Improved bacteriological surveillance of haemodialysis fluids: a comparison between Tryptic soy agar and Reasoner's 2A media

Background. Accurate microbiological surveillance in haemodialysis centres is important as end-stage renal patients can suffer from pyrogenic reaction s due to bacterial contamination of dialysis fluids. To evaluate the microb iological quality of haemodialysis fluids, special nutrient-poor culture te chniques are necessary. Although the Association for the Advancement of Med ical Instrumentation (AAMI) recommends Tryptic soy agar (TSA) as the standa rd agar, several studies have resulted in a general preference for Reasoner 's 2A (R2A) agar, as it appeared to be more sensitive in demonstrating cont amination of typical haemodialysis associated bacteria. In the Netherlands TSA is still used for culturing dialysate, while dialysis water is cultured on R2A. Therefore, the aims of our study were to evaluate bacterial yields of dialysis fluids on both media, and to qualify their use in routine micr obiological monitoring within our haemodialysis centre. Methods. Between April 1995 and March 1996, 229 samples of pre-treated and final purified dialysis water, and samples of dialysates were collected. Th e specimens were aseptically taken from the tap, various points of the reve rse osmosis (RO) water-treatment system, and the effluent tubes of 32 bicar bonate haemodialysis machines. Samples of 0.1 ml were inoculated in duplica te on spread plates with TSA and R2A agars. After 10 days of incubation at 25+/-2 degrees C, the numbers of colonies were quantified. The ranges of sp read were taken 0-100 and 0-200 colony-forming units per milliliter (c.f.u. /ml). Results. The R2A agar had significantly higher colony counts than TSA agar for both dialysis water and dialysates. Considering 100 c.f.u./ml as the up per allowable bacterial limit for all dialysis fluids, microbiological non- compliance (bacterial growth) would be missed in 16% when using only TSA me dia (TSA less than or equal to 100 c.f.u./ml and R2A >100 c.f.u./ml), while this was 3% when using only R2A (TSA >100 c.f.u./ml and R2A less than or e qual to 100 c.f.u./ml, P < 0.0001). Considering 200 c.f.u./ml as the upper limit, non-compliance would have been missed in 10% when using only TSA (TS A less than or equal to 200 c.f.u./ml and R2A >200 c.f.u./ml), and 2% when using R2A (TSA >200 c.f.u./ml and R2A less than or equal to 200 c.f.u./ml, P = 0.0011). Conclusions. Microbiological surveillance of haemodialysis fluids, includin g pre-treated dialysis water samples collected from RO treatment systems, c an be performed more precisely with R2A media than TSA, when incubated at 2 5+/-2 degrees C for 10 days.