3. At the end of each dialysis session, arterial and venous drip chambers and the filter will be inspected for visible signs of coagulation. Corresponding to the standard care at our dialysis unit, a semi-quantitative score with a range from 0 (no signs of clotting) to 4 (complete occlusion) will be used for evaluation of the dialyzer. By definition, the drip chamber will be considered clotted when a thrombus was visible after the rinse back procedure. A fibrin ring against the wall of the drip chamber will be considered normal.