The primary outcome was urinary cystine supersaturation as measured by "cystine capacity". This proprietary test (Litholink Corp., Chicago IL) is reported as a value in mg/L above or below zero, with positive values indicating urine undersaturated with cystine, while negative values indicate that the urine is supersaturated with cystine. Four 24-hour urine samples were obtained during the study: one at baseline 3-6 days prior to the measurement, one on day 3-4 of the dosing period, one on day 7-8 of the dosing period, and one 3-6 days after the washout period. Each 24 hour urine sample was sent individually for analysis by Litholink Corp, which performs the cystine assays.

Secondary outcomes included serum sodium and other electrolyte levels. Serum electrolytes for each subject before, during, and after tolvaptan treatment (Sodium/Potassium/Chloride in mmol/L).

This was a single arm, unblinded study. All participants received half of the recommended dose (weight-dependent) on Days 1 through 4 and then the full recommended dose (weight-dependent) on Days 5 through 8

Example of Dosing Schedule by Weight:

Participant Weight is between: 12.5 kg -37.49 kg Then their Dose on Days 1-4 = 7.5mg (.6mg/kg -.2mg/kg) Dose on Days 5-8 is doubled = 15mg (1.2mg/kg - .4mg/kg) Maximum dose administered was 60 mg.

The primary outcome was urinary cystine supersaturation as measured by "cystine capacity". This proprietary test (Litholink Corp., Chicago IL) is reported as a value in mg/L above or below zero, with positive values indicating urine undersaturated with cystine, while negative values indicate that the urine is supersaturated with cystine. Four 24-hour urine samples were obtained during the study: one at baseline 3-6 days prior to the measurement, one on day 3-4 of the dosing period, one on day 7-8 of the dosing period, and one 3-6 days after the washout period. Each 24 hour urine sample was sent individually for analysis by Litholink Corp, which performs the cystine assays.

Tolvaptan.