Substantial individual-level discrepancies exist between measured GFR, eGFR
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Researchers identified significant differences in eGFR and measured eGFR on an individual level, according to data published in Annals of Internal Medicine.
“Directly measured glomerular filtration rate (mGFR), the gold standard for assessing GFR, involves injecting a filtration marker and measuring plasma or urinary clearance by serial blood and urine sampling under standardized conditions. As direct GFR measurement is not practical for every patient, GFR is assessed indirectly by the serum concentrations of creatinine in routine clinical practice,” Tariq Shafi, MBBS, MHS, from the University of Mississippi Medical Center, and colleagues wrote. “For clinical decision-making, clinicians need to understand the magnitude of these individual-level differences between the mGFR and the eGFR and how they apply to each patient.”
In a cross-sectional study, researchers examined 3,223 individuals (mean age was 59; 55% were women; 58% were “non-Hispanic white”; 32% were “non-Hispanic Black”; 7% were Hispanic; 3% were another race) from four U.S. community-based epidemiologic cohort studies with mGFR.
Researchers used urinary iothalamate and plasma iohexol clearance to measure GFR, then determined eGFR from serum creatinine concentration alone (eGFRCR) and with cystatin C.
The primary analysis compared individual-level differences between mGFR and eGFRCR.
Overall, the mean mGFR among all studies was 68 mL/min/1.73 m². Although researchers observed little difference in eGFR on a population level (the median difference [mGFR – eGFR] was –0.6), they identified a large difference on an individual level.
“For example, at an eGFRCR of 60 mL/min/1.73 m², the median mGFR was 59 mL/min/1.73 m², a clinically insignificant population-level difference,” Shafi and colleagues wrote. “However, for a person with an eGFRCR of 60 mL/min/1.73 m², 95% of the directly measured GFRs are expected to range from as low as 36 mL/min/1.73 m² to as high as 87 mL/min/1.73 m², values ranging from stage 3B CKD to no CKD.”
Researchers noted that eGFR based on cystatin C did not provide significant improvement.
“In conclusion, we found that the individual-level differences between the mGFR and the eGFR are substantial. Laboratory reports that provide eGFR calculations should consider including the distribution of this uncertainty,” Shafi and colleagues wrote. “Clinicians need to recognize that the eGFR is not an mGFR replacement and consider eGFR's inaccuracy while managing individual patients. Renaming the eGFR as a population average GFR (or paGFR) merits further discussion.”
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