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Background: Urinary levels of epidermal growth factor to creatinine ratio are considered as an early predictor of interstitial kidney fibrosis but so far no data are available on their biological variation (BV) and derived parameters. The aim of this study is to determine the BV of epidermal growth factor to creatinine ratio in patients with chronic kidney disease and in healthy volunteers., Methods: This cross-sectional study included 150 patients with chronic kidney disease (CKD) and 150 healthy volunteers (HV). In both groups, spot second-morning urine samples were collected once a week for 4 consecutive weeks. Measurements of EGF were done by ELISA and expressed as EGF/creatinine ratio. The components of BV, individuality index (II), and reference change values (RCV) were calculated., Results: The analytical coefficient of variation (CVa) of EGF/creatinine ratio was 3.8% in patients with CKD and 3.9% in HV. The within-patient variation coefficient (CVw) was 11.2% in CKD and 12.1% in HV. The between-patient coefficient of variation (CVg) was 34% in CKD and 22% in HV. In both groups, CVa met the optimum analytical quality specifications for imprecision, since it was lower than 25% of CVw. There were no significant differences between CKD and HV in the analytical or in the within-patient variances of the EGF/creatinine ratio. The between-patient EGF/creatinine ratio variance was significantly higher in patients with CKD than in HV (F: 48.3, p: 0.000). The EGF/creatinine ratio showed an individuality index (II) of 0.3 in CKD and 0.5 in HV. The reference change value (RCV) was 29.2% in CKD and 31.6% in HV., Conclusions: The CVa associated with the measurement techniques used in our study meets the optimal criteria of analytical imprecision. The urine EGF/creatinine ratio shows a high index of individuality both in patients with CKD and in HV, so the comparison of an isolated value with a reference interval is of little use. In the monitoring of repeated levels in the same individual or patient, the changes can only be considered significant when they are greater than 30% in relation to the previous values.

Background: Urinary levels of EGF may be a noninvasive biomarker of the degree of interstitial fibrosis. However, all the available data are based on studies that examined the EGF/creatinine ratio in spot urine samples. The agreement between EGF/creatinine ratio and 24-hours EGF excretion has not been analyzed, neither has it been established which of these two measurements is a better predictor of the degree of interstitial fibrosis. To investigate whether the EGF/creatinine ratio can predict 24-hours EGF, and which of these two measures is a better predictor of interstitial fibrosis in patients with IgA nephropathy (IgAN)., Methods: This is a cross-sectional study including 80 patients with IgAN. EGF levels were measured by ELISA in spot second-morning and 24-hours urine samples. We analyzed the concordance between these two measures and their respective ability to predict interstitial kidney fibrosis., Results: The intraclass correlation coefficient between 24-hours and spot EGF/creatinine was 0.63 (95% CI: 0.54 - 0.70), bias was 2.7 µg/mL (95% CI: 2.1 - 7.5). Passing-Bablok regression did not show a significant deviation from linearity (p = 0.72). Bland-Altman showed a systematic and proportional error between both EGF measures. Spot EGF/creatinine ratios overestimated the 24-hours EGF at low excretion values and underestimated it at high excretion values. In univariate analyses, 24-hours excretion of EGF was a better predictor of interstitial fibrosis than spot EGF/creatinine ratio (R2: 0.43 vs. 0.30, p = 0.000). In multivariate analyses, the 24-hours excretion of EGF plus GFR, significantly improved the prediction of interstitial fibrosis when compared with GFR alone (R2: 0.52 vs. 0.39, p = 0.000). When spot-urine EGF was introduced instead of the 24-hours excretion, the model was statistically significant but had a lower predictive capacity (R2: 0.46 spot EGF/creatinine vs. R2: 0.52 24-hours EGF excretion, p = 0.000)., Conclusions: The 24-hours excretion of EGF should be considered as the first-choice measure to estimate the interstitial fibrosis. The EGF/creatinine ratio cannot accurately estimate the total EGF excretion of but it also improves the estimation of the fibrosis surface, and, consequently, could be an alternative whenever 24-hours urine samples cannot be obtained.