Method: A cross - sectional prospective study was performed over 130 patinets with renal diseases in nephrology department - Cho Ray hopital from 12/2009 to 8/2010 have serum creatinine 0, 8 - 2, 5mg / dL, serum cystatin C is determined by test Latex turbidimetry, the 24h creatinine clearance correcting with body surface area (as standard to compare), and use Diazyme method to evaluate GFR.