why filtration of drug can't be done in dialysis
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drug removal by hemodialysis factors protein binding volume of distribution molecular weight authoritative
Drug removal can occur during dialysis, but not every drug is removed well.
Hemodialysis removes drug only from the blood plasma passing through the dialyzer. A drug is poorly dialyzable if:
- Highly protein bound: Only the free, unbound drug crosses the dialysis membrane. Drug attached to albumin stays in blood.
- Large volume of distribution (Vd): Most drug has left the blood and entered tissues. Dialysis cannot reach drug stored in muscle, fat, or organs.
- Large molecular size: Large molecules cross standard dialysis membranes slowly or not at all.
- Poorly water soluble / highly lipophilic: These drugs tend to distribute into tissues rather than remain in plasma.
- Short or inefficient dialysis session: Dialysis clearance also depends on membrane type, blood flow, dialysate flow, and treatment duration.
So the key idea is:
Dialysis filters only the free drug present in the bloodstream, not protein-bound drug or drug already distributed into tissues.
For example, lithium is small, water soluble, minimally protein-bound, and has a relatively low Vd, so dialysis removes it effectively. Digoxin has a large Vd because it is mainly in tissues, so dialysis is generally ineffective. Textbook sources likewise identify molecular size, protein binding, and Vd as main determinants of drug dialyzability. Supporting pharmacokinetic evidence
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