C MANNIX1,2, AT WONG1,2, J ZHANG1,2, V LEE1,2, D HARRIS1,2, K SUD2,3, G RANGAN1,2
1Centre for Transplant and Renal Research, Westmead Institute for Medical Research, University of Sydney, Westmead, Australia, 2Department of Renal Medicine, Westmead Hospital, Western Sydney Local Health District, Sydney, Australia, 3Department of Renal Medicine, Nepean Hospital, Nepean Blue Mountains Local Health District, Sydney, Australia
Aim: To determine the relationship between random daytime spot urine osmolality (UOsm) and 24-hour (hr) UOsm in patients with autosomal dominant polycystic kidney disease (ADPKD).
Background: UOsm is a surrogate measure of hydration and dietary solute intake, regulated by levels of circulating arginine vasopressin (AVP). Monitoring UOsm may be useful to gauge adherence to fluid and diet prescriptions in ADPKD. Traditionally, UOsm is determined by 24-hr collection but patients find this method cumbersome, and hence we investigated if spot collection could be an alternative.
Methods: Spot urine was collected prospectively from ADPKD patients (18-65 years, eGFR ≥30 mL/min/1.73m²) during the Screening Visit of the PREVENT-ADPKD study (RCT investigating the efficacy of prescribed fluid intake on kidney cyst growth). Urine specific gravity (USG) and serum copeptin (a biomarker of AVP) were also analysed, followed by two 24-hr urine collections over the subsequent 12 weeks.
Results: Seventy-nine participants were included (age: 43±11 years; 54% male). The mean spot UOsm was 491±196 mosmol/L and correlated moderately with the mean 24-hr UOsm (424±173 mosmol/L; r=0.56; P<0.001). In contrast, the association between spot UOsm to urinary creatinine ratio and 24-hr UOsm was weak (r=0.16, P=0.03). As expected, spot UOsm also predicted the USG (P<0.001), which was correlated with 24-hr UOsm (r=049, P<0.001). Spot UOsm had a weak association with serum copeptin (median 4.2, range 0.9-28.1 ng/ml; r=0.26; P<0.001).
Conclusion: These data suggest that spot urine collected randomly during the day (either at a clinic visit for UOsm or by patient self-monitoring for USG) is an alternative method to estimate 24-hr UOsm in ADPKD. Further prospective studies and the outcome of the PREVENT-ADPKD trial are required to validate these findings.
Carly Mannix is a Senior Research Dietitian and PhD candidate at the Westmead Institute for Medical Research (Westmead Hospital and University of Sydney). She is presently undertaking the PREVENT-ADPKD study which is a national multicentre randomised controlled trial that is investigating the role of hydration on slowing the progression of autosomal dominant polycystic kidney disease, and this is the subject of her PhD thesis. She is an Accredited Practising Dietitian with the Dietitians Association of Australia. She holds a Master of Nutrition and Dietetics (2016) and Bachelor of Applied Science (Exercise and Sport Science) (2014) from the University of Sydney.