Commentary by Zbylut J. Twardowski, MD

PET is the most widely used peritoneal function test, because of its standardization, simplicity, and usefulness in diagnostic and prognostic purposes. An abridged test¹ looking only for D/D₀ glucose and D/P creatinine ratios, and drain volumes, is commonly used.

Based on PET, a computer model (PD ADEQUEST, Baxter Healthcare Corporation, Round Lake, Illinois, USA) has been developed that permits prediction of small solute clearances and ultrafiltration for various alternative prescriptions of peritoneal dialysis. Excellent agreement between predicted and measured values has been validated in 111 patients².

In the original PET, which established standard values for membrane categorization in 1987, the glucose solution was used for the preceding exchange^3,4. Recently many patients, particularly those with ultrafiltration problems, use polyglucose solution for nightly exchanges. Lilaj et al. found that polyglucose-containing solution used for the preceding exchange increases D/P ratios of creatinine, phosphate, and sodium, as well as glucose absorption, measured during the test. Therefore, they recommend that patients using icodextrin solution during the nighttime should perform their nighttime exchange with conventional glucose solution before a scheduled PET5.

The preceding exchange dwell time was 8 or more hours in the original PET. This was convenient when almost all patients were on continuous ambulatory peritoneal dialysis; however, now that many patients are on some form of automated peritoneal dialysis (APD), an 8-hr exchange prior to the PET requires changes in the dialysis schedule. Our recent study⁶ evaluated the differences in 2-hr equilibration curves with standard, 8-hr and 3-hr preceding exchanges. The values for D/P creatinine and urea, as well as D/D₀ glucose, were almost identical throughout the 2-hr PET dwell after long and short exchanges. D/P protein values tended to be higher in the PET after the long exchange. We concluded that for creatinine and glucose equilibrations, any dwell time between 3 and 12 hours was acceptable for the preceding exchange, and the equilibration test might be performed with a 2-hr or 4-hr dwell. The protein values obtained with a 3-hr prior dwell are different from those with a long prior exchange⁶. However, for a full characterization of peritoneal membrane function, an unabridged test as described in 1987^3,4, which includes D/P urea, protein, creatinine, and D/D₀ glucose as well as D/P potassium, sodium, and corrected creatinine, drain volume and residual volumes should be used. Although this test is extremely reliable and valuable in characterization of peritoneal function, it is rarely used because of its complexity and considerable nursing time requirement; nonetheless, it should be used in cases where full characterization of peritoneal function is necessary. An unabridged test should be performed shortly after the break-in period of peritoneal dialysis as a baseline for future comparisons.

Peritoneal Function after Long-term Exposure to Peritoneal Dialysis Solutions

Long-term peritoneal dialysis is associated with progressive loss of ultrafiltration capability. There are two possible mechanisms leading to this phenomenon:

• First, effective peritoneal surface area progressively increases in long-term PD patients. This is related to increased number and surface area of peritoneal capillaries as a response to an increase of nitric oxide synthase (NOS) activity and upregulation of vascular endothelial growth factor (VEGF).
• Second, peritoneal membrane is gradually denuded of mesothelial cells, leading to loss of mesothelial aquaporins, which are ultra-small pores responsible for free water transport. Gradual decline of Cancer Antigen (CA) 125 concentration in dialysate, a marker of mesothelial cell mass, has been found in long-term peritoneal dialysis patients⁷ and loss of mesothelial cells in patients with ultrafiltration failure has been determined by peritoneal biopsy⁸. On an unabridged PET in these patients, an initial dip of dialysate to plasma sodium concentration is missing and extremely high dialysate protein concentrations are seen.⁸

References

1. Twardowski ZJ. Clinical value of standardized equilibration tests in CAPD patients. Blood Purif 1989; 7(2-3):95-108
2. Vonesh EF, Burkart J, McMurray SD, Williams PF. Peritoneal dialysis kinetic modeling: validation in a multicenter clinical study. Perit Dial Int 1996; 16(5):471-481
3. Twardowski ZJ, Nolph KD, Khanna R, Prowant BF, Ryan LP, Moore HL, Nielsen MP. Peritoneal equilibration test. Perit Dial Bull 1987; 7(3):138-147
4. Twardowski ZJ, Khanna R, Nolph KD. Peritoneal dialysis modifications to avoid CAPD dropouts. In: Khanna R, Nolph KD, Prowant BF, Twardowski ZJ, Oreopoulos GD, eds. Advances in Continuous Ambulatory Peritoneal Dialysis. Proc Seventh Ann CAPD Conf, Kansas City, Missouri, February 1987. Toronto: Perit Dial Bull 1987; 7:171-178
5. Lilaj T, Dittrich E, Puttinger H, Schneider B, Haag-Weber M, Hörl WH, Vychytil A. A preceding exchange with polyglucose versus glucose solution modifies peritoneal equilibration test results. Am J Kidney Dis 2001; 38(1):118-126
6. Twardowski ZJ, Prowant BF, Moore HL, Lou LC, White E, Farris K. Short peritoneal equilibration test: Impact of preceding dwell time. Adv Perit Dial 2003; 19:53-58
7. Sanusi AA, Zweers MM, Weening JJ, de Waart DR, Struijk DG, Krediet RT. Expression of cancer antigen 125 by peritoneal mesothelial cells is not influenced by duration of peritoneal dialysis. Perit Dial Int 2001; 21(5):495-500
8. Dobbie JW, Krediet RT, Twardowski ZJ, Nichols WK. A 39-year-old man with loss of ultrafiltration. Perit Dial Int 1994; 14(4):384-394

Additional commentary by Todd S. Ing, MD

Universally used, Dr. Twardowski's PET test has become the gold standard for the measurement of peritoneal membrane function.