A method of modeling a patient's peritoneal dialysis drain phase includes (i) modeling a first segment of a drain phase curve as having a constant flowrate; (ii) modeling a second segment of a drain phase curve as having a decaying exponential flowrate; and (iii) incorporating a switching component into the first and second components so that (a) at a first time the first segment is active and while the second segment is inactive and (b) at a second time the first segment is inactive and while the second segment is active.