B KALMA1, RJ ELLIS1,2, L VITETTA3,4, ST WOOD1,2, GC GOBE1
1 Kidney Disease Research Group, Diamantina Institute, University of Queensland, Brisbane, QLD, 2 Department of Urology, Princess Alexandra Hospital, Brisbane, QLD, 3 Medlab Clinical Ltd., Sydney, NSW, 4 Sydney Medical School – Medical Sciences, University of Sydney, Sydney, NSW.
Aim: To evaluate whether urinary indoxyl sulphate (IS) predicted post-nephrectomy renal function.
Background: IS is an end-product of tryptophan metabolism, derived from intestinally produced indole. It is albumin-bound in plasma and primarily excreted in the urine via tubular secretion. IS is regarded as a uremic toxin and is documented to have detrimental effects to kidney and endothelial cells. Low urinary IS in patients with normal kidney function (GFR >60ml/min/1.73m2) may indicate impaired IS secretion. The detrimental effects of IS can disrupt tubular cell mitochondrial redox metabolism. However, it may also indicate low IS production due to low protein intake. It is hypothesised that low urinary IS associates with elevated serum creatinine following radical tumour nephrectomy.
Methods: Urinary IS, albumin and creatinine were evaluated in 90 patients with normal preoperative kidney function, undergoing radical tumour nephrectomy at a single centre. The primary outcome was serum creatinine percentage increase (pre- to postoperative). The ratios of IS-creatinine and albumin-creatinine (ACR) were compared as predictors of elevated creatinine by substituting them into separate multivariable linear regression models.
Results: Lower IS-creatinine ratio (β: -7.87, 95%CI: -15.5, -2.01; p=0.04) and ACR (β: -3.91, 95%CI: -7.30, -0.54; p=0.02) were independent predictors of elevated serum creatinine in their respective models.
Conclusions: Low urinary IS is a predictor of post-nephrectomy kidney function in patients with normal preoperative kidney function. When compared to ACR, it appears that IS-creatinine ratio has similar predictive utility and a larger effect size. High ACR is, counterintuitively, a protective factor in this scenario. A limitation of urinary IS for this purpose is that it cannot be evaluated reliably in patients with poor kidney function, due to significantly reduced IS excretion.