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Acute renal dysfunction (ARD) and subsequent acute renal failure after cardiac surgery are associated with high mortality and morbidity. Early therapeutic or preventive intervention is hampered by the lack of an early biomarker for acute renal injury. Recent studies showed that urinary neutrophil gelatinase–associated lipocalin (NGAL or lipocalin 2) is up-regulated early (within 1–3 h) after murine renal injury and in pediatric ARD after cardiac surgery. The authors hypothesized that postoperative urinary NGAL concentrations are increased in adult patients developing ARD after cardiac surgery compared with patients without ARD.After institutional review board approval, 81 cardiac surgical patients were prospectively studied. Urine samples were collected immediately before incision and at various time intervals after surgery for NGAL analysis by quantitative immunoblotting. ARD was defined as peak postoperative serum creatinine increase by 50% or greater compared with preoperative serum creatinine.Sixteen of 81 patients (20%) developed postoperative ARD, and the mean urinary NGAL concentrations in patients who developed ARD were significantly higher early after surgery (after 1 h: 4,195 ± 6,520 [mean ± SD] vs. 1,068 ± 2,129 ng/ml; P < 0.01) compared with patients who did not develop ARD. Mean urinary NGAL concentrations continued to increase and remained significantly higher at 3 and 18 h after cardiac surgery in patients with ARD. In contrast, urinary NGAL in patients without ARD decreased rapidly after cardiac surgery.Patients developing postoperative ARD had significantly higher urinary NGAL concentrations early after cardiac surgery. Urinary NGAL may therefore be a useful early biomarker of ARD after cardiac surgery. These findings may facilitate the early detection of acute renal injury and potentially prevent progression to acute renal failure.