The peritoneal equilibration test (PET) is a useful tool that categorizes peritoneal transport. However, the method has some inconveniences. Some authors suggest that measuring the sodium level in the dialysate (NaD) or the dialysate-to-plasma ratio for sodium (D/PNa) can substitute for the PET. We applied a mathematical analysis [Fisher intraclass correlation coefficient (FICC)] to 43 PETs performed in 43 peritoneal dialysis patients (29 males, 14 females) with a mean age of 55.3 years (range: 28-85 years). Determinations of NaD, of sodium level in plasma (NaP), and of D/PNa at times 0, 30, 60, 120, and 240 minutes were added to the usual PET determinations. After using the NaD240 and the D/PNa240 values to calculate the cut-off values for the various peritoneal transport categories, we obtained a transport distribution very similar to that of the PET dialysate-to-plasma ratio for creatinine after 240 minutes (D/PCr240). At the same time, the FICC showed good (0.69) and excellent (0.77) correlation of NaD240 and D/PNa240 respectively with the D/PCr240. Therefore either of these two methods, which are cheaper and quicker than a PET, can be used to categorize peritoneal transport with a high degree of reliability.