The glomerular filtration rate is substantially increased in pregnancy. Trimethoprin, cimetidine, quinine, quinidine, procainamide reduce creatinine excretion. Icteric samples, lipemia, and hemolysis may interfere with determination of creatinine. Since tubular secretion of creatinine is fractionally more important in progressing renal failure, the creatinine clearance overestimates GFR with high creatinine levels. While ingestion of meats may cause some increase in creatinine excretion, in practice this seems to make little difference.
Creatinine clearance is calculated on the basis of the surface area of the patient.
Have patient drink water before the clearance is begun, and continue good hydration throughout the clearance. If possible, drugs should be stopped beforehand.