Table 1 New classification of uremic toxins based on metabolic pathways and dialysis modalities [35]

Protein-bound (Protein-bound ≥ 80%)

Exogenous (Gut-derived)

< 0.5 kDa

Low-flux HD;

High-flux HD;

High-flux HDF;

Medium cutoff HDx;

High cutoff HD;

Tryptophan metabolism (IS, KYN, IAA, KYNA [2]);

Tyrosine metabolism (PCS [18]);

Methionine metabolism (Homocysteine [173]);

Maillard reaction (carboxymethyl lysine [174])

Water soluble (Protein-bound < 80%)

Exogenous and exogenous (Both gut-derived and endogenous metabolism)

< 0.5 kDa

Low-flux HD;

High-flux HD;

High-flux HDF;

Medium cutoff HDx;

High cutoff HD;

Choline, carnitine and betaine metabolism (TMAO [31]);

Arginine methylation (asymmetric dimethylarginine, symmetric dimethylarginine [175]);

Purine metabolism (uric acid [176]);

Carbamylation (carbamylated compounds [177])

Endogenous (endogenous metabolism)

0.5–15 kDa

High-flux HD;

High-flux HDF;

Medium cutoff HDx;

High cutoff HD

Cytokine (IL-8);

Structural protein (β₂-microglobulin)

> 15–25 kDa

High-flux HDF;

Medium cutoff HDx;

High cutoff HD

Cytokines (TNF, IL-18, IL-10, IL-6, FGF-2);

Hormone (prolactin);

Structural proteins (kappa-FLC, myoglobin, sTNFR2, complement factor D);

> 25–58 kDa

Medium cutoff HDx;

High cutoff HD

Cytokines (pentatraxin-3, FGF-23, CX3CL1, CXCL12, IL-2);

Structural proteins (sTNFR1, lambda-FLC, YKL-40);

Maillard reaction (advanced glycosylation end products [178])

> 58 kDa

High cutoff HD

Modified albumin