The steady state concentration of a drug can serve as the target for an orally administered drug. In addition to adjusting the maintenance dose, it is sometimes necessary to adjust the loading dose in patients with renal failure.
For many drugs given orally the decision is made to keep the dosing interval unchanged while adjusting the maintenance dose.
maintenance dose of drug in mg to administer per dosing interval =
= ((average steady state plasma concentration in mg/L) * (estimated drug clearance in L per hour) * (dosing interval in hours) / (fraction of dose absorbed)
Normally it is not necessary to modify the usual loading dose if one is used, since it is intended to intended to rapidly bring the level of drug in the body to that level seen in steady state. The volume of distribution (the main determinant of the loading dose) is usually close to normal.
If the half-life of a drug is short, then the time required to reach steady state levels is not clinically significant and no loading dose is needed.
If the half-life becomes sufficiently prolonged in renal failure, then the time to steady state levels may be quite long, and a loading dose should be considered. The eventual steady state level will be the same, since it is determined by the maintenance drug dose and drug clearance.
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