A case report of false negative Legionella test results in a chlorinated public hot water distribution system due to the lack of sodium thiosulfate in sampling bottles.

We examined samples from the showers and the central water distribution system of a public building with an indoor swimming pool. The pool was used for school and recreational activities and as a sports therapy facility for patients with coronary heart disease. The building's hot water system was contaminated with Legionella pneumophila. Due to the building's intricate piping system, several attempts to completely eliminate legionellae by thermal and chemical disinfection had failed, so an external sanitation company was charged with the installation of a continuous chlorination device in order to keep Legionella concentrations low. The laboratory which was contracted by the sanitation company to monitor bacteria levels after installation of the chlorination device used sampling bottles without sodium thiosulfate and repeatedly reported an absence of Legionella. However, up to 69,000 colony forming particles (CFP) of Legionella pneumophila (Lp) per litre and up to 171 CFP/ml of heterotrophic bacteria could be detected when parallel samples were collected in bottles containing sodium thiosulfate at standard concentrations. Laboratories, epidemiologists, public health officials and technical staff who may be in charge of delivering, preparing or using sterile sampling devices for the collection of environmental samples to be tested for legionellae should be aware that cultures can return false negative results if the sampling containers used to collect chlorinated drinking water or chlorinated pool water samples do not contain a neutralizing agent to instantly inactivate residual halogen biocides. False negative results may lead to a false sense of security regarding the safety of water systems or the success of disinfection measures, and may thus endanger public health or even hinder the epidemiological clarification of outbreaks.