An enzyme linked immunoadsorbent assay for urinary albumin using commercially available reagents is described. The assay range is 2.5-120 micrograms/l. When samples are analysed in two standard dilutions, the assayable albumin concentration range is 2.5-240 mg/l, covering the clinical range from normoalbuminuria to overt clinical nephropathy. Intra-assay variation was 2.1% and interassay variation 8.3%. Recovery of added albumin to urine was 95%-106% and dilution of urine was linear. The correlation to urinary albumin determined by immunodiffusion was excellent (n = 80, r = 0.99). Intraindividual variation of the 24 h urine albumin excretion of different days was high in patients with incipient diabetic nephropathy (51.5%) and was only slightly reduced by taking the variation of creatinine excretion into account (39.5%). No correlation was found between albumin excretion, and HbA1c or urine glucose excretion, indicating that minor metabolic variations are not responsible for the huge intraindividual day-to-day variations of UalbV. The study shows that more than one UalbV measurement must be done before classifying patients into groups with or without incipient diabetic nephropathy.

The pathogenesis of clinical nephropathy in Type 1 (insulin-dependent) diabetes was investigated by measuring renal fractional clearances of albumin, total IgG, IgG4 and beta 2-microglobulin, four plasma proteins which differ in size and charge. Seventy patients and eleven control subjects were studied. In diabetic patients with normal urinary albumin excretion (less than 30 mg/24 hr), fractional IgG clearance was two to three times higher than in control subjects, whereas fractional clearance of the anionic plasma proteins IgG4 and albumin was similar to that of control subjects. These alterations indicate an increase in anionic pore charge within the glomerular basement membrane concomitant with an increase in either pore size or impairment of tubular reabsorption. Diabetic patients, whose urinary albumin excretion has started to rise (30 to 100 mg/24 hr), had unchanged fractional IgG compared to patients with normal albumin excretion, while fractional IgG4 and albumin clearances were increased three- to fourfold; indicating unchanged glomerular pore size, but a decrease in anionic pore charge. In patients demonstrating urinary albumin excretion of greater than 100 mg/24 hr fractional IgG clearance increased to the same extent as fractional albumin clearance, indicating an increase in large pore area. Fractional beta 2-microglobulin clearances were similar to that of control subjects in the different patient groups indicating unchanged tubular reabsorption of proteins. Thus, the increase in large pore area seen in patients with clinical nephropathy is preceded by loss of anionic charge in the glomerular basement membrane. It is likely that this loss of anionic charge is due to loss of heparan sulphate-proteoglycan.

Our study revealed a progressive raise in serum sialic acid with increasing urinary albumin excretion rate in NIDDM patients. Furthermore, several modifiable cardiovascular risk factors were associated with serum sialic acid.