Most commercial insulin assays fail to detect recombinant insulin analogues

Insulin: Know what your immunoassay detects. Evaluation of two new immunoassays

BACKGROUND

Insulin is essential for glycemic regulation but diseases can cause a default or an excess of insulin secretion leading to dysregulated glycemia. Hence, measurement of insulinemia is useful to investigate hypoglycemia, determine the pathogenesis of diabetes and evaluate β-cell function. Thus, diabetic patients need supplementation with recombinant human insulin and/or insulin analogues. Analogues have primary sequences different from native human insulin and may not be detected by some immunoassays. The objective of our study was to evaluate new insulin immunoassays by determining their ability to detect different types of human insulin or analogues.

METHODS

This study compared the reactivity of two new insulin immunoassays with five well-established immunoassays on ten commercial insulins. We also measured insulin in blood samples from diabetic or pancreas transplant patients with known treatment.

RESULTS

Contrary to recombinant human insulin, there were differences in the specificity to insulin analogues. We distinguished three immunoassay categories: those recognizing all types of insulin such as the non-specific BI-INS-IRMA®, Architect® and Access® immunoassays; those recognizing human insulin only (Cobas®); and those recognizing human insulin and analogues in variable proportions (Liaison XL®, iFlash® and Maglumi®).

CONCLUSION

An accurate biological interpretation of insulinemia relies on knowledge of the specificity of the immunoassay used.

Assessment of circulating insulin using liquid chromatography-mass spectrometry during insulin glargine treatment in type 2 diabetes: Implications for estimating insulin sensitivity and β-cell function

AIM

To determine the potential impact of the cross-reactivity of insulin glargine U-100 and its metabolites on insulin sensitivity and β-cell measures in people with type 2 diabetes.

MATERIALS AND METHODS

Using liquid chromatography-mass spectrometry (LC-MS), we measured concentrations of endogenous insulin, glargine and its two metabolites (M1 and M2) in fasting and oral glucose tolerance test-stimulated plasma from 19 participants and fasting specimens from another 97 participants 12 months after randomization to receive the insulin glargine. The last dose of glargine was administered before 10:00 PM the night before testing. Insulin was also measured on these specimens using an immunoassay. We used fasting specimens to calculate insulin sensitivity (Homeostatic Model Assessment 2 [HOMA2]-S%; QUICKI index; PREDIM index) and β-cell function (HOMA2-B%). Using specimens following glucose ingestion, we calculated insulin sensitivity (Matsuda ISI[comp] index) and β-cell response (insulinogenic index [IGI], and total incremental insulin response [iAUC] insulin/glucose).

RESULTS

In plasma, glargine was metabolized to form the M1 and M2 metabolites that were quantifiable by LC-MS; however, the analogue and its metabolites cross-reacted by less than 100% in the insulin immunoassay. This incomplete cross-reactivity resulted in a systematic bias of fasting-based measures. By contrast, because M1 and M2 did not change following glucose ingestion, a bias was not observed for IGI and iAUC insulin/glucose.

CONCLUSIONS

Despite glargine metabolites being detected in the insulin immunoassay, dynamic insulin responses can be used to assess β-cell responsiveness. However, given the cross-reactivity of the glargine metabolites in the insulin immunoassay, fasting-based measures of insulin sensitivity and β-cell function are biased.

Factitious Hypoglycemia in an Infant With Undetectable Exogenous Insulin by a Commercial Insulin Immunoassay: A Diagnostic Pitfall

Factitious hypoglycemia in infancy is a rare, life-threatening manifestation of Munchausen syndrome by proxy (MSBP). The hallmark of such presentation is the detection of low c-peptide combined with high insulin at the time of hypoglycemia. We report the case of a male infant who presented with recurrent severe unexplained hypoglycemic episodes since the age of six months. Two of his siblings had similar unexplained hypoglycemia episodes at a young age. He was extensively investigated, and all were normal, for endocrine and metabolic etiologies. He underwent fundoplication and insertion of a gastrostomy tube with multiple lengthy hospital admissions. His mother had diabetes and was on insulin treatment; she also had mental health issues with family-related social stressors. His hypoglycemic attacks resolved once separated briefly from his mother on the ward, raising our suspicion of MSBP. The exogenous administration of insulin was only confirmed following a scheduled change of our local Insulin assay in our laboratory when his insulin was detectable with low C-peptide on one of his typical attacks. Apparently, our previous insulin immunoassay lacked sensitivity for his mother's long-acting insulin. We are reporting this case to raise awareness about this potential diagnostic pitfall.

Pitfalls in Diagnosing Hypoglycemia Due to Exogenous Insulin: Validation and Utility of an Insulin Analog Assay

OBJECTIVE

To overcome the limitations of commercially available insulin immunoassays which have variable detection of analog insulin and can lead to clinically discordant results and misdiagnosis in the workup of factitious hypoglycemia.

PATIENTS AND METHODS

We performed analytical validation of a liquid chromatography high resolution accurate mass (LC-HRAM) immunoassay to detect insulin analogs. We completed clinical assessment using a large cohort of human serum samples from 78 unique individuals, and subsequently used the assay in the evaluation of eight individuals with high diagnostic suspicion for factitious hypoglycemia.

RESULTS

The performance characteristics show that the LC-HRAM immunoassay can be applied to detect five commonly used synthetic insulin analogs (lispro, glulisine, aspart, glargine metabolite, and detemir) in human serum. Our clinical cases show that this assay could be used in the diagnosis of factitious hypoglycemia by identifying the analog insulin(s) in question.

CONCLUSION

The LC-HRAM immunoassay reported here overcomes a gap in our diagnostic pathway for hypoglycemia. The results obtained from our studies suggest that this method is appropriate for use in clinical laboratories when factitious hypoglycemia is considered as a differential diagnosis.

Prevalence and characteristics of factitious hypoglycaemia in non-diabetic patients in a department of endocrinology

INTRODUCTION

Factitious hypoglycaemia is defined as the surreptitious use of insulin or oral hypoglycaemic agents to deliberately induce self-harm. It represents a challenging diagnosis and misdiagnosis is associated with significant morbidity and mortality. The aim of this study was to assess the prevalence and the associated factors of factitious hypoglycaemia in non-diabetic patients.

METHODS

This was a single-centre, retrospective study including 70 non-diabetic patients who were admitted for the investigation of hypoglycaemia. All patients fulfilled the Whipple triad. Epidemiological parameters, medical history, clinical and paraclinical data and the aetiology of hypoglycaemia were collected from medical records.

RESULTS

The diagnosis of factitious hypoglycaemia was held in 11 patients (9 women and 2 men) corresponding to a prevalence of 16%. It was secondary to intentional insulin use in six patients and the ingestion of glibenclamide in five patients. The median age of the patients was 28 years (interquartile range: 21-43). Two patients with factitious hypoglycaemia had a personal history of psychiatric disorders. The other causes of hypoglycaemia were adrenal insufficiency (34%), prediabetes (24%), insulinoma (6%), iatrogenic hypoglycaemia (10%), criminal hypoglycaemia (1%) and alcohol intoxication (2%). Age ≤ 35 years (Odds Ratio = 5.6, p = .017), family history of diabetes mellitus (Odds Ratio = 1.29, p = .015), attention disorders during hypoglycaemia (Odds Ratio = 12.5, p = .017) and fasting glucose level <0.7 g/L (Odds Ratio = 5.75, p = .017) were positively associated with factitious hypoglycaemia.

CONCLUSION

Factors significantly associated with factitious hypoglycaemia were young age, family history of diabetes and a low fasting glucose level.

Explaining unexplained hypoglycemia: How LC-MS/MS can help

Explaining hypoglycaemia, especially in patients without diabetes mellitus, is challenging. Here we present a case, where the added value for clinical diagnosis of insulin determination with liquid chromatography-mass spectrometry (LC-MS/MS) is shown. By the use of LC-MS/MS the different insulin analogues can be identified. The confirmation of an insulin analogue present during hypoglycaemia facilitated in our case the discussion with the patient and his family about what happened.

The Other Face of Insulin—Overdose and Its Effects

Insulin is the most effective glycemic-lowering drug, and for people suffering from type 1 diabetes it is a life-saving drug. Its self-dosing by patients may be associated with a higher risk of overdose, both accidental and deliberate. Insulin-induced hypoglycemia causes up to 100,000 emergency department calls per year. Cases of suicide attempts using insulin have been described in the literature since its introduction into therapy, and one of the important factors in their occurrence is the very fact of chronic disease. Up to 90% of patients who go to toxicology wards overdose insulin consciously. Patients with diabetes are burdened with a 2–3 times higher risk of developing depression compared to the general population. For this reason, it is necessary to develop an effective system for detecting a predisposition to overdose, including the assessment of the first symptoms of depression in patients with diabetes. A key role is played by a risk-conscious therapeutic team, as well as education. Further post-mortem testing is also needed for material collection and storage, as well as standardization of analytical methods and interpretation of results, which would allow for more effective detection and analysis of intentional overdose—both by the patient and for criminal purposes.

Insulin as a Tool in Factitious Dysglycemia

Factitious dysglycemia is a type of self-inflicted harm that includes deliberate attempts to induce hypo- or hyperglycemia as a sickness to gain empathy. We report the cases of three Iraqi women with different motives to induce factitious dysglycemia. Two of them had used insulin to induce hypoglycemia to have their family affection centered on them again. The third woman with type 1 diabetes mellitus intentionally missed her insulin doses to induce diabetic ketoacidosis and gain familial empathy through recurrent hospital admission, with underlying suicidal ideation. The problems with all women were discovered by a thorough history, physical examination, and with family help. They were referred to have psychiatric management. This is the first case series regarding factitious dysglycemia from Iraq.

Clinical and Laboratory Aspects of Insulin Autoantibody-Mediated Glycaemic Dysregulation and Hyperinsulinaemic Hypoglycaemia: Insulin Autoimmune Syndrome and Exogenous Insulin Antibody Syndrome

Autoimmune glycaemic dysregulation and hyperinsulinaemic hypoglycaemia mediated by insulin autoantibodies is an increasingly recognised but controversial phenomenon described in both exogenous insulin naïve (insulin autoimmune syndrome) and exposed (exogenous insulin antibody syndrome) individuals. There has been a significant proliferation of case reports, clinical studies and reviews in the medical literature in recent years which have collectively highlighted the discrepancy between experts in the field with regard to the nomenclature, definition, proposed pathophysiology, as well as the clinical and biochemical diagnostic criteria associated with the condition. The essential characteristics of the condition are glycaemic dysregulation manifesting as episodes of hyperglycaemia and unpredictable hyperinsulinaemic hypoglycaemia associated with high titres of endogenous antibodies to insulin. Although the hypoglycaemia is often life-threatening and initiation of targeted therapies critical, the diagnosis is often delayed and attributable to various factors including: the fact that existence of the condition is not universally accepted; the need to exclude surreptitious causes of hypoglycaemia; the diverse and often complex nature of the glycaemic dysregulation; and the challenge of diagnostic confirmation. Once confirmed, the available therapeutic options are expansive and the reported responses to these therapies have been variable. This review will focus on our evolving understanding, and the associated diagnostic challenges - both clinical and laboratory - of this complex condition.

Expression of the Biologically Active Insulin Analog SCI-57 in Nicotiana Benthamiana

Diabetes mellitus is a growing problem worldwide; however, only 23% of low-income countries have access to insulin, and ironically it costs higher in such countries than high-income ones. Therefore, new strategies for insulin and insulin analogs production are urgently required to improve low-cost access to therapeutic products, so as to contain the diabetes epidemic. SCI-57 is an insulin analog with a greater affinity for the insulin receptor and lower thermal degradation than native insulin. It also shows native mitogenicity and insulin-like biological activity. In this work, SCI-57 was transiently expressed in the Nicotiana benthamiana (Nb) plant, and we also evaluated some of its relevant biological effects. An expression plasmid was engineered to translate an N-terminal ubiquitin and C-terminal endoplasmic reticulum-targeting signal KDEL, in order to increase protein expression and stability. Likewise, the effect of co-expression of influenza M2 ion channel (M2) on the expression of insulin analog SCI-57 (SCI-57/M2) was evaluated. Although using M2 increases yield, it tends to alter the SCI-57 amino acid sequence, possibly promoting the formation of oligomers. Purification of SCI-57 was achieved by FPLC cation exchange and ultrafiltration of N. benthamiana leaf extract (NLE). SCI-57 exerts its anti-diabetic properties by stimulating glucose uptake in adipocytes, without affecting the lipid accumulation process. Expression of the insulin analog in agroinfiltrated plants was confirmed by SDS-PAGE, RP-HPLC, and MS. Proteome changes related to the expression of heterologous proteins on N. benthamiana were not observed; up-regulated proteins were related to the agroinfiltration process. Our results demonstrate the potential for producing a biologically active insulin analog, SCI-57, by transient expression in Nb.

Clinical Utility and Cross-Reactivity of Insulin and C-Peptide Assays by the Lumipulse G1200 System

BACKGROUND

Measurement of insulin and C-peptide concentrations is important for deciding whether insulin treatment is required in diabetic patients. We aimed to investigate the analytical performance of insulin and C-peptide assays using the Lumipulse G1200 system (Fujirebio Inc., Tokyo, Japan).

METHODS

We examined the precision, linearity, and cross-reactivity of insulin and C-peptide using five insulin analogues and purified proinsulin. A method comparison was conducted between the Lumipulse G1200 and Roche E170 (Roche Diagnostics, Mannheim, Germany) systems in 200 diabetic patients on insulin treatment. Reference intervals for insulin and C-peptide concentrations were determined in 279 healthy individuals.

RESULTS

For insulin and C-peptide assays, within-laboratory precision (% CV) was 3.78-4.14 and 2.89-3.35%, respectively. The linearity of the insulin assay in the range of 0-2,778 pmol/L was R²=0.9997, and that of the C-peptide assay in the range of 0-10 nmol/L was R²=0.9996. The correlation coefficient (r) between the Roche E170 and Lumipulse G1200 results was 0.943 (P<0.001) for insulin and 0.996 (P<0.001) for C-peptide. The mean differences in insulin and C-peptide between Lumipulse G1200 and the Roche E170 were 19.4 pmol/L and 0.2 nmol/L, respectively. None of the insulin analogues or proinsulin showed significant cross-reactivity with the Lumipulse G1200. Reference intervals of insulin and C-peptide were 7.64-70.14 pmol/L and 0.17-0.85 nmol/L, respectively.

CONCLUSIONS

Insulin and C-peptide tests on the Lumipulse G1200 show adequate analytical performance and are expected to be acceptable for use in clinical areas.

Factitious Hypoglycemia Caused by a Unique Pattern of Drug Use: A Case Report

INTRODUCTION

Factitious hypoglycemia, caused by the surreptitious use of insulin and sulfonylureas, is one of the most challenging differential diagnoses of hypoglycemia. Diagnosis is usually established via exclusion with respect to the special patterns of plasma insulin and C-peptide during hypoglycemic episodes.

CASE PRESENTATION

We report a case of recurrent hypoglycemic episodes and confusing patterns of insulin and C-peptide levels. In the primary evaluations, insulinoma was suspected considering the high plasma concentrations of insulin and C-peptide, besides negative urine and plasma sulfonylureas during hypoglycemic episodes. Considering the normal imaging studies and refractory hypoglycemia to medical therapy, distal pancreatectomy was performed. The patient had no episodes of hypoglycemia after the surgery. Five months later, similar episodes recurred. Further investigations revealed different plasma concentrations of insulin and C-peptide in each hypoglycemic episode. Regarding various biochemical patterns during hypoglycemia and absence of evidence supporting other differential diagnoses, we suspected factitious causes. Close observation revealed that the patient had a history of intermittent glyburide consumption and analog insulin injection.

DISCUSSION

Most commercial insulin immunoassays can only detect human insulin and lack the ability to identify synthetic analog insulin. In addition, common detection methods for sulfonylureas and meglitinides are of low diagnostic value in the human plasma and urine. These laboratory defects can lead to the misdiagnosis of insulinoma or noninsulin-mediated hypoglycemia due to the surreptitious use of insulin secretagogues or analog insulin, respectively. Therefore, due to the lack of any definitive laboratory findings, clinical suspicion is the best strategy for diagnosis.

Serum Insulin Bioassay Reflects Insulin Sensitivity and Requirements in Type 1 Diabetes

CONTEXT

Insulin resistance could increase insulin requirements in type 1 diabetes (T1D). Current insulin immunoassays do not detect insulin analogs. Kinase insulin receptor (IR) activation (KIRA) bioassays specific for human IR isoforms A (IR-A) and B (IR-B) permit assessment of all circulating insulin bioactivity. We studied whether IR-A and IR-B KIRA assays are related to direct measures of insulin sensitivity or insulin doses in T1D.

DESIGN

We evaluated 31 adult patients with T1D (age 45.7 ± 1.6 years, body mass index 28.8 ± 0.7 kg/m2). Serum IR-A and IR-B bioactivities were measured by KIRA bioassays. Insulin sensitivity of glucose production (Ra) was measured by the euglycemic hyperinsulinemic clamp technique in which a low insulin dose (0.4 mU/kg/min for 240 minutes) was combined with D-[3-3H] glucose infusion to measure rates of Ra and utilization and insulin action on antilipolysis from suppression of serum free fatty acids.

RESULTS

Baseline circulating IR-A bioactivity was 53 ± 7 pmol/L, and IR-B bioactivity was 81 ± 11 pmol/L. Compared with baseline, insulin infusion significantly increased IR-A (P < 0.001) and IR-B (P < 0.001) bioactivities. Fasting IR-A and IR-B bioactivities were positively related to endogenous Ra (r = 0.44, P = 0.01 and r = 0.38, P < 0.05). Fasting IR-A (r = 0.43, P = 0.02) and IR-B (r = 0.47, P = 0.01) bioactivities were significantly correlated with insulin requirements and glycosylated hemoglobin (IR-A: r = 0.52, P = 0.002; IR-B: r = 0.48, P = 0.006).

CONCLUSIONS

Circulating IR-A and IR-B bioactivities are associated with insulin resistance, high insulin requirements, and poor glycemic control in T1D. Measurement of IR bioactivity by KIRA assays provides a tool to assess the amount of biologically active insulin in groups of T1D patients treated with insulin analogs.

Estimation of glucose utilization in a type 2 diabetes mellitus patient on insulin analogs with tumor hypoglycemia induced by IGF-II

We present a 38-year-old male patient with insulin requiring type 2 diabetes mellitus (DM) who had fasting hypoglycemia caused by a non-pancreatic-islet-cell mesenchymal tumor producing IGF-II. The evaluation was confounded in that there was pre-existing DM being treated with insulin analogs. Insulin levels were assessed with an immunoassay with cross reactivity with the insulin analogs. An 18-Fluorodeoxyglucose (FDG) positron emission tomography/computerized tomography (PET/CT) scan localized the 19.7×18.0×17.8cm retroperitoneal mass. A 3.25kg tumor was resected. Post-operatively insulin treatment was resumed and circulating IGF-II levels returned to normal. The maximum standardized uptake values of FDG (SUVmax) along with a steady state glucose infusion of 17.5g/h were used to determine the components of glucose utilization due to IGF-II induced muscle glucose uptake (utilization, 62%) whereas the tumor itself was responsible for approximately 22% of measurable glucose uptake. Whereas tumor induced hypoglycemia has been ascribed to preferential glucose utilization by the tumor, the predominant hypoglycemic effect was due to hormonal IGF-II induced total body glucose uptake.

Quantitation of Insulin Analogues in Serum Using Immunoaffinity Extraction, Liquid Chromatography, and Tandem Mass Spectrometry

Insulin analysis is used in combination with glucose, C-peptide, beta-hydroxybutyrate, and proinsulin determination for the investigation of adult hypoglycemia. The most common cause is the administration of too much insulin or insulin secretagogue to a diabetic patient or inadequate caloric intake after administration of either. Occasionally there is a question as to whether hypoglycemia has been caused by an exogenous insulin-whether by accident, intent, or even malicious intent. While traditionally this was confirmed by a low or undetectable C-peptide in a hypoglycemic specimen, this finding is not entirely specific and would also be expected in the context of impaired counter-regulatory response, fatty acid oxidation defects, and liver failure-though beta-hydroxybutyrate levels can lend diagnostic clarity. For this reason, insulin is often requested. However, popular automated chemiluminescent immunoassays for insulin have distinctly heterogeneous performance in detecting analogue synthetic insulins with cross-reactivities ranging from near 0 % to greater than 100 %. The ability to detect synthetic insulins is vendor-specific and varies between insulin products. Liquid Chromatography and Tandem Mass Spectrometry (LC-MS/MS) offers a means to circumvent these analytical issues and both quantify synthetic insulins and identify the specific type. We present an immunoaffinity extraction and LC-MS/MS method capable of independent identification and quantitation of native sequence insulins (endogenous, Insulin Regular, Insulin NPH), and analogues Glargine, Lispro, Detemir, and Aspart with an analytical sensitivity for endogenous insulin of between 1 and 2 μU/mL in patient serum samples.

Commercial insulin immunoassays fail to detect commonly prescribed insulin analogues

OBJECTIVES

Blood insulin and C-peptide are key investigations in the differential diagnosis of hypoglycaemia. Analogues of insulin have modified primary-sequences compared to native human insulin, as such may not cross react with insulin assays. This has important implications in detecting surreptitious or malicious insulin administration. The aim of this study is to assess the cross-reactivity of all insulins currently listed in the British National Formulary (BNF65, 2013) in clinical insulin assays currently used in UK clinical laboratories.

DESIGN AND METHODS

Sample sets were prepared for all 15 exogenous insulin classes listed in the BNF, at concentrations of 1000 pmol/L and 300 pmol/L, using pooled human serum. Samples were sent blinded to 5 participating analytical laboratories to cover analysis on the 10 major clinical insulin assays used in the UK.

RESULTS

The ability of insulin assays to detect exogenous insulin preparations was highly variable and ranged from 0% to >140% for a single exogenous insulin. Four assays were highly specific for the human insulin sequence and had no cross-reactivity with any synthetic analogue insulin. Two detected all insulin types (human sequence, animal and synthetic analogue), with the remaining having variable cross-reactivity.

CONCLUSION

The cross-reactivity of the 15 exogenous insulin preparations is highly variable in the assays used in clinical laboratories around the UK. It is important that laboratories and clinicians are aware of the limitations of their local assays to avoid missing the important diagnosis of hypoglycaemia secondary to excessive exogenous insulin. Where necessary, samples should be referred to specialist centres for insulin analysis and ideally by a validated and fully-quantitative mass spectrometry-based method.

Importance of insulin immunoassays in the diagnosis of factitious hypoglycemia

We report two cases emphasizing the importance of insulin assays for evaluation of hypoglycemia in diabetic patients. Case 1 was a 96/12-year-old female patient with type 1 diabetes mellitus and case 2 was a 1010/12-year-old male patient with DIDMOAD. Both patients were on a basal-bolus insulin regimen. Both were admitted because of persistent hypoglycemia. Analyses of serum samples obtained at the time of hypoglycemia initially showed low insulin and C-peptide levels. Recurrent episodes of unexplained hypoglycemia necessitated measurement of insulin levels by using different insulin assays, which revealed hyperinsulinemic hypoglycemia with low C-peptide levels, findings which confirmed a diagnosis of factitious hypoglycemia. Surreptitious administration of insulin should not be excluded in diabetic patients with hypoglycemia without taking into account the rate of cross-reactivity of insulin analogues with the insulin assay used.

Cross-reactivity of insulin analogues with three insulin assays

Background

Immunometric assays have recently shown higher specificity in the detection of human insulin than radioimmunoassays with almost no cross-reaction with proinsulin or C peptide. The introduction of the new insulin analogues on the market, however, has raised the need to define their cross-reactivity in these assays. Several studies have been published in this regard with different results.

Methods

The analogues studied were insulins lispro, aspart, glargine, detemir, and glulisine. Insulin concentrations were measured in Immulite® 2000 and Advia Centaur® XP (Siemens Healthcare Diagnostics), and Elecsys® Modular Analytics E170 (Roche). All samples were processed 15 times in the same analytical run following a random sequence. Those samples which showed statistically and clinically significant changes in insulin concentration were reprocessed using increasing concentrations of analogue, and this was done twice, using two different serum pools, one with a low concentration of insulin and one with a high concentration of insulin.

Results

In the Elecsys® E170 analyser, glargine showed statistical changes (comparison of mean concentrations with p < 0.05) and clinically significant changes in measured insulin (percentage difference 986.2% > reference change value: 59.8%), and the interference increased with increasing concentrations of analogue; the differences were not significant in the case of the other analogues. In the Advia Centaur® and Immulite® 2000 only the results for glulisine did not present significance (percentage difference 44.7% < reference change value 103.5%). Increasing concentrations of aspart, glargine, and lispro showed increased interference in Immulite® 2000.

Conclusions

In the Elecsys® E170 assay, relevant cross-reactivity was only detected with insulin glargine, whereas in the other analysers all analogues except glulisine showed significant interference.

Pediatric hypoglycemia

Hypoglycemia in the pediatric population is a common finding important to recognize and manage to prevent brain injury. Recent advances in molecular genetics have provided new insight into its biochemical and physiologic basis and have led to more appropriate and specific treatment. Although a major cause of brain injury in pediatrics, the ability to predict the long-term outcome in these patients remains difficult. Identification of these at-risk individuals is important. The physiologic adaptations associated with transition from fetal to neonatal life are now better understood thus allowing for improved surveillance and management. Despite these advances, analytical limitations of point-of-care testing instruments at low glucose concentration continue to persist, This review aims to address these questions and provide an overview of pediatric hypoglycemia and the molecular pathways involved.

External quality assessment of hormone determinations

Hormone determinations are of central importance to the practice of Clinical Endocrinology, and ensuring their correct use and performance is a multidisciplinary responsibility involving clinicians, laboratory staff, manufacturers of diagnostic systems and healthcare regulatory agencies. All these professional groups have, therefore, an interest in external quality assessment (EQA) as an audit tool that can identify areas where use of tests in routine practice requires improvement to reduce risks to patients. This chapter reviews the principles of EQA, and outlines its strengths and limitations, illustrated with example data from the UK National External Quality Assessment Service (UK NEQAS). The immunological nature of many hormone assays, often further complicated by heterogeneity of analyte structure and lack of suitable calibrators, presents special problems for the designers of EQA schemes in ensuring that specimens are appropriate and that target values are accurate. Laboratory users of EQA should have sufficient knowledge of the characteristics of the EQA schemes in which they participate to make informed interpretation of their data. The trend since the 1980s for in-house assays designed in individual laboratories to be superseded by automated assays provided by a small number of diagnostics manufacturers places a special responsibility on manufacturers to ensure reliable assay design and calibration. In collaboration with other parties EQA can help identify priorities for improved assay design and calibration. Although traditionally the focus of EQA has been on assessing the analytical phase it can also make some assessment of other important aspects of performance, e.g. the consistency of reference ranges and how results are interpreted. Overall, EQA has a valuable role both in laboratory accreditation and as an educational resource, thereby helping to ensure and improve the quality of laboratory services that support patient care.

Forensic aspects of insulin

Insulin or, more appropriately, hypoglycaemia gives rise to a wide variety of interactions with the law. In most cases its role is not seriously open to question occasionally however, it is. This is especially true of situations in which insulin is suspected of having been used inappropriately or maliciously. The major differences between investigation of hypoglycaemia in clinical and forensic situation are that in the latter the history is often unreliable, appropriate samples for analysis were not collected, preserved or labelled correctly and analytical results are likely to be challenged on grounds of specificity, accuracy and interpretation. Immunoassay remains the mainstay of clinical investigation of hypoglycaemia but likely to become displaced by mass-spectrometry in the forensic situation especially now that human insulin is being replaced by synthetic insulin analogues for the treatment of diabetes.

Analysis of insulin in human breast milk in mothers with type 1 and type 2 diabetes mellitus

Despite the important role that insulin plays in the human body, very little is known about its presence in human milk. Levels rapidly decrease during the first few days of lactation and then, unlike other serum proteins of similar size, achieve comparable levels to those in serum. Despite this, current guides for medical treatment suggest that insulin does not pass into milk, raising the question of where the insulin in milk originates. Five mothers without diabetes, 4 mothers with type 1, and 5 mothers with type 2 diabetes collected milk samples over a 24-hour period. Samples were analysed for total and endogenous insulin content and for c-peptide content. All of the insulin present in the milk of type 1 mothers was artificial, and c-peptide levels were 100x lower than in serum. This demonstrates that insulin is transported into human milk at comparable concentration to serum, suggesting an active transport mechanism. The role of insulin in milk is yet to be determined; however, there are a number of potential implications for the infant of the presence of artificial insulins in milk.

Insights in regulated bioanalysis of human insulin and insulin analogs by immunoanalytical methods

Despite the long and illustrious history of insulin and insulin analogs as important biotherapeutics, the regulated bioanalysis (in this article, regulated bioanalysis refers to the formalized process for generating bioanalytical data to support pharmacokinetic and toxicokinetic assessments intended for development of insulin and insulin analogs as biotherapeutics, as opposed to the analytical process used for measuring insulin as a biomarker) of these peptides remains a challenging endeavor for a number of reasons. Paramount is the fact that the therapeutic concentrations are often low in serum/plasma and not too dissimilar from the endogenous level, particularly in patients with insulin resistance, such as Type 2 diabetes mellitus. Accordingly, this perspective was written to provide helpful background information for the design and conduct of immunoassays to support regulated bioanalysis of insulin and insulin analogs. Specifically, it highlights the technical challenges for determination of insulin and insulin analogs by immunoanalytical methods that are intended to support evaluations of pharmacokinetics and toxicokinetics. In a broader sense, this perspective describes the general bioanalytical issues that are common to regulated bioanalysis of peptides and articulates some of the bioanalytical differences between conventional monoclonal antibodies and peptide therapeutics.

Update on investigating hypoglycaemia in childhood

Identification and investigation of hypoglycaemia in childhood remains an important clinical emergency. Rapid recognition and appropriate management of this clinical state continues to be important in order to prevent neurological damage or even death. The purpose of this review is to provide an update on the advances made in this area since the review by Bonham in this journal in 1993. Advances in molecular science and diagnostic techniques have assisted in understanding the mechanisms involved in the homeostasis of glucose metabolism at rest and when stressed. New disorders causing hypoglycaemia are described using the classification based upon aetiologies, which was used in Bonham's original paper. The development and use of guidelines and pre-assembled packs for investigating hypoglycaemia is also discussed.

Murder by insulin: suspected, purported and proven-a review

Murder by insulin-whether attempted, suspected or proven-is rare. Only 66 cases worldwide could be found for this review. A conviction was secured in 31 cases and additional weapon was employed in 11. Differentiation of attempted homicide from Munchausen syndrome by proxy in the young and from 'mercy killing' in the elderly was not attempted. Most perpetrators were close relatives and most victims were alive when discovered and responded to treatment. Hypoglycaemia is the first clue to homicidal insulin use in living subjects and requires the demonstration of a plasma insulin concentration of generally more than 1000 pmol/L and undetectable plasma C-peptide concentration to establish the diagnosis. Serum glucose measurements are valueless in victims found dead. The presence near the body of insulin vials, syringes or needles, loose talk by the suspected perpetrator or their ready access to insulin may be the only clue. The demonstration of insulin in tissue around an injection site by immunohistopathology or by measuring it in an extract clinches the diagnosis. Immunoassays suitable for clinical use to detect and measure insulin and C-peptide are subject to random errors and cannot be relied upon unless special precautions including separation by gel filtration or HPLC are undertaken prior to analysis. They do not detect or measure accurately a new generation of synthetic insulin analogues. Mass spectrometry will be required to do this and to validate clinical immunoassays, upon which convictions have always had to rely in the past.

Cross-reactivity of 12 recombinant insulin preparations in the Beckman Unicel DxI 800 insulin assay

Background

Insulin analogues used in the treatment of insulin-dependent diabetes may result in hypoglycaemia; however, these preparations show variable cross-reactivity in many insulin assays. In order to investigate a suspected insulin overdose, it is therefore essential to characterize the cross-reactivity of these preparations in insulin assays.

Methods

The cross-reactivity of 12 commercial insulin preparations was assessed on the Beckman DxI analyser, by comparison with the insulin calibration curve corresponding to the midpoint of the dose–response curve. This was achieved by manually plotting the relative light units generated by the analyser for each preparation.

Results

All 12 insulin preparations examined showed significant cross-reactivity with the Beckman DxI insulin assay, with 11 of the insulin preparations demonstrating cross-reactivity of 83% or greater. The lowest cross-reactivity was observed with detemir with a value of 47.6%.

Conclusion

When investigating hypoglycaemia it is important to be aware of the cross-reactivity of the recombinant insulin analogues in insulin assays. This study has established the cross-reactivity of 12 preparations in the Beckman DxI insulin assay.

Plasma insulin concentrations during a hyperinsulinaemic clamp: what do we measure?

Background

The aim of this study is to compare the performance of three commonly used insulin assays with respect to the detection of exogenous human and porcine insulin added to human or rat plasma.

Methods

The DPC Immulite human insulin assay, the Mercodia rat insulin enzyme-linked immunosorbent assay and the Linco rat insulin radioimmunoassay were tested.

Results

The mean cross-reactivity of exogenous insulin ranged from 25% to 92%. The mean cross-reactivity of Actrapid in human plasma on the DPC Immulite was 56% and was independent of the endogenous insulin concentration.

Conclusions

The measurement of exogenous insulin varies according to the source of exogenous insulin, matrix and insulin assay.

Comparison of 11 human insulin assays: implications for clinical investigation and research

BACKGROUND

The American Diabetes Association task force on standardization of insulin assays in 1996 showed wide variation in assay bias. Newer assays are specific for insulin, with several now available on automated immunoassay analyzers.

METHODS

In 2004, we compared 11 commercially available insulin assays by analyzing 150 serum samples (99 fasting/51 postprandial) from study participants with various degrees of glucose intolerance (exclusions being type 1 diabetes, insulin treatment, or presence of insulin antibodies). All assays were calibrated against International Reference Preparation 66/304. One assay was not specific for insulin and another was an RIA; 10 assays used enzyme/chemiluminescent labels. Bland-Altman difference plots were modified to use the mean insulin from all assays on the x-axis as a common comparator.

RESULTS

As in the 1996 study, insulin values from the different assays varied by a factor of 2, with the nonspecific assay ranking in the middle of the distribution. Spearman rank correlation coefficients, for ranking samples vs the mean, were 0.983-0.997. Both offsets and concentration-dependent differences were seen in the modified difference plots. Imprecision (mean CV) for automated assays (3%) was not significantly different from manual assays (5%). Similar values were obtained when one automated assay was run in laboratories in both the UK and the US. Results of 1 assay showed lower insulin concentrations in heparinized plasma than in serum.

CONCLUSIONS

Assay performance must be considered before comparing insulin results. The 2-fold variation in insulin results may be related to specificity, manufacturers' calibration procedures or conversion factors.