, and mitochondria function. Dietary intake and physical activity were also assessed. Our primary outcome was the change in peripheral insulin sensitivity, as assessed by the hyper-insulinemic euglycemic clamp.

RESULTS

This study was underpowered for the primary outcome. Pre- and post-roflumilast mean peripheral insulin sensitivity were 48.7 and 70.0 mg/g fat free mass/minute, respectively, (P-value=0.18), respectively. Among the mixed meal variables, roflumilast altered glucagon-like peptide 1 (GLP-1) hormone the most, although the average effect was not statistically significant (P=0.18). Roflumilast induced a trend toward significance in 1) decreased energy intake (from 11,095 KJ to 8,4555 KJ, P=0.07), 2) decreased fat mass (from 34.53 to 32.97 kg, P=0.06), 3) decreased total and LDL cholesterol (P=0.06 for both variables), and 4) increased plasma free fatty acids (from 0.40 to 0.50 mEq/L, P=0.09) The interval changes in adiposity and free fatty acid were significantly associated with the subject's age (P-value range= <0.001 to 0.02 for the correlations). Inflammatory and adhesion markers, though unchanged, significantly correlated with one another and with incretin hormones only after roflumilast.

CONCLUSION

We demonstrate, for the first time in humans, increasing percentage of fat mass loss from roflumilast with increasing age in adults with prediabetes and overweight/obesity. We also demonstrate novel associations among roflumilast-induced changes in incretin hormones, inflammatory markers, peripheral insulin sensitivity, and adiposity. We conclude that roflumilast's early effects on insulin sensitivity is indirect and likely mediated through roflumilast's prioritization of lipid over glucose handling.

CLINICAL TRIALS REGISTRATION

NCT01862029.

Hemolysis Affects C-Peptide Immunoassay

BACKGROUND

C-peptide is used widely as a marker of insulin secretion, and it participates in the inflammatory response and contributes to the development of coronary artery disease (CAD) in patients with type 2 diabetes mellitus (T2DM). Previous studies have reported that C-peptide measurement was unaffected by hemolysis. However, we found that hemolysis negatively affected C-peptide assay in routine laboratory practice. We further established and validated an individualized hemolysis correction equation to correct and report accurate serum C-peptide results for hemolyzed samples.

METHODS

We studied the effects of hemolysis on C-peptide assay by adding lysed self red blood cells (self-RBCs) to serum. An individualized correction equation was derived. Further, we evaluated the performance of this individualized correction equation by artificially hemolyzed samples.

RESULTS

C-peptide concentration decreased with increasing degree and exposure time of hemolysis. The individualized hemolysis correction equation derived: C-P_corr = C-P_meas /(0.969-1.5Hb_serum/plasma -5.394 ×10^-5 Time), which can correct bias in C-peptide measurement caused by hemolysis.

CONCLUSIONS

Hemolysis negatively affects C-peptide measurement. We can correct and report accurate serum C-peptide results for a wide range of degrees of sample hemolysis by individualized hemolysis correction equation for C-peptide assay. This correction would improve diagnostic accuracy and reduce inappropriate therapeutic decisions.