*The patients are to visit an external pathology collection centre for blood and urine sample collection.

†Group B patients only for safety check and titration of water prescription.

‡Patients in group B will receive scheduled telephone calls from the study nurse (with the dietitian, as needed) to review changes in health, fluid intake, USG, laboratory results, AE and provided coaching to alter water intake (if needed). These calls will be made at week 1, 3, 6 and then monthly in year 1 and 3-monthly in years 2–3 (and as required). Patients in group B will only be contacted if the pathology results meet the criteria for an AE.

§Patients randomised to group B will receive specific advice on the amount of fluid required per day to reduce the urine osmolality to ≤270 mOsmol/kg. Patients in group B will be asked to self-monitor their USG regularly at home using urine dipstick (provided to the patients), and adherence to the intervention will be quantified by patient responses to SMS text, sent twice weekly during months 0–6 and then monthly for the duration of the study, asking for the results of that day’s USG.

¶All patients will conduct two 24 hours urine collections and record their fluid intake for those 2 days prior to baseline visit. The study dietitian will provide group B patients with diet and fluid assessments and review during all study visits to ensure prescribed fluid intake is achieved and to validate a fluid intake tool.

**To be collected following the screening visit and prior to the final visit (two collections each).

††To be collected between study visits (one collection only).

‡‡To be collected prior to the study visit (one collection only).

§§To be collected at screening for PKD gene analysis.

¶¶To be collected at all study visits for exploratory renal biomarkers (stored at −80°C).

ADPKD, autosomal dominant polycystic kidney disease; AE, adverse event; BP, blood pressure; PKD, polycystic kidney disease; SMS, short message service; USG, urine-specific gravity.