T04) Find C0 from Ct, t half and t
50. Pharmacokinetics 50.2) Plasma concentrations over time 50.2.1) Plasma concentrations over time
T01: Later plasma concentration Normal 1
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A 122 -kg patient has received an IV bolus injection of a drug. A sample of blood was collected 6.3 hours after the first collection. The drug follows a first order elimination process, and its plasma half life is 5.37 hours. If the plasma concentration was 1.93 mg/L in the second sample, then what was the concentration in the initial sample?
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lb equals 4.35 mg/L kg
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For a first order process,
`C_{t}=C_{0}×e^(-kt)`
It may be re-organized as:
`C_{0}=C_{t}/(e^(-kt))`
Or, `C_{0}=C_{t}×e^(kt)` ..... (1)
In the question, k is not provided, but we can derive it from the equation below:
`k=0.693/t_{½}=0.693/(5.37 \quad h)=0.129 \quad h^{-1}`.
Plugging the values in equation (1), we get:
`C_{0}=1.93 × e^(0.129 × 6.3)`
`∴ C_{0}=4.35`
Thus, the initial concentration was: 4.35 mg/L. (Ans).