The inclusion criteria were free light chain values >100 mg/L for k chains and >50 mg/L for lambda chains. These levels were chosen arbitrarily because no cut-off levels are available in the literature for the assessment of lowered serum free light chain values during hemodialysis in patients with end-stage renal disease. Other inclusion criteria were: age >18 years, absence or <200 ml/die residual diuresis, fistula or central venous catheter with blood flow >250 ml/min. Exclusion criteria were: intradialytic hypotension during bicarbonate dialysis, multiple myeloma requiring the double PMMA filter application, poly-allergy, HIV positivity. The filters used during the study were: i) HFR17 (Bellco, Mirandola, Italy), a double chamber filter used for online HFR. The first part of the filter consisted in a polyphenylene high flux hemofilter with an ultrafiltration coefficient (Kuf) of 28 ml/h/mmHg, a surface area of 0.7 m2 and a membrane cut-off value of 35,000 daltons. The endogenous ultrafiltrate rate is obtained automatically by means of the transmembrane pressure levels in the hemofilter. These are calculated from two pressure sensors: the first is on the arterial bubble chamber and the second before the roller pump of the ultrafiltrate. The ultrafiltrate is driven from this hemofilter to a 40 g neutral styrenic resin that allows an adsorbing area of 28,000 m2. After adsorption, the ultrafiltrate is added to the whole blood that, in turn, passes into the second HFR17 filter, a polyphenylene low flux filter (Kuf 13 ml/h/mmHg, surface area 1.7 m2) where the weight loss and diffusive depuration take place. ii) PMMA (Toray Filtryzer BK-F, Tokyo, Japan) with a surface area of 2.1 m2, a membrane cut-off value of 20,000 daltons and an ultrafiltration coefficient (Kuf) of 26 ml/h/mmHg. iii) A conventional polysulfone membrane (Fresenius F7HPS, Bad Homburg, Germany) with a cut-off of 11,500 daltons, a Kuf of 16 ml/h/mmHg and a surface area of 1.7 m2 was used as a control dialyzer.