In the vast majority of cases, glomerular filtration rate (GFR) is estimated using serum
creatinine, which is highly influenced by age, sex, muscle mass, body composition, severe
chronic illness and many other factors. This often leads to misclassification of patients or potentially puts patients at risk for inappropriate clinical decisions. Possible solutions are the use of
cystatin C as an alternative endogenous marker or performing direct measurement of GFR using an exogenous marker such as
iohexol. The purpose of this review is to highlight clinical scenarios and conditions such as extreme body composition, Black race, disagreement between
creatinine- and
cystatin C-based estimated GFR (eGFR), drug dosing,
liver cirrhosis, advanced
chronic kidney disease and the transition to
kidney replacement therapy, non-kidney solid organ transplant recipients and living kidney donors where
creatinine-based GFR estimation may be invalid. In contrast to the majority of literature on measured GFR (mGFR), this review does not include aspects of mGFR for research or public health settings but aims to reach practicing clinicians and raise their understanding of the substantial limitations of
creatinine. While including
cystatin C as a renal
biomarker in GFR estimating equations has been shown to increase the accuracy of the GFR estimate, there are also limitations to eGFR based on
cystatin C alone or the combination of
creatinine and
cystatin C in the clinical scenarios described above that can be overcome by measuring GFR with an exogenous marker. We acknowledge that mGFR is not readily available in many centres but hope that this review will highlight and promote the expansion of kidney function diagnostics using standardized mGFR procedures as an important milestone towards more accurate and
personalized medicine.