Dialyzability of Drugs
To determine the likelihood that dialysis will significantly
impact clearance consult the literature for information or the following may be
used.
1. Apparent unbound Vd(L/kg) = Vd(L/kg) / fraction
unbound, if this value is greater than 3.5 l/kg an insignificant amount will
probably be removed. Unbound volume of distributions 150 liters and above have less than
20% cleared during a 4 hour
dialysis period. Only unbound drug can be dialyzed.
2.
If the medication's clearance is above 10 ml/min/kg (600 ml/hr/kg, 42 L/hr for
70 kg patient))
(metabolic plus renal clearance) dialysis probably won't significantly increase
clearance. Dialysis clearance is usually less than 150 ml/min (9 L/hr)
3.
If the molecular weight is greater than 1000 Daltons low flux dialysis won't
significantly impact clearance and if MW is greater than 5000 Daltons high flux
dialysis won't significantly impact clearance.
Drugs with low MW, limited Vd,
and that are water-soluble are most likely to be removed by HD and will require
extra dosing. Clearance during dialysis is related to blood flow rate, dialysate
flow rate, and filter. If the same flow rates and filter is used in patients of various weight the
fraction removed will be lower in higher-weight patients and higher in lower-weight patients as
the volume of distribution is proportional to body weight.
Fraction of amount in body lost during dialysis period =
(1-exp(-K(1/hr)during dialysis * Dialysis Period(hours)))
Fraction of amount in body lost during dialysis period
= (1-exp(-((Cl dialysis + Cl renal ) / Vd)*Dialysis Period(hours)))
This is only the amount lost during the dialysis period and does not include the
amount lost between dialysis sessions due to renal elimination.
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