Concordance between creatinine- and cystatin C-based eGFR in clinical practice

The mean of GFR-estimates based on serum creatinine (eGFR_crea) and cystatin C (eGFR_cys) has superior accuracy than each estimate alone. Recent studies have shown that agreement between eGFR_crea and eGFR_cys is an indicator for the accuracy of the mean of the two estimates. As long as the difference between the two (|ΔeGFR|) is below 40%, a high P₃₀ accuracy rate of more than 90% was documented in research settings using gold-standard GFR measurements. This was the case in approximately 80% of the measurements. The study was set out to explore |ΔeGFR| in a broader pediatric nephrological population and identify factors influencing the discrepancy between eGFR_crea and eGFR_cys. We retrospectively analyzed 1596 simultaneous cystatin C and creatinine measurements in 649 unique patients at the pediatric nephrology outpatient clinic of VU university medical center. The FASage equation was used to calculate eGFR_crea, FAScys for eGFR_cys. |ΔeGFR| was calculated as 100x(|eGFR_crea-eGFR_cys|)/(0.5x(eGFR_crea+eGFR_cys). ΔeGFR below 40% was considered high agreement. Patient characteristics like age, diagnosis, glucocorticosteroid use, eGFR, BMI and sex were analyzed for their effect on ΔeGFR below or above 40% using non-parametric tests and a potential explanation for measurements with low agreement was sought. Eighty-seven percent of the population had a |ΔeGFR| lower than 40%. Measurements with |ΔeGFR| above 40% were significantly more frequent from patients with neural tube defects. In 102 out of 208 measurements with low agreement, a potential explanation was found. In a broad pediatric nephrological population, |ΔeGFR| is below 40% in the vast majority of measurements. In this group, the mean of eGFR_crea and eGFR_cys can be used as an accurate estimate of GFR.