General questions 8-9 : Gentamycin and initial dose estimation

General question number eight. Here we have a 27 year old woman weighs 60 kg. Her lab values includ BUN of 20 and the serum creatinine of 1.4. She’s to be started on gentamicin. Based on the limited information provided above how would you estimate gentamicin half-life? Again we kind of used the method of elimination A we can estimate gentamicin clearance directly and convert it to half-life which is not true. B Directly convert serum creatinine to half-life Well there’s no such formula to do so and the D half-life cannot be estimated from the information provided above that is that not the case either otherwise it would not be asked to work on this problem. So C is the correct answer.

that is you can estimate creatinie clearance and then convert it to K elimination rate constant and it was K of course you can divide the half-life. and here we have the equation K is equal to 0.00293 times creatinine clearance plus 0.014 Please be careful that this equation is specific for aminoglycosides, for gentamicin as a matter of fact. So moving on to general question number nine what information is required to estimate the initial dose for renal failure as a fraction of the normal dose? And if you look at the equation printed in orange dose in uremia is equal to dose in normal times 1 minus fe times 1 minus KF.

Here fe is the fraction of the drug excreted unchanged and KF is kidney function expressed as a fraction of normal. So based on this formula you’ve realized that the C is correct . 1 and 2. You need fraction of the dose excreted unchanged and renal function as a fraction of normal in order to estimate the initial dose.