 Serum creatinine will be determined by the calibrated Jaffe method Roche, traceable to isotopic dilution mass spectrometry ID-MS, with kinetic reading to minimize interference by bilirubin. The dosage of cystatin C will be performed by the immunoturbidimetry method Roche. The coefficients of intra and interassay variation are 2.2 and 5.3, respectively. 