Urinary N-acetyl-β-D-glucosaminidase (NAG) in lupus nephritis and rheumatoid arthritis

Increased activity of urinary N-acetyl-beta-D-glucosaminidase (NAG) can be used as an early indicator of damage to the tubular epithelium. Systemic lupus erythematosus (SLE) is a multisystem autoimmune rheumatic disease. Nephritis is known as the most serious complication of SLE and the strongest predictor of poor outcome. In this study urinary NAG excretion was investigated in 24 SLE patients with normal renal function (serum creatinine <= 1.2 mg/dL) and the results were compared with those from 26 untreated patients with rheumatoid arthritis (RA) and 27 healthy controls. The SLE patients were divided into two groups according to their urinary total protein levels: group A consisted of 16 patients with <= 3.5 g/day proteinuria, and group B consisted of eight patients with nephrotic-range proteinuria (>3.5 g/day). Serum and urinary creatinine, total urinary protein levels, and urinary NAG excretion were measured in patients with SLE and RA. In addition, serum C3 and C4 levels were determined in the SLE patients. Renal biopsies were performed in all of the SLE patients. Glomerular lesions were classified according to WHO criteria for lupus nephritis (LN) I-V. The Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) was used to assess disease activity. Urinary NAG excretion was significantly higher in the SLE groups than in the healthy controls (P<0.001). In urinary NAG excretion there was also significant difference between SLE groups and RA patients (P<0.001). However, there was no significant difference in NAG excretion between the RA and control groups (P=0.062). Urinary NAG excretion was significantly higher (P<0.05) in group B compared to group A. There were no differences in SLEDAI scores, ages, and serum creatinine levels between study groups (P=0.601, P=0.285, P=0.669, respectively). Elevated SLEDAI values and hypocomplementemia were detected more often in younger patients (P<0.010, r=-0.529 and P<0.010, r=-0.569, respectively). There was a strong positive correlation between proteinuria and urinary NAG activity (P<0.001, r=0.759). These results suggest that the determination of urinary NAG activity may be a useful supplement to the routine biochemical analysis performed on the urine in cases of SLE.