Background: Multiple laboratory tests are used in the diagnosis and management of patients with diabetes mellitus The quality of the scientific evidence supporting the use of these assays varies substantially

Approach: An expert committee drafted evidence-based recommendations for the use of laboratory analysis in patients with diabetes An external panel of experts reviewed a draft of the guidelines, which were modified in response to the reviewers’ suggestions A revised draft was posted on the Internet and was presented at the AACC Annual Meeting in July, 2000 The recommendations were modified again in response to oral and written comments The guidelines were reviewed by the Professional Practice Committee of the American Diabetes Association

Content: Measurement of plasma glucose remains the sole diagnostic criterion for diabetes Monitoring of glycemic control is performed by the patients, who measure their own plasma or blood glucose with meters, and by laboratory analysis of glycated hemoglobin The potential roles of noninvasive glucose monitoring, genetic testing, autoantibodies, microalbumin, proinsulin, C-peptide, and other analytes are addressed

Summary: The guidelines provide specific recommendations based on published data or derived from expert consensus Several analytes are of minimal clinical value at the present time, and measurement of them is not recommended

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Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus

Monitoring of glycemic status, as performed by patients and health care providers, is considered a cornerstone of diabetes care. Results of monitoring are used to assess the efficacy of therapy and to make adjustments in diet, exercise, and medications in order to achieve the best possible blood glucose control.

The purpose of this review is to summarize current knowledge about the tests used most widely in monitoring the glycemic status of people with diabetes. The review addresses both patient- and physician/laboratory-based testing, and it includes tests of urine glucose and ketones and tests of blood glucose and glycated proteins (hemoglobin and serum proteins). The major emphasis is on the advantages and limitations of each test for routine clinical practice. Use of these tests for diabetes screening and diagnosis will not be addressed in this review. Since this review was first published in 1995, there have been many advances in the field, most notably standardization of glycated hemoglobin testing and new approaches to self-monitoring of blood glucose (SMBG), including minimally invasive continuous glucose monitoring over hours to days at a time. These and other advances are presented in detail in a recent report that was prepared by the National Academy of Clinical Biochemistry (NACB) and published as an American Diabetes Association (ADA) position statement (1). This review will attempt to complement, rather than duplicate, the material in the NACB report.

If there was an ideal method of monitoring glycemic status, it might be a small noninvasive device, perhaps similar to a wristwatch that people with diabetes could wear to continuously monitor their blood glucose level. The device would warn of impending hypoglycemia. It also would store blood glucose data and perform a variety of calculations such as hourly, daily, weekly, or monthly blood glucose averages. Unfortunately, such a monitoring device is …

https://care.diabetesjournals.org/content/diacare/27/7/1761.full.pdf

Tests of Glycemia in Diabetes

Glucose chemically attaches to proteins and nucleic acids without the aid of enzymes. Initially, chemically reversible Schiff base and Amadori product adducts form in proportion to glucose concentration. Equilibrium is reached after several weeks, however, and further accumulation of these early nonenzymatic glycosylation products does not continue beyond that time. Subsequent reactions of the Amadori product slowly give rise to nonequilibrium advanced glycosylation end-products which continue to accumulate indefinitely on longer-lived molecules. Excessive formation of both types of nonenzymatic glycosylation product appears to be the common biochemical link between chronic hyperglycemia and a number of pathophysiologic processes potentially involved in the development of long-term diabetic complications. The major biological effects of excessive nonenzymatic glycosylation include: inactivation of enzymes; inhibition of regulatory molecule binding; crosslinking of glycosylated proteins and trapping of soluble proteins by glycosylated extracellular matrix (both may progress in the absence of glucose); decreased susceptibility to proteolysis; abnormalities of nucleic acid function; altered macromolecular recognition and endocytosis; and increased immunogenicity.

Nonenzymatic glycosylation and the pathogenesis of diabetic complications.

Acrylamide (CH2CHCONH2), an industrially produced α,β-unsaturated (conjugated) reactive molecule, is used worldwide to synthesize polyacrylamide. Polyacrylamide has found numerous applications as a soil conditioner, in wastewater treatment, in the cosmetic, paper, and textile industries, and in the laboratory as a solid support for the separation of proteins by electrophoresis. Because of the potential of exposure to acrylamide, effects of acrylamide in cells, tissues, animals, and humans have been extensively studied. Reports that acrylamide is present in foods formed during their processing under conditions that also induce the formation of Maillard browning products heightened interest in the chemistry, biochemistry, and safety of this vinyl compound. Because exposure of humans to acrylamide can come from both external sources and the diet, a need exists to develop a better understanding of its formation and distribution in food and its role in human health. To contribute to this effort, this integrate...

https://pubag.nal.usda.gov/pubag/downloadPDF.xhtml?id=25505&content=PDF

Chemistry, biochemistry, and safety of acrylamide. A review.

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of ‘glycomedicine’. Glycosylation refers to the addition of carbohydrate chains to proteins and lipids. In this Review, the authors discuss the broad role of glycans in immunity, cancer, xenotransplantation and glomerular filtration and the potential of ‘glycomedicine’.

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Glycosylation in health and disease.

A high glucose concentration in vivo or an increased glucose of glucose 6-phosphate concentration in vitro has been found to lead to the glycosylation of epsilon-amino groups of lysine residues in bovine and rat lens crystallins. In vitro, this glycosylation imparts an increased susceptibility of the crystallins to sulfhydryl oxidation. Disulfide crosslinks result in the formation of high molecular weight aggregates and an opalescence in the crystallin solutions. The addition of reducing agents prevents as well as reverses the formation of high molecular weight aggregates and the opalescence of the crystallins. These phenomena suggest a new interpretation of previous results on cataract formation and a new approach for development of drugs to prevent cataracts.

https://www.pnas.org/content/pnas/75/6/2918.full.pdf

Diabetic cataract formation: potential role of glycosylation of lens crystallins.

Clinical studies on the minor hemoglobins (hemoglobin A1a-c) have suggested that a novel adduct may form in people abusing alcohol. Such patients were found to have an elevated concentration of minor hemoglobins, but normal or subnormal amounts of glycosylated hemoglobin (hemoglobin A1c) as determined by radioimmunoassay, Acetaldehyde, a reactive metabolite of ethanol, was postulated to form adducts with hemoglobin A that change its chromatographic properties. At physiological concentrations, acetaldehyde was found to form adducts with hemoglobin that were stable to extensive dialysis for several days. The amount of hemoglobin adducts formed were a function of the concentration and number of exposures to acetaldehyde. The reaction of acetaldehyde with hemoglobin A produced chromatographic variants, some of which migrated in the hemoglobin A1a-c region. Analysis of stable acetaldehyde-hemoglobin adducts demonstrated that valine, lysine, and tyrosine residues of globin were sites of reaction. The acetaldehyde-modified amino acid residues appear to exist in interconvertible conformations, only one of which is reducible by sodium borohydride. The amount of these adducts was found to be significantly elevated in hemoglobin from alcoholics as compared with normal volunteers.

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Acetaldehyde adducts with hemoglobin.

The formation of sugar-cataracts has been hypothesized to involve the nonenzymatic glycosylation, sulfhydryl oxidation, and aggregation of lens proteins. Cataractous lenses of diabetic and galactosemic rats were analyzed for glycosylated lysine residues in crystallins. A five- and a ten-fold increase in glycosylated lysine residues was measured in galactose and diabetic cataracts, respectively. The modification was predominant in the insoluble fraction of the lens homogenate. The proteins were further examined for the presence of disulfide bonds and high molecular weight aggregates. After careful disruption of the lens in a nitrogen environment, a cloudy solution was obtained from cataractous lenses whereas a clear solution was obtained from normal lenses. The absorbance at 550 nm of the solution of both the galactosemic and the diabetic cataracts could be decreased by approximately 50% with the addition of dithioerythritol (50 mM). The presence of high molecular weight aggregates was ascertained by sucrose gradient centrifugation and gel filtration chromatography. The proteins were heterogenous in size and showed a mol wt range of 36 to greater than 176 million daltons. Treatment with dithioerythritol induced a marked decrease in the amount of high molecular weight proteins. These data suggest that sugar cataracts of experimental animals have, in common with human cataracts, the presence of high molecular weight aggregates which are in part linked by disulfide bonds.

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Nonenzymatic glycosylation, sulfhydryl oxidation, and aggregation of lens proteins in experimental sugar cataracts*

The increased level of the glycosylated hemoglobin (hemoglobin A1c) in the diabetic patient has proved to be an interesting clue to understanding the biochemical basis of the sequelae of diabetes. This minor hemoglobin, which arises as nonenzymatic postsynthetic addition of glucose to hemoglobin A, acts as an indicator molecule for the glucose environment over a 3-5-wk period prior to measurement. Reasoning that a similar glycosylation reaction could be occurring with other body proteins, we have studied the ocular lens. The lens, like the erythrocyte, is not dependent on insulin for glucose concentration in the extracellular milieu that would be elevated in the diabetic state. These studies have revealed that a high glucose in vivo or an increased glucose or glucose-6-phosphate concentration in vitro leads to the glycosylation of epsilon-amino groups of lysine residues in bovine and rat lens crystallins. This glycosylation imparts an increased susceptibility of the crystallins to sulfhydryl oxidation. Disulfide crosslinks result in the formation of high molecular weight aggregates and an opalescence of the crystallin solutions.

Victor J. Stevens

Papers

Diabetic cataract formation: potential role of glycosylation of lens crystallins.

Acetaldehyde adducts with hemoglobin.

Nonenzymatic glycosylation, sulfhydryl oxidation, and aggregation of lens proteins in experimental sugar cataracts*

Role of nonenzymatic glycosylation in the development of the sequelae of diabetes mellitus.

Maillard reactions involving proteins and carbohydrates in vivo: relevance to diabetes mellitus and aging.