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Background: In a patient with biliary obstruction, a macromolecular complex of cystatin C with liver plasma membrane fragments, which also contain several membrane-bound enzymes, which may be removed by butanol extraction, has recently been characterised. This could lead to an underestimation of the glomerular filtration rate (GFR) from serum cystatin C concentration.
Methods: Using the particle enhanced nephelometric immunoassay (PENIA), serum cystatin C concentration was determined in 50 healthy controls, 43 patients with renal insufficiency, 68 kidney and 88 liver transplant recipients, and 60 patients with biliary obstruction. Residual cystatin C concentrations and γ-glutamyltransferase (GGT) activities after butanol extraction were also determined.
Results: In the controls and patients with renal insufficiency the residual concentrations of cystatin C after butanol extraction were always ≥ 80 %. However, in 2 renal and 7 liver transplant recipients and in 11 patients with biliary obstruction this biochemical variable was < 80 %. A significant correlation between the residual cystatin C concentrations and residual GGT activities was obtained (r = 0.386, p < 0.001). In the relationship between estimated GFR from serum cystatin C and creatinine concentrations in the liver transplant recipients and patients with biliary obstruction, the 18 cases with residual cystatin C concentrations < 80 % were included in the distribution of the total population data with no particular tendency.
Conclusions: These results suggest that in the cases with residual cystatin C concentrations < 80 % after butanol extraction, presumably due to the presence of circulating macromolecular cystatin C, the serum levels of cystatin C obtained using the PENIA assay do not lead to a systematic GFR underestimation.
DOI: Clin. Lab. 2011;57:207-212
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