Comparison of combined urea and creatinine clearance and prediction equations as measures of residual renal function when GFR is low

Background: UK, US and European guidelines recommend the decision to initiate dialysis should be based on a combination of measurements of kidney function, nutritional status and clinical symptoms. Such recommendations assume an accurate and reproducible measure of glomerular filtration rate (GFR). Methods: Prospective study of 97 patients with chronic kidney disease (CKD) and serum creatinine >200 µmol/l (2.26 mg/dl) who between them contributed 388 24 h urine collections. Our main outcome measure was the number of patients with low residual renal function identified by different tests, using widely accepted thresholds. We calculated sensitivity, specificity, positive and negative predictive values and receiver operating characteristic curves for each comparison using a combined urea and creatinine clearance of