Substantial Discrepancies found Between Estimated and Measured GFR
A cross sectional study published in Annals of Internal Medicine uncovered substantial discrepancies between individual estimated glomerular filtration rate (eGFR) and directly measured GFR (mGFR).The authors suggest that eGFR calculations on lab reports also state this distribution of uncertainty, and also that renaming the eGFR as a population average GFR (or paGFR) merits further discussion.
GFR is the standard metric used to assess and monitor kidney function. Directly measured GFR, or mGFR, requires injecting a filtration marker and measuring plasma or urinary clearance by serial blood and urine sampling under standardized conditions is not possible for every patient. So eGFR calculated from serum creatinine is often used by clinicians to predict an mGFR. Population-level discrepancies between eGFR and mGFR are low, but individual discrepancies are much higher. It is important to understand the magnitude of these individual-level differences for clinical decision making.
Researchers calculated eGFR from serum creatinine alone and cystatin C and creatinine using the Chronic Kidney Disease Epidemiology Collaboration equations for 3223 participants and compared their eGFR to their mGFR to quantify the magnitude and consequences of the individual-level differences between the two. The authors found substantial discrepancies between directly measured GFR and estimated GFR, resulting in only about 50% agreement between CKD stages. Individual-level differences between the mGFR and the eGFR did not improve substantially using cystatin C.
The authors suggested that several factors contribute to these discrepancies: creatinine and cystatin C have non-GFR factors influencing their serum concentration; variability in the mGFR can result from normal physiology and measurement error from mGFR markers and technique; and as GFR estimation models the ratio of mGFR–body surface area as a function of serum markers, it incorporates errors in mGFR and errors in body surface area calculated from height and weight.
The authors say that their findings highlight the need to make direct GFR measurements available to patients who need them. They note that implementation studies are needed in this area, and research is needed to assess how the availability and use of mGFRs change clinical management.
Source: EurekAlert!
