Proteinuria in children with sickle cell disease.

BACKGROUND: Sickle cell nephropathy is characterized by proteinuria that starts in childhood and may lead to renal failure. Microalbuminuria is used as a marker of glomerular damage. There are no data on the extent and type of proteinuria other than microalbuminuria in children with sickle cell disease (SCD). Our goal was characterization of glomerular permselectivity and tubular proteinuria in children with SCD. The improved characterization will allow earlier recognition and prevention of renal damage.
METHODS: Thirty-two stable patients with haemoglobin SS (HbSS) (15 boys and 17 girls, age 9.57 +/- 5.45 years, 8 months to 19 years) were investigated. All patients had normal renal function and tested negative for proteinuria with a dipstick method. Markers of glomerular permselectivity used were albumin (marker of charge selectivity and less severe pore-size selectivity) and immunoglobulin G (IgG, marker of more severe pore-size selectivity). The marker of tubular injury used was retinol-binding protein (RBP, marker of proximal tubular dysfunction). These proteins were measured in urine spot samples using nephelometry. We did not include a control group as values in healthy subjects were previously published.
RESULTS: Total protein excretion was elevated in 41% (13/32) of all patients and, of these 13 patients, 38.5% (5/13) had increased microalbuminuria, 15% (2/13) had increased excretion of RBP and 23% (3/13) had increased excretion of IgG. Increased total proteinuria that was not detected by testing for microalbuminuria was found in 61.5% (8/13) of patients. The youngest patient was 3 years old. Increased microalbuminuria was present in 25% (8/32) of all patients and was detected as early as 4 years of age. Of these, 62% (5/8) also had increased total protein excretion and 62% (5/8) also had increased IgG excretion. A total of 62.5% were older than 10 years. RBP excretion was elevated in 16% (5/32) of patients, all of whom were 7-14 years old. None of these patients had increased microalbuminuria or increased excretion of IgG. IgG excretion was elevated in 16% (5/32) of patients and was accompanied by increased microalbuminuria. All patients with increased IgG excretion were > or = 13 years old. We found a weak positive correlation between microalbuminuria and age (0.323, P = 0.07). We did not find a significant correlation between any type of proteinuria and disease morbidity. Ten of the thirty-two patients received hydroxyurea treatment and 60% (6/10) had no proteinuria. Twelve of the thirty-two patients received chronic exchange transfusions and 42% (5/12) had no proteinuria.
CONCLUSION: We found early glomerular selectivity damage in children with SCD, which is secondary to both size-selectivity and charge-selectivity impairment. Microalbuminuria alone does not adequately detect early renal damage in children with SCD. Proximal tubular dysfunction is seen in younger children and is independent of glomerular damage. We suggest that children with SCD be tested for both total protein and IgG excretion in the urine in addition to albumin. Knowing the extent and type of renal damage may allow earlier recognition of renal injury and prompt earlier initiation of preventive therapies.