THE 'WITHIN-DAY' VARIABILITY OF ALBUMINURIA INFLUENCES THE MONITORING OF NEPHROPROTECTIVE TREATMENT EFFECT

Abstract Background and Aims Measurement of urine proteins, mainly through the 24-hour excretion or the albumin-to-creatinine ratio (ACR) has become crucial in current clinical practice in Nephrology. Besides being a biomarker of injury, albuminuria also exerts direct pro-inflammatory and pro-fibrotic effects on renal tubules. Hence, a large number of intervention studies have been aimed at lowering albuminuria levels in patients with Chronic Kidney Disease (CKD). However, albuminuria is a measure characterized by a random variability that has been evaluated in several mechanistic and experimental studies and may be influenced by several factors, such as posture, exercise and dietary factors. The aim of the present study was to evaluate the within-day variability, specifically in a cohort of CKD outpatients who were on Renin-Angiotensin-Aldosterone-System inhibitors (RAASi) therapy. Method We enrolled consecutive CKD patients referred to the Nephrology Unit at Magna Graecia University Hospital of Catanzaro between January 1st and March 30th 2020. Inclusion criteria were: age > 18 years, diagnosis of CKD from any cause, presence of albuminuria in the range 150-3500 mg/g at screening. Patients already treated with RAASi, patients with active malignancy/signs of glomerulonephritis requiring immunosuppressive therapies, were excluded. At screening visit, patients with increased albuminuria were started with an Angiotensin Converting Enzyme inhibitor (ACEi) or an Angiotensin II Receptor Blocker (ARB). A second ambulatory visit was scheduled 1 month after start of RAASi therapy. During this visit, patients were asked to stay in the Renal Unit for 12 hours. They collected urine void for ACR, protein-to-creatinine ratio (PCR) and urine creatinine (Ucreat) assessment at 8am,1pm and 6pm. For each patient, comparisons between ACR, PCR and Ucreat were assessed by Kruskal-Wallis test and Friedman post-hoc, using the Benjamini-Hochberg as adjustment method. Coefficients of variation (CV,standard deviation/mean) were also computed. Results Final analysis included 43 patients (46.5% of males). Mean age was 59.6±16.7 years and median eGFR 41 [21-74] mL/min/1.73m2. A high cardiovascular (CV) risk profile was testified by the prevalence of type 2 diabetes (30.2%) and previous CV disease (34.8%). When patient characteristics were compared across ACR categories (<30, 30-300, >300 mg/g), systolic blood pressure was increased (p=0.020) and mean age decreased from 68.0 to 54.8 years on average (p=0.038). ACR values collected were 189 [38-759], 252 [51-1685] and 229 [56-1185] mg/g at 8am, 1pm and 6pm, respectively, with a CV of 24.6% (95%CI 12.4-39.1). 8am ACR was significantly different from 1pm ACR (p<0.001) and from 6pm ACR (p<0.001). 1 pm ACR was significantly different from 6pm ACR (p=0.002). Median PCR were 335 [115-932], 429 [146-1811] and 447 [151-1465] mg/g respectively at 8am, 1pm and 6pm, with CV being 17.8% (95%CI 9.0-26.6). 8am PCR differed significantly from 1pm and 6pm PCR (p<0.001 for both), while 1pm ACR was lower than 6pm ACR (p<0.001). Ucreat was 66 [53-103], 63 [47-96] and 69 [50-90] mg/dL respectively at 8am, 1pm and 6pm with no significant variation trend. Individual within-day ACR and PCR trends are depicted in the Figure . Conclusion The present analysis showed a significant individual within-day variability of albuminuria, measured as both ACR and PCR, suggesting that another parameter, in addition to those already known, may be responsible for the day-by-day albuminuria variation. Moreover, the absence of variability in Ucreat, namely the denominator of the ACR/PCR formula, suggests that the true variation is likely dependent on the urine protein values. Thus, albuminuria reduction in response to RAASi treatment should be carefully evaluated by measuring ACR/PCR preferentially at a similar time of the day for each patient. This would allow to avoid under- or overestimation of the actual treatment effect.