Glomerular filtration rate, or GFR, is generally accepted as the best overall index of kidney function, and a decrease in GFR has important implications for prognosis in patient management.  GFR is most commonly estimated by calculation, using the serum concentration of an endogenous filtration marker such as creatinine and demographic variables such as age, sex, and race.  In the March 2019 issue of Clinical Chemistry, a paper investigated the possibility of developing a more accurate estimate of GFR, using a panel of metabolites measured by quantitative liquid chromatography, tandem mass spectrometry without creatinine, cystatin C, or demographic variables.