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Cornell Pediatrics

Estimated Steady-State Trough Level

This calculator is in beta-testing.
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.

Peak level mcg/mL
Trough level  mcg/mL
Time to peak * hrs
Time to trough * hrs
Initial Dose mg/kg
Initial Frequency hrs
Proposed Dose mg/kg
Proposed Frequency hrs
The levels were drawn off the first dose
The levels were obtained under conditions of steady state

 * Times are measured from the initiation of the dose.

 
kelim
t hrs
Vd L/kg
Estimated SS Trough =  mg/L

Warning: Exercise caution with these estimates, but particularly if the elimination rate is high. High rates of elimination (high kelim or short t1/2) may invalidate the estimate of Vd, 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 kelim and the Vd. 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:

  • The timing of the dose administration and the levels are reasonably accurate.
  • The first level was drawn at a time that allowed for completion of alpha distribution.
  • The volume of distribution is constant, e.g. the patient is not becoming dehydrated or edematous.
  • The clearance is constant, e.g. the renal function is stable in the case of vancomycin.
  • Each dose is infused over the same time interval as all the others.

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 4-5 half-lives 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:

kelim = ln(Cpeak) - ln(Ctrough)
ttrough - tpeak

t = 0.693 / kelim

Vd = D = CL
Cp0 kelim

Css,min =
F •  D
Vss

 

  • e-kt

1 - e-kt

kelim    elimination constant k
Cpeak    peak concentration in plasma/serum
Ctrough    trough concentration in plasma/serum
ttrough    time of trough level (relative to dose initiation)
tpeak    time of peak level (relative to dose initiation)
t1/2    half-life time
Vd or Vss   volume of distribution
  dose
CL   clearance
Cp0    concentration in plasma/serum at time zero
Css,min    minimum (trough) concentration at steady state
  bioavailable fraction of dose (1 for IV medications)
  time
tintervel   time interval between doses (24 / frequency)
Cavg   average drug concentration

 


Created: October 14, 2013
Revised: December 1, 2013.

Weill Medical College of Cornell University
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