Drug Monitoring

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Therapeutic Drug Monitoring

CL/F = dosing rate/Css (measured)
  • CL/F can be used to adjust the maintenance dose to achieve the desired target concentration
  • Changes in effects of drugs may be delayed relative to changes in plasma concentration due to a slow rate of distribution - eg. digoxin often reaches toxic plasma levels soon after oral administration, but does not cause toxic effects
  • Sampling during steady state allows modification of CL/F estimate and therefore by the above formula dosing can be changed
  • When the goal of measurement is adjustment of dose, levels should be taken just before the next planned dose (trough levels)
  • When the goal is to determine whether efficacious concentrations of drugs are being achieved, a sample taken soon after administration can be helpful
  • When the goal is to look for accumulation due to low clearance, trough levels are useful
  • Sometimes peak and trough levels can be taken to give a better pharmacokinetic idea of the drug
  • Steady state of drugs is reached only after 4 t1/2s have passed
  • If a sample is obtained too soon it will not represent clearance accurately, if it is obtained too late the damage may already be done
  • Dose modifications can be done using the following formula:
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which can be rearranged to:
Screen shot 2012-11-26 at 4.06.10 PM.png