T08) Find t from Ct, C0 and half-life
50. Pharmacokinetics 50.2) Plasma concentrations over time 50.2.1) Plasma concentrations over time
T01: Later plasma concentration Normal 1
Total tried: Correct: Wrong:
A patient (126 Kg) is receiving an IV bolus administration of a drug. The plasma concentration was found to be 3.5 mg/L, when the blood was first collected. If the drug follows a first order elimination and the elimination half life (t1/2) is 2.89 hr, then how long will it take to reach the plasma concentration to 0.238 mg/L?
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lb equals 11.2 h kg
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For a first order process,
`C_{t}=C_{0}×e^(-kt)`
It may be re-organized as:
`ln(C_{t})=ln(C_{0})-kt`
`\implies t = ln(C_{0}/C_{t})/k` ..... (1)
However, the question did not provide k, but we can derive it from:
`k=0.693/t_{1/2}=0.693/2.89`.
Plugging values in equation (1),
`t =(ln(3.5/0.238))/(0.693/2.89)`
Or, `t = 11.2`
Thus, required time is `11.2 \quad h`.(Ans).