The following experience is exclusively based on ePTFE prostheses. Their surface is uneven, fibrillary, and hydrophobic with pore sizes 50–70 μm, varying with the respective manufacturers. Only after exceeding a critical pressure may fluid permeate the wall. This pressure is called the hydrophobic barrier. According to manufacturers it is around 300 mmHg.

In practice, however, in some patients plasma leaks from grafts at much lower pressures. Our lab tests showed that the protein and electrolyte content of this leaking fluid corresponds to that of blood plasma. Consequently there is a filtration process rather than a tissue reaction to the prosthesis, but it may also be called perigraft reaction in so far as this filtration also leads to alterations of the surrounding tissue. This filtration is caused by the conversion of the ePTFE graft surface from a hydrophobic to a hydrophilic state brought about by plasma molecules.

Before 1992 when we started selective sequential fibrin sealing of the outer graft surface, we observed the intraoperative filtration of prostheses in almost a third of our dialysis patients. In patients who did not require hemodialysis, we could observe a clinically relevant filtration of non fibrin-sealed ePTFE grafts (peripheral bypass surgery) in around 1 %. Damage to the ePTFE wall and also to its potential fibrin coating (e.g., by clamping or grasping) favors filtration. As we take care to avoid this damage as much as possible, the experience reported here mostly refers to patients who had already been treated elsewhere.

The clinical presentation depends on the relation of the filtered volume (filtrate) to the absorptive capacity of the surrounding tissue and the duration of the filtration.

According to the clinical presentation and course we distinguish between five types of graft filtration.