Background and importance Osmolarity is one of the pharmaceutical controls carried out on the nominative parenteral nutrition (NPN) compounded at the hospital pharmacy. A previous validation method for calculation of the theoretical osmolarity of standardised parenteral nutrition (SPN) was extrapolated to NPN. Osmolarity was determined using the Pereira Da Silva equation¹ (PDS) when osmolarity is >1453 mosmol/L, or the manufacturer’s data (MD) equation, which is the addition of the different osmolarities of the components. After 2 years, a non-conformity osmolarity rate of 8.9% was observed.

Aim and objectives The aim was to determine the best model to calculate the theoretical osmolarity of the NPN to decrease the non-conformity rate.

Material and methods A retrospective analysis of the NPN osmolarity values was performed on the last 27 months’ production. Mean relative errors (MRE) between the theoretical osmolarities calculated with the PDS and MD equations and the measured osmolarity were compared using a Student’s t test. NPN was divided into seven ranges according to the osmolarity measured by freezing point depression with the OsmoPro osmometer (Advanced Instruments).

Results 2572 NPN were analysed. Osmolarities were distributed as follows: 1.7% from 500 to 749 mosmol/L, 19.6% from 750 to 999 mosmol/L, 25.5% from 1000 to 1249 mosmol/L, 18.4% from 1250 to 1499 mosmol/L, 15.5% from 1500 to 1749 mosmol/L, 15.3% from 1750 to 1999 mosmol/L and 4.0% over 2000 mosmol/L. Between 500–749 and 750–999 mosmol/L, the MRE of osmolarities were similar with both equations (p=0.99 and p=1). However, there was a significant difference in MRE in favour of the PDS equation between 1000 and 1249 mosmol/L (p=0.027), 1250 and 1499 mosmol/L (p=6.5×10^-45), 1500 and 1749 mosmol/L (p=2.4×10^-12), 1750 and 1999 mosmol/L (p=2.05×10^-129) and over 2000 mosmol/L (p=1.66×10^-36).

Conclusion and relevance From 500 to 999 mosmol/L, both equations can be used to predict NPN osmolarities. For NPN with osmolarities from 1000 to over 2000 mosmol/L, the PDS equation was significantly more accurate. Therefore, the actual theoretical osmolarity calculation method should be revised in favour of the MD equation for NPN with osmolarities <1000 mosmol/L and the PDS equation for NPN with osmolarities >1000 mosmol/L.

References and/or acknowledgements

1. Pereira-da-Silva L. A simple equation to estimate the osmolarity of neonatal parenteral nutrition solutions.

Conflict of interest No conflict of interest