What is considered a high cut-off filter in dialysis, and is the F180 (Fresenius 180) filter considered high cut-off?

High Cut-Off Dialysis Filters: Definition and F180 Classification

High cut-off (HCO) dialyzers are defined as membranes with pore sizes up to 50 kDa (compared to 5 kDa in standard dialyzers), and the F180 (Fresenius Theralite™) is indeed considered a high cut-off filter with a molecular weight cut-off of approximately 45-60 kDa. 1, 2, 3

Technical Definition of High Cut-Off Filters

Standard vs. High Cut-Off Membranes:

• High-flux dialyzers are defined by β2-microglobulin clearance of at least 20 mL/min and molecular weight cut-offs around 5 kDa 1
• High cut-off dialyzers have molecular weight cut-offs up to 50 kDa, allowing removal of molecules significantly larger than conventional membranes 1, 4
• The molecular weight retention onset (MWRO) provides a classification system that clearly differentiates low-flux, high-flux, protein-leaking, and high cut-off membrane families 5

The F180/Theralite™ as a High Cut-Off Filter

Specific Characteristics:

• The Fresenius Theralite™ (F180) filter is specifically designed to remove plasma substances with molecular weights up to 45 kDa, including free light chains 2, 3
• This filter can reduce serum free κ light chains by more than 75% over 6 weeks of treatment 2
• During hemodialysis with the Theralite™, phenytoin clearance averaged 80.1 mL/min, demonstrating its enhanced removal capacity for larger molecules 3

Clinical Applications and Important Caveats

Primary Uses:

• Myeloma cast nephropathy: HCO dialyzers can reduce serum free light chain levels by >70%, with two randomized trials (MYRE and EuLITE) showing mixed results on renal recovery 1
• The MYRE trial demonstrated significantly higher renal recovery at 6 months (56.4% vs 35.4%, OR 2.37, p=0.04) and 12 months (60.9% vs 37.5%, OR 2.59, p=0.02) with HCO dialysis 1
• Removal of proinflammatory cytokines in sepsis and larger middle molecules in end-stage renal disease 4

Critical Limitations:

• Albumin loss is the major concern - HCO membranes are more permeable than the glomerular filtration barrier before blood exposure, though they become tighter after blood contact 5, 6
• Albumin losses with HCO dialysis are prohibitive against long-term use in chronic hemodialysis patients 6
• The EuLITE study showed inferior age-adjusted overall survival (HR 2.63, p=0.03) with HCO dialysis, possibly due to higher infectious complications (31% vs 9% pulmonary infections) 1

Practical Distinction from Standard High-Flux

Key Differences:

• High-flux membranes (20 mL/min β2-microglobulin clearance) are recommended for reducing β2-amyloidosis and may reduce cardiac mortality in patients dialyzed >3.7 years 1
• High cut-off filters go beyond high-flux capabilities, targeting molecules 10-fold larger (up to 50 kDa vs 5 kDa) 1, 4
• HCO filters require supplementation with low-flux filters or careful monitoring to prevent excessive albumin loss 6

The F180/Theralite™ is definitively a high cut-off filter, not merely high-flux, based on its 45-60 kDa molecular weight cut-off and clinical applications targeting free light chains and larger molecules. 2, 3, 4