Pet

In 1987 Twardowski presented the peritoneal equilibration test for the determination of small molecular mass transport and ultrafiltration behaviour (Wiegard- Szramek 2019) for the first time. In later years some modifications were made (Haag- Weber 2017). La Milia et al. presented the so-called mini PET test in 2005. With this test the free water transport can be assessed easily and quickly (La Milia 2005).

A peritoneal equilibration test is a functional test to optimize therapy or to select the best procedure in peritoneal dialysis (see also APD) (Herold 2020). With PET, the peritoneal transport properties are evaluated by determining the transfer rates for urea, creatinine and glucose (Kasper 2015).

There are now three forms of the test:

• Traditional PET: This is performed with a 2.27% glucose solution.
• Mini-PET: This reduces the duration of the test to 1 hour. With the mini-PET one can make statements about the determination of the free water transport (La Milia 2005)
• Combined modified PET: This test is performed with 3.86% glucose solution. Additionally the sodium sieving is determined (Haag- Weber 2017).

In 2000, the International Society of Peritoneal Dialysis recommended combined modified PET (Geberth 2011). Since then, this test has been the most commonly used functional test for patients on peritoneal dialysis (Bruck 2017).

In peritoneal dialysis, the peritoneal transport properties should be checked regularly using the combined modified peritoneal equilibration test (PET). According to the recommendation, this should be done for the first time 4 weeks after the start of therapy, then once a year (Herold 2020) and additionally after every case of peritonitis (4 weeks after discontinuation of antibiotics [Geberth 2011]) (Kasper 2015).

Test procedure (combined modified PET): The test is standardized and is performed by changing dialysate with a total volume of 2,000 ml and a total retention time of 4 h (Kasper 2015).

For diabetics a good blood glucose adjustment is necessary before the test starts.

Before the test, a dialysis solution containing glucose should be left in the abdominal cavity for at least 60 min. This is drained completely and 2 l of a 3.68 % glucose solution is instilled. The end of the enema represents the so-called PET start = time 0 min.

Dialysate samples are taken from the abdominal cavity at precisely defined times (Kuhlmann 2015).

• At time 0 min: 200 ml are taken immediately after enema, of which 10 ml remain as a sample. The remaining 190 ml are allowed to run back again.
• After 60 min.: The dialysate is completely drained off, 10 ml of which are used as a sample. The dialysate, which has been noted down in terms of quantity, is then let back in. A blood sample is also taken.
• After 4 h: The entire dialysate is drained off again and 10 ml is taken as a sample. Afterwards, the patient can continue his usual dialysis procedure. (Bruck 2017)

During the test, the time is determined in which the concentration of a dissolved substance is equalized between the dialysate and the plasma. This ratio is called D/P.

The peritoneal transport type (see below) can thus be determined by means of standard membrane permeability diagrams (Bruck 2017).

The high-percent glucose used in the test stimulates ultrafiltration via aquaporin channels. In the first 2 h of the test, this results in a predominant transport of electrolyte-free water, since the sodium is retained (also known as "sodium sieving").

The complete discharge of dialysate after 1 h allows conclusions to be drawn about the function of the aquaporins due to the drop in sodium in the dialysate (Bruck 2017).

The prognostic statement on the cardiovascular and infectious mortality of patients can possibly be better determined by the peritoneal protein clearance than by the peritoneal transport properties of small molecular substances. It has been shown that a high protein loss describes a higher correlation regarding the above mentioned mortality than D/P creatinine (Bruck 2017).

Test result: Following the test, 4 different transport types can be differentiated:

1st type: fast transporter: In fast transporters the glucose is rapidly absorbed, but there is a poor ultrafiltration at long residence times (Hörl 2003). Therefore, automated peritoneal dialysis (APD) is the standard procedure for peritoneal dialysis (Keller 2010).

2nd type: high normal transporter: For high normal transporters, continuous ambulant peritoneal dialysis = CAPD or automated peritoneal dialysis (APD) can be used (Keller 2010).

3rd type: low-normal transporter: Continuous ambulatory peritoneal dialysis = CAPD is suitable for low-normal transporters. If there is sufficient residual renal function, automated peritoneal dialysis (APD) can be used as an alternative (Keller 2010).

4th type: slow transporter: These patients resorb glucose slowly. Ultrafiltration is optimal, but the elimination of small to medium molecular weight substances is often insufficient (Hörl 2003). For slow transporters continuous ambulant peritoneal dialysis = CAPD is suitable. Automated peritoneal dialysis (APD) should not be performed. If there is a loss of residual renal function, a switch to hemodialysis is recommended (see renal replacement treatment) (Keller 2010).

The decrease in sodium sieving and free water transport are currently the best available parameters for early detection of encapsulating peritoneal sclerosis (EPS). A marker for peritoneal fibrosis is the decrease of free water transport (Haag- Weber 2017).

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