Clinical pharmacokinetic pearls: bolus versus infusion equations

Using a nonprogrammed calculator to solve lengthy pharmacokinetic equations is time consuming and carries the potential for entry error. Shorter equations reduce both the time and the risk. The aminoglycosides and vancomycin are usually administered as short infusions. The pharmaco-kinetic equation describing multiple-dose administration of short infusions is much longer than the corresponding equation for bolus dosing. This study examined the percentage errors that would occur using the bolus equation instead of the infusion equation at various drug half-lives. When the half-life is 5 or more times the duration of infusion, the error in predicting concentration is 6.8% or less. Although clinicians should make their own decisions on how much error is acceptable and whether introducing guaranteed error is appropriate to save time and reduce risk of potential error, it appears that the simpler bolus equation may be used safely when the half-life is at least 5 times the duration of the infusion.