“Let’s Talk About Peritoneal Dialysis” Post 1: The Peritoneal Equilibration Test (PET)

Welcome to a series of posts titled “Let’s Talk About Peritoneal Dialysis.” Today, we will be discussing the Peritoneal Equilibration Test (PET) that is performed in peritoneal diaysis (PD) patients. I distinctly recall my first time as a fellow being asked about a patient’s PET and responding with the avidity of his lymph nodes from a positron emission tomography–computed tomography (PET-CT)  to a chuckling attending.  Hopefully I can prevent this for a few other fellows!

The purpose of the PET is to provide a standardized measure with which to compare the transport characteristics of the peritoneal membrane.  The test has been used historically to determine suitability for peritoneal dialysis, tailor a patient’s PD prescription, aid in the diagnosis of solute or ultrafiltration failure.

The PET was first described by Twardoski in 1987 and is classically preceded by an 8-12 hour dwell and followed by drain and a blood draw. Then the test begins. A 2.5% dextrose solution is infused into the patient’s abdomen in portions of 400 mL every 2 min over a total of 10 min (total 2L infused).  The patient rolls from side to side after each 400 ml portion. After the infusion is complete, 200 ml of the solution is drained, a 10 ml sample is stored, and the remaining 190 ml reinfused.  

Next, the patient ambulates.  The original PET described a protocol in which samples of dialysate were collected with the above technique after 30, 60, 120, and 180 minutes of dwell time. Samples are taken after 120 and 240 minutes of dwell time in today’s standard PET. At 240 minutes, the patient is drained completely and total volume is also measured to calculate ultrafiltration. The ratio of the dialysate creatinine to the plasma creatinine (D/P) allows for transport classification as a “low” or “high” transporter, with a higher ratio suggesting higher transport. Conversely, lower glucose dialysate levels suggest higher transport, while higher levels suggest lower transport.  

The first PET is typically performed 4-8 weeks after initiation of PD to determine the characteristics of the peritoneal membrane by patient response to empiric dwell time adjustments.  If the test is performed properly, the results can be compared to the following chart to determine transport classification. If the creatinine and glucose classifications are not congruent, the test is to be repeated.

Figure 1. Dialysis Prescription Based on PET Results

While there are no absolute guidelines to determine candidacy for continuous ambulatory PD (CAPD)  or automated APD (APD) based on PET testing, it is generally accepted that high transporters benefit from shorter dwells (usually APD or nocturnal intermittent PD)  and low transporters from longer dwell times (usually CAPD). Of note, dwell times based on PET results can be adjusted with all PD modalities to try to fit the patient’s preference.

Finally,  the PET results have been shown to be associated with important patient outcomes.  A study published in 2015 that looked at patient outcomes in a large dialysis organization (LDO) database found that as D/P rises, so too does mortality and hospitalization rate – both in a linear fashion.  This relationship was present in both patients undergoing CAPD and automated PD. It remains to be seen if peritoneal membrane transport characteristics may present a possible target for treatment.

Post by: Ankur Shah, MD  (@nephroshah)

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