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NDT Advance Access originally published online on November 16, 2006
Nephrology Dialysis Transplantation 2007 22(4):1267-1268; doi:10.1093/ndt/gfl658
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© The Author [2006]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Renal handling of cystatin C

Email: dr.poege{at}nephrologie-bonn.de

Sir,

Recently, van Rossum et al. [1] published an interesting report on renal extraction of cystatin C (cys C). However, we have some concerns regarding the results and interpretation of these.

First, the authors showed a high correlation between 1/cys C and glomerular filtration rate (GFR) measured as iothalamate plasma clearance. In contrast to that, cys C excretion ratio (Ecyst C) did not correlate with iothalamate excretion ratio (Ethal, r = 0.05; P = 0.6, calculated from the data given in (Table 1) of the article). Since correlation of 1/cys C with GFR suggests renal elimination of Cys C and concomitantly renal iothalamate excretion ratio determines iothalamate GFR, it is not understandable why Ethal shows no correlation with Ecyst C.

Secondly, in addition to the missing correlation of Ecyst C and Ethal, some patients (e.g. patients 28 and 40) showed extreme divergences of Ecyst C between left and right, whereas iothalamate excretion ratios were similar (e.g. patient 40: Ecyst C: 0.44 vs 0.03, Ethal 0.13 vs 0.18; patient 28: Ecyst C: 0.33 vs 0.07, Ethal 0.16 vs 0.20). This phenomenon is not discussed appropriately and raise concerns with respect to the validity of the applied techniques.

Thirdly, based on the presumption that sieving coefficient of cys C is lower than that of iothalamate, the authors concluded that cys C may undergo tubular secretion, since mean Ethal and Ecyst C were similar. However, the similarity disappears when median or absolute mean difference are considered (Ethal – Ecyst C: median = 5 and absolute bias = 11.3). Moreover, median Ethal was slightly higher than Ecyst C (20 vs 16) which seems to be in line with the presumed lower sieving coefficient of Cys C. Thus, in our view, the presented data do not support a tubular secretion of Cys C.

Conflict of interest statement. None declared.

Ume Pöge, Thomas Gerhardt and Rainer P. Woitas

Department of Internal Medicine I
University of Bonn
Sigmund-Freud-StraÒe 25
D 53105 Bonn, Germany

References

  1. van Rossum LK, Zietse R, Vulto AG, de Rijke YB. (2006) Renal extraction of cystatin C vs 125I-iothalamate in hypertensive patients. Nephrol Dial Transplant 21:1253–1256.[Abstract/Free Full Text]

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This Article
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