Therapeutic Drug Monitoring
Non-Steady State

IV Infusion - Linear One Compartment

A 50 yo patient (V.D. 59.5 Kg) was started on intravenous antibiotic therapy at a dose of 700 mg over 60 minutes, twice a day, at 0700 and 1900 hr. On Day 2, three blood samples were collected and analyzed for drug concentration. These sample were determined to contain 24.8 mg/L (at 0615), 41.1 mg/L (at 0915) and 33.3 mg/L (at 1830).

Day/Time (hhmm) Dose (mg/hr) Sample (mg/L)
Day 1 at 0700 700 over 1 hr  
Day 1 at 1900 700 over 1 hr  
Day 2 at 0615   24.8
Day 2 at 0700 700 over 1 hr  
Day 2 at 0915   41.1
Day 2 at 1830   33.3

Calculate the kel, V, Clearance and t1/2 for this drug in this patient.

Hint: Sketch the expected concentration versus time curve for the second day.

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Some Equations:

The elimination rate constant, kel can be calculated using Equation 6.2.1

kel from two time points

Equation 6.2.1 An equation for determining kel from two data points

Remember drug concentration at future times can be determined using Equation 6.2.2.

Concentration at a second time

Equation 6.2.2 Concentration at another time

The concentration due to the next infusion can be calculated from Equation 6.2.3.

Equation 6.2.3 Concentration versus time, post infusion

Rearranging Equation 6.2.2 and 6.2.3 gives us an equation for the apparent volume of distribution, Equation 6.2.4.

Equation 6.2.4 Apparent volume of distribution

Problem by Dr Vincent Dennis, programming by Dr David Bourne.

Last update: Sat 27 Jul 2024 09:17:51 pm
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