Background It is generally not recommended to give two drugs together or drugs in addition to total parenteral nutrition in parallel infusion unless compatibility has been documented. The possibility of performing simple compatibility testing in the hospital pharmacy has attracted attention, and visual examination using a focused light source (Tyndall beam) has been suggested as a simple tool for this purpose¹.

Purpose To investigate the validity and reliability of the Tyndall method using two different light sources, to determine the visual detection limit in terms of particle size and concentration and to evaluate the suitability of the method for simple compatibility testing in a hospital pharmacy setting.

Materials and methods A panel of 20 inspectors examined 20 samples, with and without particles, using two different light sources. The samples contained particles of different origin, varying size and concentrations. Light obscuration measurements were used as reference. The samples were classed as accept, grey or reject and the validity (sensitivity, specificity and likelihood ratios (LR)) and reliability (Fleiss’Kappa and Gwets’AC1) of the classifications were calculated.

Results Gross particles and massive precipitations were easily detected, but for smaller particles in low concentrations, the inspectors were more uncertain. The LR + ratios were high, indicating that if a sample is rejected, there is a high probability that the samples actually contain particles. The LR- ratios were not low enough, meaning that if a sample is accepted there is still a considerable chance that the sample contains particles. The Fleiss' Kappa and AC1 indicated only fair to moderate agreement between the inspectors.

Conclusions The validity and reliability were not satisfactory using either of the light sources. The visual detection limit seemed to be around 5 µm, but a more data is required. The visual examination method cannot be recommended as the only method for determining compatibility, and it is not suitable for simple compatibility testing at the hospital pharmacies.

Acknowledgements Vigdis Staven is a PhD student of Hospital Pharmacy North, Norway, and Dept. of Pharmacy, University of Tromsø, Norway, sponsored by Helse Nord RHF. We gratefully acknowledge the financial support from the Norwegian Medicines for Children Network, Bergen, Norway. Thank you to Frode Skjold, Dept. of Pharmacy, University of Tromsø, for valuable discussions on the statistical issues, to Margaret Aarag Antonsen, Hospital Pharmacy North and the employees of the Hospital Pharmacy of Tromsø, Norway, for discussions, help and practical support.

Reference

1. Veggeland, T. & Brandl, M. Evaluation of a Simple Method for Visual Detection of Microprecipitates in Blends of Parenteral Drug Solutions Using a Focused (Tyndall) Light Beam. International Journal of Pharmaceutical Compounding 2010;14:78-81.

No conflict of interest.