title: Renal extraction of cystatin C vs 125I-iothalamate in hypertensive patients.
abstract: BACKGROUND
  Several markers are available to estimate the glomerular filtration rate (GFR) in patients. Cystatin C is a relatively new marker and has been suggested as an alternative for creatinine. Numerous studies have been performed to evaluate the usefulness of cystatin C to estimate GFR. The aim of this study is to compare the renal extraction of cystatin C with that of 125I-iothalamate in hypertensive patients.
  
  
  METHODS
  Forty hypertensive patients with unilateral renal artery stenosis, and who used at least two antihypertensive agents, were studied. For the determination of the renal extraction ratio, blood samples were drawn simultaneously from the renal vein and the abdominal aorta. The renal extraction ratio was calculated as ([A]-[V])/[A], in which A is the plasma concentration of the compound from the abdominal aorta, and V is the plasma concentration of the compound from the renal vein.
  
  
  RESULTS
  The mean difference between the renal extraction ratio of cystatin C and that of 125I-iothalamate was 0.002. The 95% confidence interval (CI) for the mean difference was -0.036 to 0.032, which was not statistically significant. However, the limits of agreement were large (-0.271 and 0.267).
  
  
  CONCLUSIONS
  Despite a lower reported glomerular sieving coefficient of cystatin C, the mean renal extraction of cystatin C was equal to the mean renal extraction of 125I-iothalamate in hypertensive patients, suggesting tubular secretion of cystatin C. Combined with the large variation in the renal extraction of cystatin C, these findings cast doubts on its usefulness as a glomerular filtration marker.
authors.affiliation: Department of Pharmacy, Erasmus MC, Dr Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. l.vanrossum@erasmusmc.nl
authors.forename: Lyonne K
authors.initials: LK
authors.lastname: van Rossum
authors.forename: Robert
authors.initials: R
authors.lastname: Zietse
authors.forename: Arnold G
authors.initials: AG
authors.lastname: Vulto
authors.forename: Yolanda B
authors.initials: YB
authors.lastname: de Rijke
mesh_terms: D000328:Adult; D001795:Blood Pressure Determination; D015331:Cohort Studies; D016001:Confidence Intervals; D055316:Cystatin C; D015891:Cystatins / Q000493:pharmacokinetics* / Q000494:pharmacology; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D006973:Hypertension / Q000150:complications / Q000175:diagnosis*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics; D007483:Iothalamic Acid / Q000493:pharmacokinetics* / Q000494:pharmacology; D007677:Kidney Function Tests; D008297:Male; D008875:Middle Aged; D011336:Probability; D012078:Renal Artery Obstruction / Q000150:complications / Q000175:diagnosis*; D012680:Sensitivity and Specificity
chemical_list: C525030:CST3 protein, human; D055316:Cystatin C; D015891:Cystatins; D007457:Iodine Radioisotopes; D007483:Iothalamic Acid