Amikacin drug monitoring

The amikacin dosing regimen is best determined using therapeutic drug monitoring. The suggested peak concentration of amikacin (1 h after administration) is 25 xg/ml and the trough concentration should be <5 xg/ml. 

The steady-state maximum plasma concentration, Cmaxss, of gentamycin and tobramycin are 6 to 10 mcg/mL. The Cmax ss of amikacin is 25 to 30 mcg/mL. The Cmin ss of both gentamycin and tobramycin is 0.5 to 1.5 mcg/mL, while that of amikacin is 5 to 8 mcg/mL. In order for these drugs to be effective, it is important to closely monitor their therapeutic concentrations. An important observation of these antibiotics is that with prolonged therapy, the Cminsl, values increase. This increase is due to the renal impairment. In the case where Cmin ss is less than the desired Cmin ss, the dose may be insufficient. 

The variability associated with drug absorption from the gastrointestinal tract can be overcome by using a parenteral preparation (dosage form). It should preferably be administered either by intravenous infusion or slow intravenous injection to avoid circulatory overload. Intraosseous administration is a useful alternative to intravenous injection of some antimicrobial agents (e.g. sodium ampicillin or amoxycillin, cefotaxime, ceftriaxone, gentamicin or amikacin sulphate) in neonatal foals (Fig. 7.1) (Golenz et al, 1994) and puppies (Lavy et al, 1995). This particularly applies when the neonate is in a state of septic shock and/or dehydration. Total plasma protein concentration is an inaccurate index of hydration status unless monitored (repeatedly measured) and interpreted in conjunction with packed cell volume (PCV). 

Monitoring plasma drug levels is important when aminoglycosides are used. In this case the patient seems to be improving, so a decrease of the amikacin dose in proportion to decreased creatinine clearance is most appropriate. Since creatinine clearance is only one-third of the starting value, a dose reduction should be made to one-third of that given initially. The answer is (B). 

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