While fructosamine represents an index of the concentration of stably glycated serum proteins (ketamine, mainly albumin, lipoproteins, and glycated globulins), GA represents a unique post-translational product of albumin alone with plasma glucose.
Although HbA1c remains the gold standard for the diagnosis and monitoring of diabetes mellitus, several studies have shown that other biomarkers of nonenzymatic glycation, such as GA and fructosamine, can be used both in clinical conditions in which HbA1c dosage does not reflect glycemic compensation (chronic renal failure, anemia, and hemoglobin variants) and in other clinical settings in which information on blood glucose in the previous 2–3 weeks is needed (poorly controlled diabetes, gestational diabetes, postprandial hyperglycemia, anemia, hemoglobin variants). Moreover, in other clinical settings, it is necessary to have information on blood glucose in the previous 2–3 weeks (poorly controlled diabetes, gestational diabetes, postprandial hyperglycemia, “fluctuating” diabetes, and gastrectomized patients). GA and fructosamine are particularly sensitive to recent changes in blood glucose and sudden f luctuations.
Preanalytical Aspects
Using these markers in clinical practice requires considering some preanalytical variables that may affect the result. The literature regarding this aspect is quite limited and sometimes conflicting. In summary, all those clinical conditions that alter the normal metabolism of albumin limit the use of fructosamine and GA: thyroid dysfunction, nephrotic syn drome, liver cirrhosis, and nonalcoholic steatohepatitis (NASH). Nephrotic syndrome, hyperthyroidism, NASH, Cushing’s syndrome, and glucocorticoid intake decrease in AG and fructosamine values, while liver cirrhosis and hypo thyroidism determine an increase in the values of these markers. Physiological or lifestyle variables such as age, gender, race, body mass index (BMI), hyperuricemia, and cigarette smoking also influence the values of these markers.
The intra- and interindividual biological variability of GA was 2.1% and 10.6%, while that of fructosamine was 2.3% and 6.3%.
GA values have an inverse correlation with BMI, fat mass levels, and cigarette smoking, while they correlate positively with age and are higher in females. Some authors also report an increase in GA values throughout pregnancy, while others report a decrease in values in the third trimester.
Fructosamine values show a modest decrease with gestational age and a modest increase with maternal age and correlate inversely with smoking. Higher values are found in males, and differences have been reported in different races.
Regarding the biological matrix used for assaying these markers, serum or plasma can be used. However, it has been reported that the values found in plasma are lower than those obtained in serum for fructosamine. It is therefore recommended that the same biological matrix is always used for monitoring.
As shown in two studies, GA is stable for 24 h on separated serum and plasma stored at room temperature, up to 1 week when stored at 4 °C, and up to 2 months when stored at −80 °C.
Stability for up to 2 weeks with serum stored at 4 °C and up to 5 weeks when stored at −20 °C has been demonstrated for fructosamine.
Analytical Aspects
Several methods have been developed to evaluate fructosamine in serum and plasma. The most widely used and best standardized are colorimetric assays that typically exploit the property of fructosamine to be a reducing agent under alkaline conditions. The first technique, developed in 1983, was based on reducing the nitroblue tetrazolium dye to formazan. The spectrophotometric method monitored the rate of formazan formation, which was directly proportional to the fructosamine concentration. Later, a nonionic deter gent containing uricase was added to eliminate interference from uric acid and polylysine, thus allowing a more accurate and sensitive measurement. The assay thus modified is currently available and widely used in clinical laboratories. Although it is rapid, technically simple, inexpensive, and available for automation, the method is affected by ambient temperature variations and remains poorly standardized. In addition, many molecules with reducing activity, such as bilirubin and vitamins, may interfere with the measurement.
GA concentration can be measured by boronate affinity chromatography, ion exchange chromatography, HPLC, and immunoassays (ELISA or radioimmunoassays). An enzymatic method with good analytical performance and excel lent specificity has recently been automated (Lucica GA-L kit, Asahi Kasei Pharma, Tokyo, Japan). The method is based on the initial elimination of endogenous glycated amino acids and peroxides by a ketamine oxidase and a peroxidase. The GA is then hydrolyzed by an albumin-specific proteinase, and the products of this reaction are oxidized by a ketamine oxidase. The H2O2 produced is then measured quantitatively by a colorimetric method. In parallel, the con centration of albumin is measured by the bromocresol violet method so that the final result can be expressed as the ratio of GA to total albumin. This method can be implemented on major automated analytical platforms and offers good analytical performance.
