First, I apologize for being so late replying to this question about PETs.
ISPD standards indicate a PET should be performed after the patient is on PD a month, giving the peritoneal membrane time to adjust to the presence of dialysis solutions. THere is probably not much clinical difference if you perform the PET earlier, but in a purely evidence based question, there have been statistical differences in PETs performed earlier than 1 month versus after 1 month. At any rate, an initial PET should be done to assist in the best prescription for the patient to meet appropriate target clearances and as a baseline for further evaluations of changes in the membrane (see ISPD.org guidelines on solute and fluid removal by Lo et al). While it is true that clinical changes will most likely indicate changes in membrane function, it is important to know where the patient is at baseline. If a clinic is omitting initial PETs because of the large time required by the staff to perform the standard PET, a short PET can be done with little inconvenience for either patient or nurse. Published studies have shown no difference in results with standard vs short PET.