Eligible patients at the end of week 4 ( those with a baseline potassium level of 5.5 to < 6.5 mmol per liter in whom the level decreased to 3.8 to < 5.1 mmol per liter ) entered an 8-week randomized withdrawal phase in which they were randomly assigned to continue patiromer or switch to placebo ; the primary efficacy end point was the between-group difference in the median change in the serum potassium level over the first 4 weeks of that phase .