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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 148 Kg patient was intravenously administered a drug. The plasma concentration was found to be 0.74 mg/L, when the blood was first collected. If the drug follows a first order elimination and the plasma half-life of the drug is `21 \quad h`, then determine the plasma concentration after 11.6 hours.


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lb equals 0.505 mg/L kg

    For a first order process,
    `C_{t}=C_{0}×e^(-kt)`
    In the question, k is not provided, but we can derive it from the equation below:
    `k=0.693/t_{½}=0.693/(21 h)=0.033 h^{-1}`.
    Plugging the values in this equation, we get:
    `C_{t}=0.74 × e^(- 0.033 × 11.6)`
    `∴ C_{t}=0.505`
    Thus, the later concentration is: 0.505 mg/L. (Ans).