Uromodulin, released from tubular cells of the ascending limb into the blood, may be associated with kidney function. This work studies the relevance of plasma uromodulin as a biomarker for kidney function in an observational cohort of chronic kidney disease (CKD) patients and subjects without CKD (CKD stage 0). It should be further evaluated if uromodulin allows the identification of early CKD stages.Plasma uromodulin, serum creatinine, cystatin C, blood-urea-nitrogen (BUN) concentrations, and estimated glomerular filtration rate (eGFR CKD-EPIcrea-cystatin) were assessed in 426 individuals of whom 71 were CKD stage 0 and 355 had CKD. Besides descriptive statistics, univariate correlations between uromodulin and biomarkers/eGFR were calculated using Pearson-correlation coefficient. Multiple linear regression modeling was applied to establish the association between uromodulin and eGFR adjusted for demographic parameters and pharmacologic treatment. Receiver-operating-characteristic (ROC) analysis adjusted for demographic parameters was performed to test if uromodulin allows differentiation of subjects with CKD stage 0 and CKD stage I.Mean uromodulin plasma levels were 85.7 ± 60.5 ng/mL for all CKD stages combined. Uromodulin was correlated with all biomarkers/eGFR in univariate analysis (eGFR: r = 0.80, creatinine: r = −0.76, BUN: r = −0.72, and cystatin C: r = −0.79). Multiple linear regression modeling showed significant association between uromodulin and eGFR (coefficient estimate β = 0.696, 95% confidence interval [CI] 0.603–0.719, P < 0.001). In ROC analysis uromodulin was the only parameter that significantly improved a model containing demographic parameters to differentiate between CKD 0° and I° (area under the curve [AUC] 0.831, 95% CI 0.746–0.915, P = 0.008) compared to creatinine, cystatin C, BUN, and eGFR (AUC for creatinine: 0.722, P = 0.056, cystatin C: 0.668, P = 0.418, BUN: 0.653, P = 0.811, and eGFR: 0.634, P = 0.823).Plasma uromodulin serves as a robust biomarker for kidney function and uniquely allows the identification of early stages of CKD. As a marker of tubular secretion it might represent remaining nephron mass and therefore intrinsic “kidney function” rather than just glomerular filtration, the latter only being of limited value to represent kidney function as a whole. It therefore gives substantial information on the renal situation in addition to glomerular filtration and potentially solves the problem of creatinine-blind range of CKD, in which kidney impairment often remains undetected.

BackgroundAn ELISA to analyse uromodulin in human serum (sUmod) was developed, validated and tested for clinical applications.MethodsWe assessed sUmod, a very stable antigen, in controls, patients with chronic kidney disease (CKD) stages 1–5, persons with autoimmune kidney diseases and recipients of a renal allograft by ELISA.ResultsMedian sUmod in 190 blood donors was 207 ng/mL (women: men, median 230 versus 188 ng/mL, P = 0.006). sUmod levels in 443 children were 193 ng/mL (median). sUmod was correlated with cystatin C (rs = −0.862), creatinine (rs = −0.802), blood urea nitrogen (BUN) (rs = −0.645) and estimated glomerular filtration rate (eGFR)–cystatin C (rs  =  0.862). sUmod was lower in systemic lupus erythematosus-nephritis (median 101 ng/mL), phospholipase-A2 receptor- positive glomerulonephritis (median 83 ng/mL) and anti-glomerular basement membrane positive pulmorenal syndromes (median 37 ng/mL). Declining sUmod concentrations paralleled the loss of kidney function in 165 patients with CKD stages 1–5 with prominent changes in sUmod within the ‘creatinine blind range’ (71–106 µmol/L). Receiver-operating characteristic analysis between non-CKD and CKD-1 was superior for sUmod (AUC 0.90) compared with eGFR (AUC 0.39), cystatin C (AUC 0.39) and creatinine (AUC 0.27). sUmod rapidly recovered from 0 to 62 ng/mL (median) after renal transplantation in cases with immediate graft function and remained low in delayed graft function (21 ng/mL, median; day 5–9: relative risk 1.5–2.9, odds ratio 1.5–6.4). Immunogold labelling disclosed that Umod is transferred within cytoplasmic vesicles to both the apical and basolateral plasma membrane. Umod revealed a disturbed intracellular location in kidney injury.ConclusionsWe conclude that sUmod is a novel sensitive kidney-specific biomarker linked to the structural integrity of the distal nephron and to renal function.

Serum uromodulin predicted GL equivalently compared to conventional biomarkers of glomerular filtration.

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