Analysis of short-term changes in reversibly and irreversibly glycosylated haemoglobin AI: relevance to diabetes mellitus

What is "tight control" of diabetes? Goals, limitations, and evaluation of therapy

Complete normalization of the blood glucose concentration is the ideal goal in patients with diabetes. Because of deficiencies in therapeutic modalities for diabetes, achievement of approximation of normoglycemia is usually a satisfactory result. Generally, blood glucose ranges of 70 to 120 mg/dl preprandially and less than 160 to 180 mg/dl 90 minutes postprandially in nonpregnant patients with diabetes are considered appropriate. Diurnal variations in glycemia can be caused by the size and composition of meals, the time of day a meal is eaten, stress, and exercise. Self-monitoring of blood glucose by the patient and measurement of glycosylated hemoglobin have been useful in achieving control of diabetes.

Glycosylated haemoglobins: biochemical evaluation and clinical utility

A review is given of the biochemical background of the glycosylated haemoglobins, their methods of determination, and their clinical significance. Special attention is paid to the sample preparation. For all methods except the colorimetric TBA-method, the removal of the labile pre-HbA1c fraction is essential. Under proper conditions, high-performance liquid chromatography, agar-gel electrophoresis and affinity chromatography are suitable methods for use in the clinical laboratory for the estimation of HbA1c and HbA1. However, the colorimetric TBA-method must be considered to be the method of choice. The clinical utility of the test is stressed with special respect to the management of diabetic pregnancies, the control of home-monitoring of blood glucose, and the objective measurement of the effect of changing diabetic therapy.

Recent advances in glycosylated hemoglobin measurements

Glycosylated hemoglobins have gained wide acceptance as an accurate index of long-term blood glucose control in diabetes mellitus. A variety of glycosylated hemoglobin assays is available. There is a high degree of correlation between results determined by these assays. The ideal laboratory method for measuring glycosylated hemoglobin in the diabetic should be accurate, precise, easily standardized, inexpensive, and rapidly performed. Unfortunately, none of the currently used methods meet all of the criteria necessary to be considered the ideal laboratory method. The most widely used methods for quantitating glycosylated hemoglobins--including ion exchange chromatography, electrophoresis, isoelectric focusing, thiobarbituric acid colorimetry, and affinity chromatography--are reviewed with respect to the important advantages and disadvantages of each method for the clinical laboratory. Techniques for quantitating glycosylated proteins other than hemoglobins, such as albumin, are also discussed.

Clinical importance of the reversible fraction of haemoglobin A1c in Type 2 (non-insulin-dependent) diabetes

Two studies demonstrating the clinical relevance of the labile fraction of haemoglobin A1c are presented. Haemolysates, dialysed at 37 degrees C against 0.9% NaCl, showed an average decrease of haemoglobin A1c value of 7.0% of total glycosylated haemoglobin, post-dialysis. There was a close correlation between the pre- and post-dialysis haemoglobin A1c values (r = 0.9644). In four Type 2 (non-insulin-dependent) diabetic subjects, blood glucose and pre- and post-dialysis haemoglobin A1c values were determined at weekly intervals over an 18-week period, including 6 weeks when oral hypoglycaemic therapy was withheld. During this time, pre- and post-dialysis haemoglobin A1c values responded synchronously to changes in blood glucose concentration. The mean coefficient of variation of the pre- and post-dialysis haemoglobin A1c values did not differ significantly over the 18-week period. It is concluded that the routine elimination of the labile fraction of haemoglobin A1c is not necessary to assess control in Type 2 diabetes.

Increase in stable glycosylated haemoglobin after induction of poor glycaemic control

Eight insulin-treated diabetic patients in good glycaemic control were studied as out-patients with frequent determinations of stable glycosylated haemoglobin (HbA1c) before, during and after 1 week of induced poor glycaemic control. Stable HbA1c was determined by cation exchange chromatography after elimination of the labile fraction by incubation in saline (0.15 mol/l). The increase in mean blood glucose was significant on the first day of reduced insulin therapy and greatest after 1 week (6.9 +/- 3.9 mmol/l above basal values). Stable HbA1c increased significantly on day 7 of the reduced insulin treatment. The increase represented, on average, 0.009% of total haemoglobin per mmol/l increase in mean blood glucose per 24 h during the period of induced hyperglycaemia. After restoring insulin therapy, a significant decrease in blood glucose was achieved on day 1 and after 2 days, the blood glucose level was similar to before the study. There was no significant decrease in stable HbA1c within the first 2 weeks of improved glycaemia.