SO091URINE-TO-PLASMA UREA RATIO, AS SURROGATE MARKER FOR URINE CONCENTRATING CAPACITY, IS ASSOCIATED WITH DISEASE PROGRESSION IN ADPKD
Authors: Heida Judith E , Gansevoort Ronald , Messchendorp Lianne , Meijer Esther , Casteleijn Niek F , Boertien Wendy E , Zittema Debbie 
Published: June 2020

Abstract
Introduction Predicting disease progression in autosomal dominant polycystic kidney disease (ADPKD) patients poses a challenge, especially in early stage disease when renal function is not yet affected. The ongoing formation and growth of cysts causes the urine concentrating capacity to decrease from early on in the disease. We therefore hypothesized that the easy and inexpensive to measure urine-to-plasma urea ratio (UPU ratio), which is assumed to be surrogate for maximal urine concentrating capacity, can be used as marker to predict disease progression in ADPKD.
Methods The UPU ratio was calculated by dividing urea concentration in a fasting morning spot urine sample by plasma urea concentration adjusted for plasma creatinine concentration. First, we validated the UPU ratio in 30 ADPKD patients who underwent a prolonged water deprivation test to measure the maximal urine concentrating capacity. Thereafter, the association of the UPU ratio with renal outcome was evaluated in 583 ADPKD patients participating in the DIPAK observational cohort (inclusion criteria: age&gt;18 years, eGFR &gt;15 mL/min/1.73m2, no concomitant diseases affecting eGFR, without V2 receptor antagonist prescription). Kidney function was assessed as eGFR by the creatinine based CKD-EPI formula, height adjusted total kidney volume (htTKV) by MRI and copeptin (surrogate for vasopressin) by ELISA.
Results In the water deprivation test participants (n=30), the UPU ratio was strongly correlated with maximal urine concentrating capacity (R = 0.67, p&lt;0.001). This association remained significant after correcting for sex, age, htTKV and eGFR (st. β = 0.53, p = 0.007). In these subjects maximal urine concentrating capacity as well as UPU ratio were associated with the rate of eGFR decline during a median follow-up of 6.3 yr (12 eGFR assessments per patient) assessed using linear mixed modeling, also when corrected for sex, baseline age and eGFR (β = 0.009, p = 0.04, and β = 5.56, p&lt;0.001, resp.). We subsequently corroborated in the larger DIPAK observational cohort (n=583, 58% female, mean age 47 yr median eGFR 60 mL/min/1.73m2 and htTKV 898 ml/m), that the UPU ratio was significantly associated with rate of eGFR decline during a median follow-up of 4.0 yr (6 eGFR assessments per patient): β = 0.23, p = 0.005. This association remained significant when corrected for sex, baseline age and eGFR (β = 0.32, p&lt;0.001) and even when additionally corrected for Mayo class, PDK mutation and copeptin (β = 0.40, p &lt;0.001). Stepwise backward multivariate regression analysis resulted in a final model including the UPU ratio, PKD mutation, Mayo Class and copeptin. Cox survival analysis showed that a lower baseline UPU ratio (indicating less urine concentrating capacity) was significantly associated with a higher risk to develop the combined renal endpoint of incidence of start of kidney replacement therapy, eGFR &lt;15 mL/min/1.73m2 or eGFR decrease &gt;40% during follow-up (adjusted Hazard Ratio per SD = 1.39, p = 0.007). Limiting the aforementioned analyses to the subgroup of patients with relative early stage disease (n=122, age &lt;40 yr and eGFR &gt;60 mL/min/1.73m2) rendered essentially similar results, with an adjusted β for UPU ratio in the final model of 0.33 (p = 0.04). In this subgroup with a limited number of events (n=10), Cox survival analysis did not reach formal significance (adjusted HR per SD: 2.67, p = 0.055).
Conclusion The UPU ratio, which is calculated from routine laboratory measurements, predicts renal prognosis in ADPKD in addition to other, more laborious to measure and expensive risk markers. Notably, this marker of urine concentrating capacity also shows promise in early-stage disease.