1. There are significant differences between measured and estimated glomerular filtration rate (mGFR and eGFR) at the individual patient level.
2. The differences between mGFR and eGFR were small and insignificant at the population level.
Evidence Rating Level: 1 (Excellent)
Study Rundown: GFR is the standard metric for monitoring kidney function in patients. Specifically, mGFR is the gold standard for quantifying GFR in a clinical setting. However, mGFR is not practical for every patient as it requires more time and resources to determine than eGFR. Thus, estimated GFR (eGFR) is often used instead. eGFR is the predicted average mGFR based on a formula applied to large derivation cohorts. Patient values of serum creatinine, age, and sex are used. Though the average population difference between mGFR and eGFR is insignificant, there is a knowledge gap in understanding the magnitude and consequences of individual patient differences between mGFR and eGFR. The present study aimed to measure individual variations in mGFR and eGFR, and compare this to population average differences. In summary, it found that individual-level differences between mGFR and eGFR were substantial. Notably, this study was limited by unquantifiable sources of variability in the measurement of mGFR and serum markers without short-term replicates. Nevertheless, the study’s findings are significant, as they demonstrate that substantial and significant individual-level differences between mGFR and eGFR exist, and possible clinical implications must be further investigated
In-Depth [cross-sectional study]: The present study was conducted based on data from four United States community-based epidemiologic cohort studies. The four cohort studies (GENOA, ECAC, ALTOLD, and CRIC) included patients with chronic kidney disease, essential hypertension, or living kidney donors. The primary outcome measures were mGFR using urinary iothalamate and plasma iohexol clearance and eGFR calculated from serum creatinine concentration alone (eGFRCR) and with cystatin C. Outcomes in the primary analysis were assessed via quantile regression models and multivariable logistic regression. Based on the primary analysis, the population level difference between mGFR and eGFR was negligible, with a median difference of -0.6 (95% Confidence Interval, -1.2 to -0.2). However, the individual-level differences between mGFR and eGFR were large and significant. With an eGFR of 60, 50% of the mGFRs ranged from 52 to 67, 80% from 45 to 76, and 95% from 36 to 87. With an eGFR of 49 to 59, 36% had an mGFR greater than 60 while 20% had an mGFR less than 45. In patients with an eGFR of 15 to 29, 30% of patients had an mGFR greater than 30 and 5% had an mGFR less than 15. Overall, this study demonstrates that significant individual-level differences exist between mGFR and eGFR, and suggests that labs reporting eGFR should also consider including the possibility of this uncertainty to quantify patient kidney function accurately.
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