• Frank Heller, Sandra Frischmann, Maria Grünbaum, Walter Zidek, and Timm H Westhoff.
• Department of Nephrology, Charite´-Campus Benjamin Franklin, Berlin, Germany.
• Clin J Am Soc Nephrol. 2011 Oct 1;6(10):2347-55.

Background And ObjectivesTo date there is no reliable marker for the differentiation of prerenal and intrinsic acute kidney injury (AKI). We investigated whether urinary calprotectin, a mediator protein of the innate immune system, may serve as a diagnostic marker in AKI.Design, Setting, Participants, & MeasurementsThis was a cross-sectional study with 101 subjects including 86 patients with AKI (34 prerenal, 52 intrinsic including 23 patients with urinary tract infection) and 15 healthy controls. Assessment of urinary calprotectin concentration was by ELISA and immunohistochemistry of kidney biopsy specimens using a calprotectin antibody. Inclusion criteria were: admission to hospital for AKI stage 1 to 3 (Acute Kidney Injury Network); exclusion criteria were: prior renal transplantation and obstructive uropathy.ResultsMedian urinary calprotectin was 60.7 times higher in intrinsic AKI (1692 ng/ml) than in prerenal AKI (28 ng/ml, p <0.01). Urinary calprotectin in prerenal disease was not significantly different from healthy controls (45 ng/ml, p = 0.25). Receiver operating curve curve analysis revealed a high accuracy of calprotectin (area under the curve, 0.97) in predicting intrinsic AKI. A cutoff level of 300 ng/ml provided a sensitivity of 92.3% and a specificity of 97.1%. Calculating urinary calprotectin/creatinine ratios did not lead to a further increase of accuracy. Immunostainings of kidney biopsies were positive for calprotectin in intrinsic AKI and negative in prerenal AKI.ConclusionsAccuracy of urinary calprotectin in the differential diagnosis of AKI is high. Whereas calprotectin levels in prerenal disease are comparable with healthy controls, intrinsic AKI leads to highly increased calprotectin concentrations.

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