Microbiological monitoring of dialysis water systems--which culture method?

PROBLEM: Assurance of adequate water quality is one of the most important aspects of ensuring a safe and effective delivery of haemodialysis. There are several different culture methods recommended in the various guidelines on the microbiological testing of water for dialysis.

AIMS: The aim was to investigate whether or not there was a significant difference in the microbiological results obtained using the various recommended culture methods.

METHODS: Current guidelines suggest that samples for microbiological analysis should be cultured using a low nutrient media such as Reasoner's 2A but vary in recommending temperature and time. Over a six month period, samples were sent to an independent laboratory and cultured using three different times and temperatures. Samples were cultured at 22 degrees C for 7 days (ERA-ERCA, EDTNA-ERCA), 37 degrees C for 2 days (AAMI and ISO) and 30 degrees C for 5 days (ISO, EP).

RESULTS: The bacterial culture method used was R2A media at the incubation temperatures and times indicated. During an extensive microbiological survey of several water systems, the results from the laboratory gave conflicting results for the three methods. Results showed that several of sample sets returned results with a difference in recovery by a factor of between 10 and 1000.

CONCLUSIONS: Culturing using the stated temperature and times produces large variations in reported levels of contamination. Culturing at the different temperatures and incubation times can produce results which may give a false sense of security by not indicating a contamination problem. Standardising on a single method is desirable in order to produce consistently valid results.