title: Should kidney tubular markers be adjusted for urine creatinine? The example of urinary cystatin C.
abstract: BACKGROUND
  Evaluation of specific urinary markers with respect to urine creatinine (uCreat) is common. However, as uCreat is a function of both glomerular filtration and tubular secretion, using uCreat for specific tubular markers, suggests that glomerular function is normal, and there is no tubular secretion. Thus, adjusting values of any tubular marker to uCreat, especially in patients with acute or even moderate chronic renal failure, can be misleading.
  
  
  METHODS
  Using urine cystatin-C (uCST3) as a model tubular marker for following 120 kidney graft recipients daily, we evaluated the utility of either uCST3 alone or the uCST3/uCreat ratio to detect tubular damage. All positive kidney biopsies were always associated with a uCST3>0.18 mg/L.
  
  
  RESULTS
  Using the uCST3/uCreat ratio, discrepancies regarding biopsy status were observed in nine patients (4 false positive, 5 false negative results). In two patients, variability of uCreat appeared to be the most important factor causing inconsistent uCST3/uCreat ratios. With a negative predictive value (NPV) of 85.7%, uCST3/uCreat can lead to errors in clinical interpretation. These errors can be avoided when estimates of tubular damage are based on uCST3 concentrations alone (NPV=100%).
  
  
  CONCLUSIONS
  We recommend using the uCST3 value to evaluate the extent of renal tubular damage. Indeed, our conflicting results on uCST3/uCreat can be extended to every marker of tubular function. Evaluating a urine marker specific for renal tubular damage to a second urine marker that is itself strongly dependent upon glomerular or other renal or non-renal conditions, impairs its clinical relevance and may lead to incorrect interpretations. Correction with uCreat can be performed only in pure glomerulopathy, when specific markers of glomerular function are measured (i.e., urinary albumin). In all other cases of renal diseases, such correction is inappropriate and should be avoided. Clin Chem Lab Med 2009;47:1553-6.
authors.affiliation: Clinical Biochemistry Laboratory, APHP Bicêtre University Hospital, Le Kremlin Bicêtre, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Conti
authors.forename: Stéphane
authors.initials: S
authors.lastname: Moutereau
authors.forename: Liliane
authors.initials: L
authors.lastname: Esmilaire
authors.forename: Cédric
authors.initials: C
authors.lastname: Desbene
authors.forename: Karim
authors.initials: K
authors.lastname: Lallali
authors.forename: Michel
authors.initials: M
authors.lastname: Devanlay
authors.forename: Antoine
authors.initials: A
authors.lastname: Durrbach
authors.forename: Philippe
authors.initials: P
authors.lastname: Manivet
authors.forename: Pascal
authors.initials: P
authors.lastname: Eschwège
authors.forename: Sylvain
authors.initials: S
authors.lastname: Loric
mesh_terms: D015415:Biomarkers / Q000652:urine*; D003404:Creatinine / Q000652:urine*; D055316:Cystatin C / Q000652:urine*; D006801:Humans; D007684:Kidney Tubules / Q000378:metabolism*; D057230:Limit of Detection
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D003404:Creatinine