What characteristics determine whether a drug is removed by dialysis?
• Molecular weight. As a general rule, smaller molecular weight substances pass through the dialyzer membrane much more easily than larger weight molecules. In general, free drug molecules with a molecular weight of less than 500 Daltons (D) are removed efficiently by hemodialysis.
• Protein binding. Decreased protein binding may increase the amount of free drug available for removal during dialysis. In the setting of an overdose, the amount of ingested drug may exceed the normal protein-binding capacity. This would allow removal of the excess drug by hemodialysis, even though dialysis has a minimal effect when the drug is used at normal doses.
• V D . Drugs with large volumes of distribution are not removed effectively by dialysis. Lipid-soluble drugs usually have large volumes of distribution, making significant removal of the drug difficult because the plasma volume is rapidly replenished from other tissues (e.g., cyclosporine and digoxin).
• Water solubility. Drugs with high water solubility will be dialyzed to a greater extent than those with high lipid solubility.
• Dialyzer membrane. The pore size, surface area, and geometry are the primary factors in determining whether a dialysis membrane will clear a specific drug. Historically, standard dialysis membranes did not effectively remove vancomycin (molecular weight, 3300 D) given its size. Currently, high-flux membranes that remove larger-molecular-weight molecules have become standard of care in dialysis practice. Therefore vancomycin and many other antibiotics are removed by these membranes. Dosing of these medications should be held until after dialysis on those days.
• Blood and dialysate flow rates. Increased flow rates during hemodialysis will increase drug clearance. Patients who cannot tolerate standard blood flow rates will require less replacement dosing of a drug after hemodialysis.
