Methodological problems with the assay of fructosamine in serum--standardization, matrix effects, and dependence on buffer pH--have been minimized with a method involving colorimetric assay of each specimen and subsequent re-assay after standard addition of 1-deoxy-1-morpholinofructose. Absorbance at optimum wavelength of 540 nm varies linearly with fructosamine concentration to at least 5.5 mmol/L, and between-run precision is about 6% for both patients' specimens and quality control materials. Correction of fructosamine to serum albumin of 40 g/L minimizes the effect of albumin while maintaining transferability of data and reference values. From data on biological variation, the analytical goal for precision (CV) is less than or equal to 2.6%. The square root of the ratio of intra- to interindividual variance is low, indicating that fructosamine concentrations have a high index of individuality; thus conventional population-based reference values are of limited use. Although this assay may be useful in monitoring disease, we doubt that it provides a valid screening test for diabetes.

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Type 1 diabetes mellitus is associated with an increase in total exchangeable body sodium. To delineate a site of possible altered sodium handling, proximal tubular sodium reabsorption (PTRNa) was measured in 30 diabetic children, age 12.0 (range 7-16) yr, duration of diabetes 4.5 (range 0.2-12) yr, and compared with 10 non-diabetic children, age 10.0 (range 8.6-12.5) yr. PTRNa was calculated from the fractional clearance of lithium, which was determined from a single blood sample and a random untimed urine sample, taken between 0700 and 0830 h at home, fasting, before insulin therapy. PTRNa was significantly increased in the diabetic children compared with the non-diabetic children (81.6(SE 1.0) vs 74.2(2.6)%, p = 0.014). There was no relationship of PTRNa with age, duration of diabetes, metabolic control (glycosylated haemoglobin, plasma and urinary glucose, plasma lactate), or urinary protein excretion (albumin, N-acetyl-beta-D-glucosaminidase). Elevated sodium reabsorption in the proximal renal tubule may account for the high total exchangeable body sodium found in Type 1 diabetic patients.

Assessment of the relative clinical usefulness of glycosylated haemoglobin (HbA1) and serum fructosamine is complicated by their markedly different half-lives. They have therefore been compared using a number of indices derived from data on biological variation, as applied in a study of blood glucose control in newly diagnosed diabetic patients. The within-subject (CVI) and between-subject (CVG) variation of fructosamine and HbA1 were assessed in 8 stable diabetic patients. A slightly smaller relative change, or critical difference, is required between serial HbA1 results than between fructosamine results before a significant change can be said to have occurred. The heterogeneity of within-subject variance is also less for HbA1, rendering the critical difference more generally applicable. HbA1 may therefore be more appropriate for long-term monitoring. Monitoring of blood glucose control of a group of 26 newly diagnosed diabetic patients before the commencement of therapy and 1, 2, and 3 months later by serum fructosamine or HbA1 provided very similar information. Fructosamine responds more rapidly with changes of blood glucose control. However, whether this confers any clinical advantage will depend upon the frequency of testing.