A number ofthreads' of comment about creatinine clearance ran on the ACB mailbase from April 1999. (There is a mail archive on the ACB web site, http://www.acb.org.uk.) Topics included the validity of urine measurement in patients with olig- uria; the effect of meat in the diet on creatinine production; formulae for calculating creatinine clearance from plasma creatinine, age and weight without urine collection; `correction' of measured creatinine clearance for body surface area; and the role of serum cystatin C in the assessment of glomerular filtration rate (GFR). The first question might be rephrased to ask whether creatinine clearance is ever a valid mea- sure of GFR. Inulin clearance is the gold standard. Clearance values for creatinine are higher than those for inulin and diverge widely. For example, under experimental rather than clinical conditions, at an inulin clearance of 50 mL/min, creatinine clearance ranged from about 50 to 140 mL/min. 1