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This calculator is in betatesting.
Use with extreme caution.
Calculate the expected trough level of a drug from measured peak and trough levels. This calculator was designed for estimating vancomycin dosing regimens but can be used for other drugs. Please take care to determine if the assumptions of this model hold true for your patient. Violating these assumptions will invalidate the calculations.
Warning: Exercise caution with these estimates, but particularly if the elimination rate is high. High rates of elimination (high k_{elim} or short t_{1/2}) may invalidate the estimate of V_{d}, making it much lower because a significant portion of the dose would be eliminated before the first level is drawn. Measuring the amount of drug excreted in the urine up to the time the peak level is drawn, and subtracting this amount in mg/kg from the initial dose can significantly improve the estimate.
Note: The "peak" and the "trough" levels need not be a "true" peak and trough. They need only be two levels separated by time. Generally the longer time interval between the two levels the better. The peak should be obtained after sufficient time has elapsed after the dose has been completely administered to allow for distribution.
The two levels separated in time are used to calculate the k_{elim} and the V_{d}. If standard assumptions hold true, these values can be used to calculate the trough concentration at steady state for any dosing regimen. These assumptions are:
If the levels were not obtained after the first dose, the usual calculation of the Vd would be invalid. This calculator can adjust for this if conditions of steady state have been met. The conditions of steady state would be met if 45 halflives have elapsed since the last change of the dosing regimen to the measurement of the first level. Alternatively, if two measured trough levels are the same, the patient would be in steady state.
If the rate of elimination is very rapid, as is common in pediatric patients, small deviations in the timing of the doses can result in significantly different calculations. Please use caution interpeting these numbers.
The formulae used are:
k_{elim} =  ln(C_{peak})  ln(C_{trough}) 
t_{trough}  t_{peak} 
t_{½} = 0.693 / k_{elim}
V_{d} =  D  =  CL 
C_{p}^{0}  k_{elim} 
C_{ss,min} = 

• e^{kt} 

1  e^{kt} 
k_{elim}  elimination constant k  
C_{peak}  peak concentration in plasma/serum  
C_{trough}  trough concentration in plasma/serum  
t_{trough}  time of trough level (relative to dose initiation)  
t_{peak}  time of peak level (relative to dose initiation)  
t_{1/2}  halflife time  
V_{d} or V_{ss}  volume of distribution  
D  dose  
CL  clearance  
C_{p}^{0}  concentration in plasma/serum at time zero  
C_{ss,min}  minimum (trough) concentration at steady state  
F  bioavailable fraction of dose (1 for IV medications)  
t  time  
t_{intervel}  time interval between doses (24 / frequency)  
C_{avg}  average drug concentration 
Created: October 14, 2013
Revised: December 1, 2013.