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What is the efficacy of a filter needle in the retention of typical bacterial pathogens?


The use of filter needles to prepare medication is suggested as a way of preventing particulate contamination of infusions, which is regarded as a source of infection or inflammation in critical care patients [1]. To assess the impact of such needles purely on bacterial contamination we carried out a comparison of the retention of four bacterial pathogens through a standard 25-swg needle (BD), a standard fill needle with 5 μm filter (BD) and the filter of an Epidural minipack system (Portex). Comparisons were made at high and then low ('real world') bacterial contamination levels.


We prepared four bacterial suspensions mixed together in peptone saline to produce Staphylococcus aureus (2.35 × 106 colony-forming units (cfu)/ml, Bacillus cereus (5.77 × 105 cfu/ml), Escherichia coli (4.38 × 106 cfu/ml) and Pseudomonas aeruginosa (3.86 × 106 cfu/ml) per 10 ml test fluid. This volume was then injected through each type of needle and the epidural filter, after which the filtrate was taken for culture. Small standard amounts were plated on Columbia blood agar while the remainder of the sample (9.9 ml) was mixed with double-strength nutrient broth to be incubated for 24 hours at 37°C. This was repeated with three sets of needles and filters. We then prepared a low-density inoculate of 1 ml equating to a total density of 2.63 × 102 cfu of an equal mix of the above bacteria. This was injected through six sets of the devices in question, and both quantitative counts and cultures were performed. Student's t test was used to compare counts.


No bacteria could be cultured following the use of the 0.2 μm epidural filter either from Columbia blood agar or from broth at high or low contamination levels. In contrast, it was easy to isolate all four pathogens from both needles. In quantitative counts there was no difference in the mean counts (175 cfu/ml vs 190 cfu/ml) between filtered and unfiltered needles.


In terms of preventing bacterial transmission where an infusion is contaminated (even at low levels), the use of a 5 μm filter needle is no better than a normal needle. In contrast a 0.2 μm filter is highly efficient at preventing transmission, at least when resisting a single challenge.


  1. Stein HG: Medsurg Nurs. 2006, 15: 290-294.

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Elbaz, W., Moore, J., Goldsmith, C. et al. What is the efficacy of a filter needle in the retention of typical bacterial pathogens?. Crit Care 12 (Suppl 2), P227 (2008).

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