New Dialyzer Classification in Japan

The most common type of dialyzer worldwide is currently the synthetic membrane used in high-flux dialyzers. In Japan, however, super high-flux dialyzers have long been the mainstream, and those with enlarged pore sizes can remove large-molecule toxins such as α1-microglobulin. Up to 2012 in Japan, dialyzers were classified into five types based on β₂-microglobulin (β2MG) clearance: Type I (<10 mL/min), Type II (≥10-30), Type III (≥30-50), Type IV (≥50-70), and Type V (≥70). Type IV and Type V dialyzers are defined as super high-flux dialyzers and have been associated with good prognosis. In the new classification, dialyzers with a β2MG clearance <70 mL/min are classified as Type I, and those with a clearance of ≥70 mL/min are classified as Type II. Furthermore, for the albumin sieving coefficient, values below and above a boundary value of 0.03 were subdivided into Type a (albumin impermeable/low permeable) and Type b (albumin permeable), respectively. Based on β2MG clearance and albumin sieving coefficient, dialyzers are then classified into four types: I-a, I-b, II-a, and II-b. Type I-a is denoted as standard flux, Type I-b as high flux-albumin leaking, Type II-a as super high flux, and Type II-b as albumin leaking-super high flux. In addition, Type S dialyzers have been defined, which have high biocompatibility, improved solute removal efficiency by adsorption, and anti-inflammatory and antioxidant effects. In the current classification, super high-flux dialyzers with a β2MG clearance of ≥70 mL/min and Type S dialyzers are suggested to contribute to improved prognosis. This review provides an overview of the new classification of dialyzers in Japan.