Patients suffering from acute renal failure must be diagnosed and treated quickly so that a physician can confidently prescribe either peritoneal dialysis or hemodialysis. In one way of quickly treating the patients, software is used to calculate a suitable peritoneal dialysis prescription without regard to how short or how long a dialysis cycle is used, and without regard to a total dialysate fluid volume for a therapy. For patients with suitable peritoneal membrane transport properties, the software program suggests that, at least over a short period of time, unexpectedly high ultrafiltrate volumes and high clearances may be achieved.