The definition of eGFR is generally done from a random serum measurement (if you measure it in the classical but rarely done fashion, then you don't have to cal

The definition of eGFR is generally done from a random serum measurement (if you measure it in the classical but rarely done fashion, then you don't have to call it an estimate). Also, creatinine is measured in serum to estimate gfr. Urine creatinine is more used as part of a test to look for nephrotic syndrome. Creatinine is produced constantly, filtered and not resorbed, without active secretion, which is why it is an excellent proxy for glomerular filtration rate.