SummaryMannitol is often added to the cardiopulmonary bypass pump prime to reduce the incidence of renal dysfunction, but studies so far have been inconclusive. Urinary excretion of microalbumin and retinol binding protein are more sensitive than routine biochemical tests of renal function after cardiac surgery. We performed a double-blind, randomised, controlled trial in cardiac surgical patients with pre-operative plasma creatinine < 130 lmol.l )1 . Twenty patients received 0.5 g.kgof mannitol in the pump prime, whereas 20 control patients received an equivalent volume of Hartmann's solution. Blood and urine samples were taken on the day before surgery and daily for 5 days postoperatively for measurement of plasma urea and creatinine, urinary creatinine, retinol binding protein and microalbumin. We found no differences between the mannitol and control patients for any measured variable, and conclude that mannitol has little impact on renal function in patients with normal pre-operative plasma creatinine concentrations. Acute renal failure (ARF) affects around 5% of patients following cardiac surgery, with a mortality of 10-20% compared to 1% mortality in similar patients without ARF [1]. Even mild postoperative renal dysfunction is associated with increased morbidity and mortality [2,3]. ARF requiring dialysis occurs in 1-2% of patients, with a significantly higher mortality of 50-60%. ARF increases total hospital stay, duration of time spent in critical care, and triples the likelihood of discharge to extended care facilities [4][5][6][7], increasing the overall cost of hospital care. Mannitol was the earliest pharmacological prophylactic agent used to prevent ARF during cardiac surgery because it is an osmotic diuretic [8]. Mannitol is used in the pump prime in 37% of adult cardiac surgical units in the UK [9], but its use to preserve renal function in this setting remains controversial because the evidence of benefit is contradictory [8,[10][11][12][13][14][15]. This is due in part to the design of some previous studies, but also to the previous lack of sensitive tests for renal dysfunction [15].Retinol binding protein (RBP) is freely filtered at the glomerulus and is normally completely reabsorbed by proximal tubular cells [16], so only trace amounts are excreted. Urinary excretion of RBP is a sensitive and accurate indicator of early proximal tubular damage [17][18][19][20], with the advantage over other indicators of stability at all values of urinary pH [21]. Urinary microalbumin excretion indicates glomerular leak. The aim of this study was to assess the effect of mannitol on urinary concentrations of microalbumin and RBP as markers of renal dysfunction following CPB in patients with normal pre-operative plasma creatinine concentrations. MethodFollowing approval by the Local Research Ethics Committee, we studied 40 patients scheduled for first-time elective cardiac surgery. None of the patients had received radiographic contrast media in the preceding month. Patients whose weight was below 60 kg were

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