High prevalence of immuno-unreactive intact albumin in urine of diabetic patients

Abstract

Background: Intact albumin in urine may exist in two forms, immunoreactive and immuno-unreactive. Previous estimates of albuminuria in diabetic urine have only detected immunoreactive forms. Methods: High performance liquid chromatography (HPLC) was used in this study to measure both forms of intact albumin (termed total intact albumin) to provide a more accurate measurement of albuminuria compared with radioimmunoassay (RIA) on 97 fresh urine samples from patients with diabetes. Eighty-six control urine samples from volunteers without diabetes were also tested. Results: There was no significant difference between the two methods for nondiabetic controls. For diabetic urine samples, 91.6% of samples showed a greater concentration of albumin measured by HPLC than RIA. For normoalbuminuric diabetic samples, HPLC gave a mean albumin excretion rate of 12.5 ± 4.4 μg/min (SD), whereas RIA gave a rate of 8.0 ± 6.7 μg/min (P = 0.004; N = 28). For microalbuminuric samples, there also was a statistically significant difference: HPLC albumin excretion rate, 82.0 ± 49.9 μg/min, and RIA, 49.0 ± 34.6 μg/min (P = 0.004; N = 30). Thirty-two urine samples were normoalbuminuric by RIA (albumin, 11.4 ± 3.9 μg/min), but in the microalbuminuric range as determined by HPLC (albumin, 38.5 ± 14.4 μg/min). For urine samples in the macroalbuminuric range, there was no statistically significant difference between HPLC and RIA. Immuno-unreactive albumin was confirmed as albumin, analyzed by two-dimensional electrophoresis and matrix-assisted laser desorption ionization mass spectrometry. Conclusion: These studies show that to determine microalbuminuria accurately, there is a need to assess urinary total intact albumin, rather than simply immunoreactive albumin. Am J Kidney Dis 41:336-342.