Updated: 2/14/2020

Dosage Calculations

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Dose Regimens
  • Dose Regimens 
    • a dose regimen is a plan for drug administration over a period of time
    • an optimal dose regimen results in achievement of therapeutic levels of a drug in blood without the exceeding minimum toxic concentration
    • if it is necessary to achieve target plasma levels of a drug rapidly, a loading dose is used to "load" Vd with drug
    • to maintain plasma concentration of drug within a specified range over long periods of therapy, a schedule of maintenance doses is used
  • Definitions
    • Cp = target plasma concentration
    • F = bioavailability
      • F = 1 with IV administration 
Loading Dose
  • Loading dose = Cp x Vd/F 
    • loading dose is dose required to achieve a specific plasma drug concentration level with a single administration
    • loading dose will change depending on the volume of distribution in the patient 
    • if loading dose is very large (Vd much larger than blood volume), dose should be given slowly to prevent toxicity due to excessively high plasma levels during distribution phase
    • in a patient with impaired kidney/liver function, loading dose remains unchanged
Maintenance Dose
  • Maintenance dose = Cp x CL/F 
    • maintenance dose is dose required to maintain a target plasma drug concentration level
    • in a patient with impaired kidney/liver function, maintenance dose decreases
      • post-antibiotic effect: once plasma drug concentration falls, there is still antibiotic effects that suppress bacterial growth
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