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Loading Dose
Loading dose
Purpose
- A loading dose is used to quickly achieve a high plasma concentration of drug.
- Without a loading dose, it can take quite a bit of time to reach steady state: 4-5 half-lives, which (as in the case of refractory seizures) may be too long.
Equation
Overview
- Loading dose is a measurement of distribution x concentration.
- It dose is the dose of a drug we use to achieve a high plasma concentration (targeting steady state concentration), quickly; we show a syringe because it is often given intravenously.
Loading dose equation:
Loading dose = (VdTC)/F
- Vd = Volume of Distribution
- TC = Target Concentration
- F = Fraction of Absorption (aka Bioavailability)
Equation meaning:
- The loading dose is calculated by multiplying the Volume of Distribution by the Target Plasma Concentration and dividing that amount by the Bioavailability. This means that:
- The more a drug leaches out into the tissues (ie, the higher its volume of distribution), and the higher our target concentration, and the lower the absorption of the drug, the larger the loading dose required to achieve the desired effect.
- On the contrary, the better the drug stays in the blood plasma (ie, the lower its volume of distribution), the lower our target concentration, and the better its systemic circulation absorption, the less of it we’ll have to use.
Equation Units
Vd Volume (L) x TC(Mass (mg)/Blood (L)) / F* Percentage (%)
- The liter units cancel.
- We are left with a mass (mg) for the loading dose.
Example
- We calculate the following loading dose wherein:
- Vd = 50 L
- TC = 5 mg/L
- Bioavailability = 50%
- Mass (mg) = 50 (L) x 5 (mg/L) / 50%
- The liter units cancel.
- Answer: 500mg
- Note that by definition, IV medications have a loading dose of 100% but oral medications can have a wide-ranging bioavailability, so it's important to pay attention to the bioavailability when calculating loading dose.