Glycated albumin as biomarker: Evidence and its outcomes

Glycemic control markers are important for the diagnosis and treatment of diabetes. Hemoglobin A1c (A1C) is an important marker that is mandatory in routine medical examinations; however, it is well known that it has some limitations. In this review, we focus on the limitation of A1C and introduce a relatively new marker, glycated albumin (GA), which can be used to complement A1C. First, for a better understanding of the characteristics of each marker, we sort the similarities and differences of glycemic control markers as well as the characteristics of each marker. Second, we point out the limitation of A1C, introduce GA as an alternative indicator, and discuss the limitations of GA. Finally, we summarize important evidence regarding the utility of GA. We hope that this review provides useful information that permits more effective usage of GA as well as other glycemic control markers.

Abundance matters: role of albumin in diabetes, a proteomics perspective

INTRODUCTION

Human serum albumin (HSA) is a multifaceted protein with vital physiological functions. It is the most abundant plasma protein with inherent capability to bind to diverse ligands, and thus susceptible to various post-translational modifications (PTMs) which alter its structure and functions. One such PTM is glycation, a non-enzymatic reaction between reducing sugar and protein leading to formation of heterogeneous advanced glycation end products (AGEs). Glycated albumin (GA) concentration increases significantly in diabetes and is implicated in development of secondary complications. Areas covered: In this review, we discuss in depth, formation of GA and its consequences, approaches used for characterization and quantification of GA, milestones in GA proteomics, clinical relevance of GA as a biomarker, significance of maintaining abundant levels of albumin and future perspectives. Expert commentary: Elevated GA levels are associated with development of insulin resistance as well as secondary complications, in healthy and diabetic individuals respectively. Mass spectrometry (MS) based approaches aid in precise characterization and quantification of GA including early and advanced glycated peptides, which can be useful in prediction of the disease status. Thus GA has evolved to be one of the best candidates in the pursuit of diagnostic markers for prediction of prediabetes and diabetic complications.

Advantages and pitfalls of fructosamine and glycated albumin in the diagnosis and treatment of diabetes

The efficient diagnosis and accurate monitoring of diabetic patients are cornerstones for reducing the risk of diabetic complications. The current diagnostic and prognostic strategies in diabetes are mainly based on two tests, plasma (or capillary) glucose and glycated hemoglobin (HbA1c). Nevertheless, these measures are not foolproof, and their clinical usefulness is biased by a number of clinical and analytical factors. The introduction of other indices of glucose homeostasis in clinical practice such as fructosamine and glycated albumin (GA) may be regarded as an attractive alternative, especially in patients in whom the measurement of HbA1c may be biased or even unreliable. These include patients with rapid changes of glucose homeostasis and larger glycemic excursions, and patients with red blood cell disorders and renal disease. According to available evidence, the overall diagnostic efficiency of GA seems superior to that of fructosamine throughout a broad range of clinical settings. The current method for measuring GA is also better standardized and less vulnerable to preanalytical variables than those used for assessing fructosamine. Additional advantages of GA over HbA1c are represented by lower reagent cost and being able to automate the GA analysis on many conventional laboratory instruments. Although further studies are needed to definitely establish that GA can complement or even replace conventional measures of glycemic control such as HbA1c, GA may help the clinical management of patients with diabetes in whom HbA1c values might be unreliable.

Serum fructosamine concentration in nondiabetic and diabetic cats

Differentiating transient hyperglycemia from diabetic hyperglycemia can be difficult in cats since single blood glucose measurements reflect only momentary glucose concentrations, and values may be elevated because of stress-induced hyperglycemia. Glycated protein measurements serve as monitors of longer-term glycemic control in human diabetics. Using an automated nitroblue tetrazolium assay, fructosamine concentration was measured in serum from 24 healthy control cats and 3 groups of hospitalized cats: 32 euglycemic, 19 transiently hyperglycemic, and 12 diabetic cats. Fructosamine concentrations ranged from 2.1 - 3.8 mmol/L in clinically healthy cats; 1.1 - 3.5 mmol/L in euglycemic cats; 2.0 - 4.1 mmol/L in transiently hyperglycemic cats; and 3.4 to >6.0 mmol/L in diabetic cats. Values for with-in-run precision at 2 fructosamine concentrations (2.64 mmol/L and 6.13 mmol/L) were 1.5% and 1.3%, respectively. Between-run coefficient of variation was 3.8% at a fructosamine concentration of 1.85 mmol/L. The mean fructosamine concentration for the diabetic group differed significantly (P=0.0001) from the mean concentrations of the other 3 groups. Poorly regulated or newly diagnosed diabetic cats tended to have the highest fructosamine values, whereas well-regulated or over-regulated diabetic cats had values approaching the reference range. As a single test for differentiating nondiabetic cats from diabetic cats, fructosamine was very sensitive (92%) and specific (96%), with a positive predictive value of 85% and a negative predictive value of 98%. Serum fructosamine concentration shows promise as an inexpensive, adjunct diagnostic tool for differentiating transiently hyperglycemic cats from poorly controlled diabetic cats.

Monitoring diabetic control in developing countries: a review of glycated haemoglobin and fructosamine assays

Measurement of glycated proteins, especially haemoglobins, is now a routine procedure for monitoring previous glycaemic control in diabetic patients in the developed world. Until recently, however, the assays were sophisticated and costly and, therefore, were only rarely used in developing countries. With easier and cheaper tests now available, the measurement of glycaemic control could be also introduced into regional hospitals in developing countries. This paper reviews feasible methods for clinical laboratories with limited resources. Recent experiences from African countries are included.

Serum fructosamine correlates with gingival index in children with insulin-dependent diabetes mellitus (IDDM)

Fructosamine assay, which is used in diagnosing and monitoring diabetic patients, is compared with the hemoglobin and plasma glucose assays in children and adolescent insulin-dependent diabetes mellitus patients. We demonstrated that the gingival index scores were correlated with fructosamine values in insulin-dependent diabetes mellitus patients but not in non-diabetic controls. We also found that there was no correlation between gingivitis scores and fasting plasma glucose and HbA1c values. Periodontitis was found to be rare in diabetic children and adolescents.

Serum immunoglobulin concentrations in diabetic patients

The relationship between glycated haemoglobin (an index of long-term diabetic control), fructosamine (an index of intermediate-term diabetic control), and serum IgA, IgG, and IgM was studied in 110 diabetic patients (41 Type 1 and 69 Type 2) and compared with 111 healthy non-diabetic subjects. Significant increases in serum IgA (by 82.7%, p < 0.001) and IgG (by 35.2%, p < 0.001) concentrations were observed whereas the concentration of IgM was significantly decreased (by 46.7%, p < 0.001) in diabetic patients compared with non-diabetic subjects. Using Spearman's rank correlations, IgA correlated with fructosamine (r = 0.77, p < 0.001), HbA1 (r = 0.76, p < 0.001), and albumin (r = -0.58, p < 0.001) for the entire population sample but only fructosamine (r = 0.19, p < 0.05) and HbA1 (r = 0.28, p < 0.001) correlated with IgA in diabetic patients, respectively. It is concluded that abnormal levels of IgA, IgG, and IgM are very common in diabetic patients in whom serum IgA concentrations are influenced by the degree of glycaemic control. Whether changes in IgA and other immunoglobulins are implicated in the pathogenesis of diabetic complications (such as susceptibility to infection) deserve further study.

Thiobarbiturate and fructosamine assays: significance and interest of the borohydride blank

The acute-phase reaction (APR) induces the production by the liver of short-lived glycoproteins. The carbohydrate moiety of these proteins is thought to interfere with the thiobarbiturate (TBA) and nitroblue tetrazolium colorimetric tests which are used for assaying non-enzymatic glycosylation (NEG) of serum proteins. The aim of the present study was to assess the effect of the APR on the specificity of the colorimetric tests in non-diabetic and diabetic subjects. A positive correlation was found between C-reactive protein (CRP), an APR glycoprotein, and non-specific TBA reactivity as determined after borohydride reduction (BH4-resistant TBA, BR-TBA), both in non-diabetics (r = 0.61; P < 0.01) and diabetics (r = 0.68; P < 0.01). The BH4-sensitive specific TBA (SP-TBA) was not influenced by glycoproteins, and its increase in diabetics was correlated with the nitroblue tetrazolium assay (r = 0.89; P < 0.01). An independent effect of diabetes and APR on non-specific TBA was also demonstrated, suggesting an effect of hyperglycaemia on both protein glycation and glycosylation. TBA with borohydride reduction is an attractive tool for the study of complex glycoproteins in diabetes.

Fructosamine as a possible monitoring parameter in non-insulin dependent diabetes mellitus patients with periodontal disease

Fructosamine assay is a new test used in the diagnosis and monitoring of diabetic patients. This assay may be of interest to the periodontist for, while the traditional plasma glucose value would give a general view and information about diabetic control at a certain point, the fructosamine concentration gives an indication of the plasma glucose level over a considerable period of time, such as 1 to 3 weeks. We investigated whether there was any relation between the diseased state of the periodontal tissues and plasma fructosamine and the plasma glucose values in non-insulin dependent diabetes mellitus (NIDDM) patients. We found that fructosamine correlated with the degree of gingival bleeding, however serum glucose levels had little or no correlation.

Measurement of glycated albumin by the nitroblue tetrazolium colorimetric method

A method has been developed for the measurement of glycated albumin (albumin-fructosamine) by the nitroblue tetrazolium (NBT) colorimetric method. In this method, polyethylene glycol was added to the serum and then the mixture was centrifuged to separate globulin proteins from albumin proteins. This made it possible to measure the glycated albumin in the supernatant by the NBT colorimetric method, without the interference of globulin proteins. This measurement method correlated with the measurement of glycated albumin using boronate affinity chromatography with an r value of 0.942 (P < 0.001). Our method using polyethylene glycol permits easy measurement of albumin fructosamine and is therefore useful as an index of diabetic control and for diabetic screening.

Serum fructosamine concentration in patients with nephrotic syndrome and with cirrhosis of the liver: the influence of hypoalbuminaemia and hypergammaglobulinaemia

We have investigated the influence of variation of the concentrations of serum albumin and immunoglobulins on serum fructosamine concentration in 33 patients with nephrotic syndrome, and 18 patients with cirrhosis of the liver. Protein alterations were evident in these patients and they were compared with 109 normal subjects, 43 patients with type II diabetes mellitus and nine diabetic patients with nephrotic syndrome. The mean serum fructosamine concentration in diabetic patients (2.76 +/- 0.53 mmol/L) was significantly increased (P less than 0.001) by comparison with normal subjects (1.93 +/- 0.20 mmol/L) and the other patients studied. Patients with diabetic nephropathy had higher (P less than 0.01) serum fructosamine concentrations (2.23 +/- 0.54 mmol/L) than non-diabetic patients with the nephrotic syndrome (1.57 +/- 0.37 mmol/L) but remained with the normal range. Positive correlations were observed between fructosamine and immunoglobulins G and M in nephrotic and cirrhotic patients. Serum immunoglobulin A was also directly correlated with serum fructosamine in patients with cirrhosis of the liver. An inverse correlation between albumin and fructosamine in serum of patients with cirrhosis of the liver was also noted. We conclude that the fructosamine assay is not useful in the assessment of glycemic control in patients with cirrhosis of the liver, nephrotic syndrome or in any other clinical situation in which protein metabolism is altered.

Influence of serum proteins on fructosamine concentration in multiple myeloma

Serum fructosamine levels in 36 subjects with various types of multiple myeloma and in 64 normal controls were evaluated by means of a Nitroblue tetrazolium colorimetric assay. Only the IgA myeloma group showed significantly raised serum fructosamine values (P less than 0.001). In the IgG myeloma group, which showed a higher mean serum protein concentration, serum fructosamine levels were not significantly different from controls. The study shows that elevated IgA levels do influence serum fructosamine and this effect should be taken into due consideration in order to avoid possible misinterpretations in evaluating this widely used index of glucose metabolism.

Abnormal serum immunoglobulin concentrations in patients with diabetes mellitus

Since the recently reported relationship between serum fructosamine and IgA concentrations appears to throw doubt on the clinical utility of fructosamine as a measure of hyperglycemic status if IgA concentration is not taken into account, we studied serum immunoglobulin concentrations in 169 diabetics and their relationship with various clinical and analytical parameters. Over 41% of the patients studied had abnormal serum IgA concentrations. Serum IgA concentration was negatively correlated with serum albumin, and among IDDM patients was positively correlated with age (so that the prevalence of abnormal IgA was 57.7% among IDDM patients aged over 30 years). Among NIDDM patients, abnormal IgA concentrations were especially prevalent among those being treated with oral hypoglycemics. Abnormal IgA was also more frequently found in both IDDM and NIDDM patients, who had been under treatment for 10 years or more. Abnormal IgG concentrations were found in 11.8% of the diabetics, and the mean IgM concentration found in the patients was 41.6% lower than in the normoglycemic group. We conclude that abnormal serum IgA concentrations are very common in diabetic patients and that further research should be carried out to verify whether the determination of serum immunoglobulins, IgA in particular, is of clinical use for monitoring diabetes or evaluating its secondary effects.