What is the impact of calibration and inter-laboratory variation of serum creatinine assays on the estimation of GFR?
The most commonly used assay for serum creatinine, the alkaline picrate (“Jaffe”) assay, detects a color change when creatinine interacts with picrate under alkaline conditions and is subject to interference from substances other than creatinine (“non-creatinine chromogens”), such as proteins and ketoacids. Newer enzymatic methods improve upon some of the non-specificities of the alkaline picrate assay but some are subject to other interferences. Calibration of creatinine assays to adjust for this interference is not standardized across methods and laboratories. Since the concentration of non-creatinine chromogens does not increase as GFR declines, the difference in serum creatinine assays between laboratories is greater at low serum creatinine values; therefore, differences in GFR estimates related to differences in calibration are most apparent at higher levels of GFR.9 Figure 1 shows the GFR estimates with and without such calibration differences for a hypothetical 60-year-old black
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