Finding Phosphorus !

Introduction
Our pharmacy prepares on-demand parenteral nutrition (PN) bags for neonatology. After finding out an hyperphosphoremia of one of his patients under PN, a pediatrician asked us if it was possible to add phosphorus (P) to usual dosages.
In routine sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) are measured, using a microwave plasma atomic emission spectrometer: MP-AES 4210 (Agilent^®).
So, we attempted to set up the dosage of P in the PN bags routinely.

Materials et Methods
We set up the P dosage the same way we did for the 4 others elements using our MP-AES 4210, with a P recommended wavelength at 213,618 nm.
Firstly, we created a P calibration range corresponding to the usual low, medium and high concentrations of our production bags, then we tested samples from our production, we also made solutions of known concentration, from the industrial P standard.
We obtained concentrations that we compared to the expected theoretical concentrations, we aimed a relative bias under 5%.

Results
We tested 30 PN bags, and 10 solutions (P: 1, 5 and 20 mg/L) over a year.
There were 3 points of P on our calibration range: 1, 10 and 20 mg/L. We obtained a relative bias of 20%.
The laboratory informs us that P signal intensity is weaker than the other elements: 200 times less intense than Na and K, 500 times less intense than Ca and 20 times less intense than Mg.
So, we had to use higher concentrations of P in our calibration range: our 3 points were at 100, 200 and 500 mg/L.
After those modifications our relative bias was at 10%, a better result but we didn’t reach our 5% objective.
However, another problem appeared: our routine tests are multi-elementary, every point of our calibration range contains the 4 elements, and we dose them at the same time in one sample. The fact that we have to use a high quantity of P interferes with the dosage and distort the results.

Discussion/Conclusion
The routine dosage is not possible in our work conditions. However, the medical issue raised remains relevant. We will continue to improve results precision to offer our practitioners dosage on an ad hoc basis, in mono-elementary.
Two tracks are to study: a longer reading of the signal, and the possibility to less dilute our samples while respecting functional and technical constraints of the device.

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