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Suppose a patient with severe renal dysfunction receives a 1 gm dose of vancomycin , and the peak concentration , drawn 1 hr after the infusion , is 40 mg/L. A concentration determined 24 hr later is 34 mg/L, but we want to wait till the concentration reaches 10 mg/L before administering a second dose. When will concentration reach 10 mg/L?

Question

Suppose a patient with severe renal dysfunction receives a 1 gm dose of vancomycin , and the peak concentration , drawn 1 hr after the infusion , is 40 mg/L. A concentration determined 24 hr later is 34 mg/L, but we want to wait till the concentration reaches 10 mg/L before administering a second dose. When will concentration reach 10 mg/L?

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Solution

To answer this question, we need to understand the concept of drug half-life. The half-life of a drug is the time it takes for the concentration of the drug in the body to be reduced by half.

In this case, we are given that the concentration of vancomycin drops from 40 mg/L to 34 mg/L in 24 hours. This is a decrease of 6 mg/L in 24 hours.

However, we want to know when the concentration will reach 10 mg/L.

First, we need to calculate the half-life of the drug. The formula for half-life is:

t1/2 = 0.693 / k

where k is the elimination rate constant. The elimination rate constant can be calculated as:

k = (ln(C1) - ln(C2)) / (t2 - t1)

where C1 and C2 are the concentrations of the drug at times t1 and t2, respectively.

In this case, C1 is 40 mg/L, C2 is 34 mg/L, t1 is 1 hour (the time at which the peak concentration was measured), and t2 is 24 hours (the time at which the second concentration was measured).

Substituting these values into the formula gives:

k = (ln(40) - ln(34)) / (24 - 1) = 0.0171 hr^-1

Substituting this value into the formula for half-life gives:

t1/2 = 0.693 / 0.0171 = 40.5 hours

This means that the concentration of the drug in the body is halved every 40.5 hours.

To find out when the concentration will reach 10 mg/L, we can use the formula for exponential decay:

C = C0 * e^(-kt)

where C is the final concentration, C0 is the initial concentration, k is the elimination rate constant, and t is time.

Rearranging this formula to solve for t gives:

t = -ln(C / C0) / k

Substituting the given values gives:

t = -ln(10 / 40) / 0.0171 = 92.3 hours

Therefore, the concentration of vancomycin will reach 10 mg/L approximately 92.3 hours after the initial dose.

This problem has been solved

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