Aim To determine the impact of displacement values on doses in paediatric patients when using parenteral. To assess the option of using average displacement values (DV) in the preparation of parenteral medicines.

Method Over 500 Medusa1 monographs were analysed and 42 medicines were identified with a displacement value not indicated as negligible. The following were calculated: the percentage difference in dose if the DV was not taken into account for each drug and brand, the range of percent differences where there was more than one brand for each strength and the percentage difference in dose incurred if an average DV was used.

Results The 42 drugs were separated into 3 groups. The first group of 27 drugs had DVs causing less than 5% dose variation for all brands. The second group of 7 drugs had DVs resulting in more than 5% dose variation. The third group of 8 drugs had wider variations i.e. including drugs where the different brands had DVs both below and above 5% (2 to 27.9%) in dose variations for varying brands of a drug and strength.

A total of 64 preparations had less than 5% dose variation. For these the DVs could potentially be disregarded as it is unlikely to have a significant clinical effect. However there are other sources of errors when administering parenteral medications (e.g. dose rounding on prescribing, inaccuracy when preparing and drawing up the dose) and this may further contribute to cumulative dosing inaccuracies. An alternative option would be to provide an average DV for each drug and vial strength. Due to the use of an average DV under or over dosing can occur depending on the preparation. However if this method was used the dose difference is only 0.31% on average (0.47–0.98%), significantly less if the DV was ignored.

For the second group of preparations with dose differences greater than 5% (5–18%), the DV should not be ignored as it can be clinically significant. However where there is more than one manufacturer available for a particular strength of drug the range in difference of the dose variation was small (0.7–4.1%). When an average displacement value was attributed this resulted in a maximum dose difference of 2.5% (0–2.5%). For the third group there are varying differences in the range of dose variation. Where the range is small and an average was assigned the average range in dose variation was 0.30% (0–4.6%). However there are preparations that the range was too wide for an average to be safely used (2.8–9.5%).

Conclusion For the majority of drugs an average DV can be used safely. Using average DVs would simplify the preparation process for nurses and reduce the risk of them inadvertently using the wrong displacement value.