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Dose Adjust Pre & Post Calculator

Weight =
PRE Level Date mm/dd/yyyy = 0/0/0
PRE Level Time hr.mm [military] =
PRE Level =
POST Level Date = 0/0/0
POST Level Time hr.mm [military] =
POST Level =
Infusion Time = 0.5
Dosing Interval (Tau) =
Time difference for ke calc =
K elimination =
Half-Life =
Concentration (Css) =
Change in Time to Css =
DOSE =
Volume of Distribution =
Patient identifier =

Help

An equation that lets you use the results of a Pre drug level and a Post drug level that were drawn around the SAME dose to determine the patient's pharmacokinetic parameters and modify their dose.
First Order Elimination Only.
This equation should not be used if the Pre level was drawn AFTER the Post level. Use the Concentration Steady-State or Half Life and Ke with Vd template in these other cases.
This calculation assumes that doses were given at the identified Dosing interval (TAU) and that levels were at steady-state.
Do not use this calculation if the doses were not given at the frequency that was ordered. Do not use this calculation if the half-life or volume of distribution appears unusual for the patient's clinical characteristics. This is based on the Css equation for first order elimination only.

Rows

- Weight]: the patient's weight
- PRE Level Date: the date as mm/dd/yyyy chosen from the date wheels
- PRE Level Time hr.mm [military]: the time that the Pre level was drawn in hours.min format.
- PRE Level: the Pre drug level concentration in mg/L (or mCg/mL).
- POST Level Date: the date as mm/dd/yyyy chosen from the date wheels
- POST Level Time hr.mm [military]: the time that the Post level was drawn in hours.min format.
- POST Level: the Post drug level concentration in mg/L (or mCg/mL).
- Infusion Time: the dose infusion duration in hours.
- Dosing Interval (Tau): the frequency of dose administration (in hours)
- Time difference for ke calc: the time difference that is used for the k elimination calculation. The template will solve for this. This time difference is usually = [TAU - Infusion time - hours before dose that the Pre level was drawn - hours after dose infusion ended that the Post level was drawn]
- K elimination: the elimination rate constant [in "/hr"]
- Half-Life: the number to hours to eliminate 50% of the drug
- Concentration [Css]: the steady-state concentration in mg/Liter or mcg/mL [This can be any steady-state concentration (peak, trough, Cpmax, or C anytime - see notes below]
- Change in Time to Css: the number of hours from the end of the dose infusion until the measured Css concentration.
- DOSE: the mg dose of medication given
- Volume of Distribution: the apparent volume of distribution in Liters/kg
- Patient identifier: You may add initials or a name to the results to help you identify whose they are. This identifier will be included when you email results. This may also be left blank.

Conversions

Amikacin umol/L x 0.585 = mCg/mL
Gentamicin umol/L x 0.478 = mCg/mL
Phenobarbital umol/L x 0.232 = mCg/mL
Tobramycin umol/L x 0.468 = mCg/mL
Vancomycin umol/L x 1.449 = mCg/mL

Examples

CB is a 35kg 8 year old child who is started on Gentamicin 70 mg IV Q 8 hours. The first dose is at 1100 on 12/20 and infused over 30 minutes. The next dose is given at 1900 on 12/20, with the third dose given at 0300 on 12/21. A Pre level drawn 30 minutes before the third dose (12/21/10 @ 0230) was 0.7 mg/L. A Post level drawn 30 minutes after the third dose finished infusing (12/21/10 @ 0400) was 5.0 mg/L. You believe that the levels are at steady-state based on the dosage given and the patient's age and renal function. Your goal for the Post level is ~ 8 mg/L with a Pre level goal of < 1 mg/L. Input the following:
- Weight: 35kg
- PRE Level Date: December 21, 2010
- PRE Level Time hr.mm (military): 2.3
- PRE Level: 0.7 mg/L
- POST Level Date: December 21, 2010
- POST Level Time hr.mm (military) : 4.0
- POST Level: 5 mg/L
- Infusion time: 0.5 hrs
- Dosing interval (Tau): 8 hrs
The calculator finds a Time difference for ke calc of 6.5 hours. The calculator then solves for K elimination giving a value of 0.302 and solves for Half-Life giving a value of 2.3 hours.
Input the following:
- Concentration (Css): 5
- Change in Time to Css: 0.5 hrs
- DOSE: 70 mg
The calculator solves for Volume of distribution [L/kg] giving a value of 0.35 L/kg.
If you determine that the half-life of 2.3 hours and Volume of Distribution of 0.35 L/kg sound reasonable for this patient's characteristics, you can proceed to answer this next question:
How should you modify the dose to achieve levels within your goal range? Input the following:
- Concentration (Css): 8
Tap the "?" next to DOSE and the calculator gives a dose of 112 mg.
To make this a more reasonable dose to measure, modify:
- DOSE: 110 mg
Tap "?" next to Css to find the Post level that will occur with 110 mg Q 8 hours. The calculator gives an answer of 7.9 mg/L.
CAN'T REPRODUCE
This is additional example content that Bill Buss had provided but I can't figure out how to make this work:
- Change in Time is what variable? There isn't one specifically denoted as that.
- Then it says the Pre Level is re-calculated, but there is no equation for Pre Level.
I didn't get any further so remove it and hid it in case I need this section later.
To find what the new Pre value will be on this dose, re-input the following:
- Change in Time: 7.5 hrs
Tap "?" next to Css. The calculator gives an answer of 0.9 mg/L for the new Pre level. Your calculations suggest that CB's dose may be changed to 110 mg every 8 hours to achieve a Post level of 7.9 mg/L and a Pre level of 0.9 mg/L.
If you wanted a lower Pre value, you could now modify the TAU to a longer dosing interval such as 12 hours; modify change in time to 0.5 hours; modify Css to 8 mg/L; and solve for DOSE to find what dose given every
12 hours would give the desired Post level. The calculator would give the answer of ~ 120 mg. You would then modify change in time to 11.5 hours and solve for Css to find that the new Pre level on 120mg every 12 hours would be 0.3 mg/L

Keywords

Weight
PRE Level Date mm/dd/yyyy
PRE Level Time hr.mm [military]
PRE Level
POST Level Date
POST Level Time hr.mm [military]
POST Level
Infusion Time
Dosing Interval (Tau)
Time difference for ke calc
K elimination
Half-Life
Concentration (Css)
Change in Time to Css
DOSE
Volume of Distribution
Patient identifier
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