Indirect measurement of lymphatic absorption in CAPD patients is not influenced by trapping

The disappearance rate of intraperitoneally administered macromolecules is often used to calculate lymphatic absorption during CAPD. The possible contribution of local accumulation (trapping) of such solutes in the tissues surrounding the peritoneal cavity, leading to overestimation of lymphatic flow, was investigated in eight CAPD patients. They were studied twice during a four hour dwell, glucose 1.36%, to which polydisperse neutral dextran 70 1 g/liter had been added for measurement of lymphatic flow. After the test on day 1 dextran 130 mg/kg was given intravenously and also dextran 1 g/liter was added to every following dialysis bag until the second test on day 3. This was done to saturate the tissues surrounding the peritoneal cavity and thereby to create a steady state condition. In one patient the dextran administration was continued until a third study was done on day 5. Dextran in serum during day 3 was 1.3 +/- 0.5 g/liter (mean +/- SD). No difference in peritoneal clearance of dextran was found between day 1 and day 3 (1.11 +/- 0.56 versus 0.97 +/- 0.41 ml/min). Also no difference was found between day 1 (0.32), day 3 (0.62), and day 5 (0.42 ml/min). Trapping would have influenced the first but not the second test, as the second time all tissues were saturated with dextran. As the dextran absorption rate remained the same, this indicates that trapping is of no importance and that lymphatic absorption can be measured by the disappearance of a macromolecular marker.