Background Comparison of two recommended European Pharmacopoeia 9th Ed methods of sterility assay (membrane filtration versus direct seeding (DS)) highlights an important difference of sensitivity threshold. This threshold by membrane filtration method is at least lower or equal to 0. 14UFC/mL.

Purpose The aim of the study is to determine the sensitivity threshold of the DS method. This method is currently used in our Hospital Pharmacy Unit (HPU) for binary parenteral nutrition (PN) bags.

Material and methods For this study, four bags of 30 mL, with a representative composition to those produced each day were each sown by a calibrated strain Bioball^® 30 UFC (Candida albicans, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa) in order to obtain a concentration of 1UFC/mL. Then, four levels of dilution were carried out from each PN bag: 0.8 UFC/mL, 0.6 UFC/mL, 0.4 UFC/mL, 0.2 UFC/mL. For each level of dilution, 5 × 1 mL of each bag were taken in order to sow five cultures media, sabouraud dextrose agar slope (biomérieux) or Brain-Heart Infusion Broth (BHIB – comic) according to the strain. Culture media BHIB were incubated for 3 days at 37°C, then 11 days at 22°C, while the culture media sabouraud were incubated for 14 days at 22°C.

Results After 14 days of incubation, for each strain tested, there were between 0 and 4 growths according to the dilution’s level. No concentration made it possible to have 100% of positivity, so sensitivity’s threshold is thus higher than 1UFC/mL. Compared to membrane filtration, the detection’s threshold by DS is at least seven times superior.

Conclusion Even if this study did not estimate with precision a sensitivity threshold for the DS method, it has highlighted the limits for sterility assay of binary parenteral nutrition bags. So it possible to confirm the superiority of the membrane filtration method. Tests with higher micro-organisms’ concentrations in binary PN bags are planned to establish with precision the minimal detection threshold.

No conflict of interest