Choosing a predictive model of osmolarity to control parenteral nutrition solutions (PNS)
INTRODUCTION
Osmolarity, number of osmotically active particles per liter of solution, will become a routine control for our PNS. Firstly, it determines the type of venous approach (central venous catheter if it’s over 800 mOsm/L). It also reflects macronutrients and electrolytes composition of PNS, revealing manufacturing errors. It’s difficult to calculate theoretical osmolarity value. The literature reports many predictive models of osmolarity including equation with osmotic coefficients and molarity for both components or equations with multiplicative factors according glucose, proteins and a few electrolytes concentrations (most osmotically active components) like Pereira Da Silva (PDS)*, ASPEN guidelines** equations.
The study aims to choose the best osmolarity model to control our PNS (standard and nominative).
METHODS
Osmolality is measured by OsmoPRO® (Advanced Ins.) osmometer by using freezing point depression. PNS theoretical osmolarity is calculated according to 4 models: equation with both components osmotic coefficients; sum of components osmolarity based on manufacter data; PDS and ASPEN Guidelines equation. We analysed 30 samples for each of 6 standard PNS formulations. Mean relative error (MRE) between each measured osmolarity and predicted value has been calculated. A model is satisfactory without analytical value deviating by more than 15% from predicted value; less than 5% of analytical values deviating by more than 10% and MRE close to 0.
RESULTS
PDS equation is satisfactory for 3 of our 6 standard PNS (MRE = 0.045 [0.016; 0.074]; -0.015[-0.006; 0.038]; 0.002 [-0.028; 0.024]). Model based on osmotic coefficients or manufacters data is satisfactory for 4 of 6 PNS (MRE = 0.035[0.008; 0.062]; -0.069 [-0.010; -0.003]; -0.048 [-0.075; -0.020]; 0.002 [0.004; 0.040]).
CONCLUSION
For PNS with normal to high macronutrients concentration, PDS equation is reliable whereas models based on osmotic coefficients or manufacters data are reliable for PNS with low macronutrients concentrations. Considering their formulation, PDS equation is more appropriate to control our nominative PNS with normal to high macronutrients concentrations.
* Pereira-da-Silva L JPEN 2004;28:34-37.
** Mirtallo J et al. JPEN 2004;28:S39-S70.