The pathophysiologic mechanism behind microalbuminuria, a potential atherosclerotic risk factor, was explored by measuring fractional clearances of four endogenous plasma proteins of different size and electric charge (albumin, beta 2-microglobulin, immunoglobulin G, and immunoglobulin G4). Twenty-eight clinically healthy individuals with microalbuminuria, defined as a urinary albumin excretion of 6.6-150 micrograms min-1, and 60 matched control subjects were studied. Fractional immunoglobulin G clearance was higher (geometric means (95% confidence intervals)) 3.0 (2.3-3.9) x 10(-6), n = 28, vs. 2.1 (1.8-2.4) x 10(-6), n = 60; P = 0.02), whereas the ratio immunoglobulin G clearance/immunoglobulin G4 clearance was lower (geometric means (95% confidence intervals)) 1.8 (1.4-2.2), n = 28, vs. 2.3 (2.0-2.5), n = 60; P = 0.03) in microalbuminuric than in normoalbuminuric individuals. Fractional beta 2-microglobulin clearance was similar in the two groups. Since total IgG and the IgG4 subclass are of similar size and configuration but electrically neutral and negative, respectively; these findings indicate that microalbuminuria is associated with decreased size- and charge-selectivity of the glomerular vessel wall. Hypothetically, such alterations may reflect generalized vascular abnormalities linking microalbuminuria to atherogenesis.

When investigating glomerular changes in the early stages of diabetic renal disease, it is important to be able to estimate low concentrations of urinary immunoglobulins, as well as the albumin/immunoglobulin ratio. For this reason there is a need for highly sensitive and specific routine assays for urinary immunoglobulins. A 'sandwich' enzyme-linked immunoadsorbent assay (ELISA) for the quantitation of urinary immunoglobulin G is described, in which microtest plates are used as the solid phase. The assay is specific, sensitive and simple, and only used commercially available reagents. The assay range was 5-200 micrograms/l. The relative standard deviations within and between assays were 5 and 9%, respectively. Recovery of IgG added to urine was 100-102% (n = 12), and dilution of urine was linear. The assay range allowed for the quantitation of IgG in human urine samples, covering the clinical range from physiological to raised pathological values.

An enzyme-linked immunoadsorbent assay for the determination of beta 2-microglobulin in serum and urine using microtest plates as solid phase is described. All reagents are commercially available. The assay has a high capacity and it is inexpensive using only 3% of the amounts of antibodies required in a previously described ELISA based on tubes as solid phase.

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.