Objective: N-acetyl-β-D-glucosaminadase (NAG) is a enzyme which exists in the lysosomes of renal proximal tubular cells. Increased NAG activity in urine is accepted as non-invasive early index of tubular damage. Lupus nephritis is known to be the most important complication of Systemic Lupus Erythematosus (SLE) and predicts of poor outcome. The aim of study is to investigate usefullness of NAG activity measurements in urine of SLE patients for determination and monitoring of renal tubular damage in early stage of disease.

Materyal and Methods: In this study, urinary NAG activities were investigated in SLE patients with nephrotic range proteinuria (≥3.5 g/24 hour), which have <1.2 mg/dL (group A) and ≥1.2 mg/dL (group B) serum creatinine levels and compared with healty controls. In SLE patients NAG activity, protein exretion and creatinine levels were determined in 24-hour urine samples and C3, C4 levels and creatinine were measured in serum. Renal biopsies were evaluated in all of the patients with SLE. The renal biopsy specimens were classified according to the World Health Organisation (WHO) criteria. SLE Disease Activity Index (SLEDAI) has been used to determine of disease activity.

Results: In group A and B urinary NAG activities were significantly high than healthy controls (p=0.0001). There were no significant differences in urinary NAG activity and protein excretion when compared group A and B (p=0.908 and p=0.223 respectively). The SLEDAI scores and ages in these groups were similar (p=0.293 and p=0.602 respectively). A positive correlation was seen between the increase of urinary NAG activity and the extend of proteinuria (p=0.029, r=0.563). Low serum C3 levels were found in the younger patients (p=0.047, r=0.520) besides there was a positive correlation between SLEDAI score and serum creatinine level (p=0.034, r=0.549).

Conclusion: These results provide that the measurements of NAG activity in SLE patients give more valuable information than urine protein assay alone and indicate the importance of early determination of tubular damage in devolepment of chronic renal failure.