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 197 -kg patient has received an IV bolus injection of a drug. A sample of blood was collected 7.2 hours after the first collection. The drug follows a first order elimination process, and its plasma half life is 2.99 hours. If the plasma concentration was 0.862 mg/L in the second sample, then what was the concentration in the initial sample?


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lb equals 4.58 mg/L kg

    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/(2.99 \quad h)=0.232 \quad h^{-1}`.
    Plugging the values in equation (1), we get:
    `C_{0}=0.862 × e^(0.232 × 7.2)`
    `∴ C_{0}=4.58`
    Thus, the initial concentration was: 4.58 mg/L. (Ans).


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