I have an inquiry if I may about the final volume of IV preparations; if for example we dilute a 4.5g pip/taz vial with 20mL water for injection, and then inject it in a 100mL D5W IVPB, when we want to calculate a fractional dose from this preparation, do we calculate it from a final volume of 100mL or 120mL (considering the volume of the injected drug)? And why? Our current practice is to calculate it from the original volume of the piggyback.
The difference maybe negligible for adult patients, but for pediatrics and neonates there can be a considerable difference in the administered dose between the two approaches.
As per our practice, we remove an equivalent diluent volume. I strongly recommend the calculation based on the final volume as one of the patient rights is right dose.
As you mentioned, we can’t always guarantee that this small fraction is not significant for all preparations considering, for instance; electrolytes and narrow therapeutic index drugs. I’m not sure if there’s a documented evidence for a contrary opinion.
Also it’s advised to make sure that the final volume is within the concentration stability window as per known references and manufacturer recommendations. Attached is an interesting paper showing supporting examples.
Apologies for the late reply. I work in the paediatric setting and we usually calculate doses in the following manner.
To continue with your example, once we diluted the vial of pipercillin/tazobactam 4.5g strength vial with 20mL water, we would withdraw the required dose right away – for example if the patient weighs 10kg and we are giving 100mg/kg/dose = 1gram required, we would take out 5mL then further dilute this into a compatible fluid then infuse over 30 minutes. In which case, this 5mL volume is very small hence negligible. This is usually the practice for most drugs, unless huge volumes are required to be diluted further which is pretty uncommon.