Impact of calcium concentration on the determination of magnesium in individualized parenteral nutrition preparations for neonatal intensive care service at a general hospital

Introduction
Our hospital pharmacy prepares personalized parenteral nutrition bags for neonatal intensive care service. Cation assay (Na+, Ca2+, K+, Mg2+) is one of the release controls performed by enzymatic method in the biochemistry laboratory. Abnormally high levels of magnesium [Mg2+] were observed in some preparations, including those that contain none. The refabrication of these pockets by excluding each component one by one which highlighted the impact of the addition of calcium gluconate on the dosages of Mg2+. The aim of this task is to indicate that the observed deviation in Mg2+ assay is correlated with calcium [Ca2+] concentrations in the bag, and to analyse this interference in order to define rules for the pharmaceutical validation of the Mg2+ results.

Material and méthod
The study focuses on bags produced between March 2017 and December 2018. Production was manual (Group 1) until December 2017 and then automated (Group 2). The [Ca2+] and [Mg2+] theoretical and dosed values were extracted from the Hesiode^® software. Cancelled, bags-test, or whose results were not found were excluded. The study was carried out in 3 steps:
1/ Checking the interaction’s nature by comparating Mg assay’results with enzymatic method and reference method (atomic emission spectrometry) from manually prepared pockets by varying the [Ca2+] and [Mg2+]
2/ Analysing Mg2+ dosages of prepared bags without Mg2+
3/ Investigating the impact of Ca2+ on Mg2 determination and precision of the direction of this effect by the calculation of correlation coefficients (site BiostaTGV)

Résults
3384 pockets were studied: 1105 from Group 1 and 2279 from Group 2
1/ When there is only Ca2+ in the bag, the enzymatic method detects Mg2+ not detected by the reference method
2/ In the pockets without Mg2+ of the 2 groups (n=279), the result of the Mg2+ assay increases with the [Ca2+] (linear plot, R2=0.9)
3/ In the presence of Mg2+ a correlation between [Ca2+] and the error in the dosage of Mg2+ is observed. The Pearson correlation test gives [Mg2+] (mmol/L) values between (0.5-0.99), (1-1.99), (2-2.99), (3-3.99), (4-4.99) respectively a coefficient of 0.91-0.81-0.66-0.3-0.02 for group 1 and 0.82-0.87-0.84-0.33-0.37 for group 2. P-values are significant and < 0.05

Discussion/Conclusion
There is a strong correlation (relationship) between the rate error observed on the Mg2+ assay and the [Ca2+]. When [Mg2+] increase, the interaction is prone to disappear. This analysis will allow us to develop decision rules for the pharmaceutical pockets release. The advantage of keeping Mg2+ dosage in our release controls is to be able to detect a tangible error in volume taken by the automaton.

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