Measurement of GFR: pitfalls & quality control (a practical approach)

For certain clinical scenarios accurate and precise GFR measurement is required. In the majority of situations this is calculated from the plasma clearance of an exogenous marker that undergoes glomerular filtration, using the slope-intercept (SI) or single-sample (SS) methods. This paper is intended to complement a previous Seminars paper with a specific focus on practical ways to avoid potential pitfalls and quality control measures that can be implemented to minimize the risk of GFR measurement errors. The underlying causes of GFR errors can be understood in terms of measurement errors or procedural errors. All measurements are accompanied by inherent errors that are related to the finite precision of the measuring instruments used. Kept within an acceptable range, measurement errors make a small contribution to GFR error when compared to expected biological variation. Procedural errors consist of a diverse group of deviations from correct procedure ranging from selection of an inappropriate methodology through to missteps at many points in the investigation procedure. Procedural errors can significantly affect the accuracy of GFR measurements. Errors are primarily avoided by regular equipment quality control and careful adherence to clear standard operating procedures. Quality control parameters integrated into a calculation spreadsheet can play a useful secondary role to flag potential measurement or procedural errors.