| Background: Renal disease is a common manifestation of systemic lupus erythematosus (SLE). Both glomerular and tubular functions could be affected. The tubular function can be measured by different methods including urinary retinol binding protein (RBP).
Objective: The study was aimed to assess the urinary RPB level in SLE patients with and without evidence of renal disease and to determine whether its measurement would be of value in early diagnosis and subsequent monitoring of renal disease in SLE.
Methods: We studied 22 female patients with SLE aged between 6 and 18 years (mean ±SD: 14.4±3.6 years) in comparison to 18 healthy age and sex matched subjects. Patients were categorized into two groups: Group I (non-renal SLE patients) which included 7 patients, who had no clinical or laboratory evidence of renal disease; Group II (renal SLE patients) which included 15 patients with lupus nephritis. They were subjected to determination of disease activity using the SLE Disease Activity Index (SLEDAI) and laboratory investigations including complete urine analysis, ESR, serum ANA, anti-DNA and C3 in SLE patients, and corrected creatinine clearance, urinary total protein, urinary microalbumin, and urinary RBP by ELISA.
Results: The urinary RBP (mg/g Cr) was significantly higher in SLE patients as a whole than controls. It was higher in renal patients than both non-renal patients and controls (1.1±0.32, 0.75±0.15, 0.5±0.08 respectively, t = 3.6, p<0.001; t = 7.11, p<0.001 respectively). Also, it was higher in non-renal patients than controls (t = 4.1, p<0.001). Urinary RBP was inversely correlated to corrected creatinine clearance(r=-0.55, p<0.05) and positively correlated to SLEDAI score, ESR, total protein and albumin in urine (r = 0.38, p<0.05; r = 0.41, p<0.05; r = 0.64, p<0.05; r = 0.58, p<0.05 respectively). From the 7 non-renal SLE patients who had urinary total protein<0.2 gm/24 hrs and no increase of albumin in urine, there were 5 patients (71.4%) with increased urinary RBP. The diagnostic sensitivity of urinary RBP, total protein and albumin in urine were 82%, 59% and 77% respectively. So, RBP held the best predictive value among other parameters in this study. From a prognostic point of view, Z score analysis of studied parameters revealed the importance of RBP in the follow up of non-renal SLE patients.
Conclusion: Urinary RBP is increased in SLE patients whether demonstrating evidence of renal disease or not. The increased urinary RBP in a large proportion of patients who had no other evidence of renal involvement could reflect early subclinical nephropathy. In renal SLE patients, RBP correlated positively to other parameters of disease activity and severity such as SLEDAI score, urinary total protein and albumin, and correlated negatively to corrected creatinine clearance. So its measurement seems to be useful in early diagnosis and subsequent monitoring of renal disease activity in SLE. |