Key Points

Indexed eGFRcr-cys outperformed eGFRcr or eGFRcys across the body mass index (BMI) spectrum, improving GFR classification and treatment decisions.Nonindexed eGFRcr-cys led to further but smaller improvement across the BMI range that was not consistently related to BMI category.These findings inform clinical decisions on how to estimate GFR in the large segment of society living with low or high extremes of BMI.

Background

The presence of a low or high body mass index (BMI) in patients may influence the accuracy of eGFR. This study evaluates the performance of eGFR equations across the range of BMI.

Methods

This is an observational study of 4707 adults (7503 repeated observations) referred for measured GFR (mGFR) in Stockholm, Sweden. We calculated indexed eGFR (in ml/min per 1.73 m2) and nonindexed eGFR (in ml/min) with validated equations that use creatinine (eGFRcr), cystatin C (eGFRcys), or both (eGFRcr-cys). We assessed equation performance against indexed and nonindexed mGFR across categories of BMI with median bias, P30 (the percentage of estimated values within 30% of mGFR), and classification of GFR categories, and modeled the implications of choice of filtration marker and indexing on clinical decisions regarding dose adjustment or eligibility for treatment.

Results

The mean age (SD) was 57 (16) years (39% female), and the median (interquartile range) indexed and nonindexed mGFR were 59 (39–79) and 65 (42–87) ml/min, respectively. In total, 9% of participants were underweight (BMI

Conclusions

In a clinic population of northern European individuals referred for GFR measurement, indexed eGFRcr-cys was more accurate than indexed eGFRcr across the BMI spectrum. Using nonindexed eGFRcr-cys further improved accuracy for some treatment decisions.