Diabetic patients have been shown to excrete increased quantities of albumin, which is undetectable by conventional albumin antibodies (immunounreactive) using high-performance liquid chromatography (HPLC) (1). Furthermore, the lead time for the development of microalbuminuria (albumin excretion rate [AER] >20 μg/min) measured by HPLC has been shown to occur 3.9 and 2.4 years earlier than that determined by radioimmunoassay (RIA) for type 1 and type 2 diabetic patients, respectively (2). This study not only identified that progression from normo- to microalbuminuria is associated with an increase in urinary immunounreactive albumin, but also raises the possibility that measurement of total albumin (immunoreactive plus immunounreactive) may allow earlier detection of progression to kidney disease.
The aim of this study was to determine whether a group of type 2 diabetic patients with a low glomerular filtration rate (GFR) of <60 ml · min−1 · 1.73 m−2, as measured by a single-injection isotopic technique using 99mTc-DTPA (3), and normoalbuminuria, as measured by RIA, excrete increased quantities of immunounreactive albumin. Total albumin was measured by HPLC analysis (1,2), and immunoreactive albumin was measured by RIA on two to three consecutive urine samples collected from 38 type 2 diabetic patients attending the Austin & Repatriation Medical Centre, Victoria, Australia. Patients who had recurrent urinary tract infections or hematuria, known nondiabetic renal disease, or severe intercurrent illness, such as a malignancy or symptomatic cardiac failure, were excluded from the study.
The major finding of this study is that 24% (9 of 38) of patients had an AER >20 μg/min and 37% (14 of 38) of patients had an AER >15 μg/min, as measured by HPLC, in comparison with RIA analysis, which detected 0% (0 of 38) of patients with an AER >20 μg/min and 13% (5 of 38) of patients with an AER >15 μg/min. There was no significant difference between HPLC and RIA analysis of albumin in urine from nondiabetic subjects (1). These results identify that type 2 diabetic patients with a GFR <60 ml · min−1 · 1.73 m−2, and therefore presumably some form of kidney dysfunction, have an increased prevalence of urinary immunounreactive albumin. The possible pathogenesis of increased urinary immunounreactive albumin in these patients is limited given the absence of renal ultrastructural data. In fact, there is a paucity of information available regarding the renal morphology of normoalbuminuric patients with type 2 diabetes, regardless of their GFR. Nevertheless, the discrepancy between the HPLC and immunochemical assays demonstrates that conventional albumin assays may provide a relatively late diagnosis of incipient kidney disease at a threshold of 20 μg/min.
The combination of impaired renal function and normoalbuminuria in patients with diabetes was first highlighted by Lane et al. (4). For healthy nondiabetic individuals, the rate of decline in GFR with age has been reported to range from 0.6 to 1.0 ml · min−1 · 1.73 m−2 · year−1 (5). We have previously shown that the rate of decline in renal function for normoalbuminuric type 2 diabetic patients is −5.5 ± 1.0 ml · min−1 · 1.73 m−2 · year−1, which is clearly greater than that related to aging alone (6). It should also be noted that the rate of decline in patients who had a GFR of <60 ml · min−1 · 1.73 m−2 and normoalbuminuria was similar to that observed for micro- and macroalbuminuric patients (6).
In summary, this study demonstrates that urine from diabetic patients with a low GFR contains a high prevalence of immunounreactive albumin as measured by HPLC. This indicates that HPLC analysis of albumin components in the urine may provide a better indication of kidney dysfunction than the conventional immunoassays currently available.
