Background: Roche Diagnostics has issued new c-fas calibrators for its automated systems. These produce creatinine values that are more comparable with those obtained by high-performance liquid chromatography. However, this results in an underestimation of measured creatinine at concentrations below 155 μmol/L and an overestimation at concentrations above this value.
Methods: Serum and urine creatinine concentrations were prospectively determined on samples from 60 patients using the new (compensated) and old (uncompensated) c-fas calibrators, and Passing-Bablok regression analysis was performed. The regression equations thus determined were then used retrospectively to determine the compensated creatinine results (i.e. those results that would have been obtained using the new calibrator) in those serum and urine samples analysed in the previous year using the old uncompensated c-fas calibrator. The compensated creatinine results were then used to estimate the glomerular filtration rate (GFR) by calculating creatinine clearance. This was done by using the formula: UV/Pt, in which U represents the urinary creatinine concentration (μmol/L), V the urinary collection volume (mL), P the serum creatinine concentration (μmol/L) and t the urinary collection time (min). It was also calculated using the abbreviated Modification of Diet in Renal Disease study group (MDRD) formula.
Results: The creatinine clearance as determined using either the UV/Pt calculation or the MDRD formula overestimated GFR by ~30% and ~50%, respectively, in normal individuals with a serum creatinine concentration below 155 μmol/L. However, in patients with mild to moderate renal failure (serum creatinine from 155 to 500 μmol/L), changes in creatinine clearances determined by the two procedures were minimal.
Conclusion: When laboratories introduce this new, compensated calibrator into practice, it may be appropriate to discuss its potential impact with clinical staff who monitor patients using creatinine clearance.
