The predictive value of total coliforms in drinking water using life table analysis

Total coliform monitoring in distribution systems is no longer a required parameter in many countries but is still mandated in Canada and the USA. The value of reporting total coliform results in drinking water is a topic of debate. Total coliform and Escherichia coli results from small drinking water systems tested over a 3-year period in British Columbia were analyzed using life table analysis. Small drinking water systems that have a non-E. coli total coliform positive result have a slightly higher probability that a subsequent sample will contain E. coli compared to small drinking water systems with no prior total coliforms detected in the distribution system (relative risk = 2.04). One month after a non-E. coli total coliform positive test, the probability of the system having an E. coli-positive test was nearly four times that of systems with no prior total coliforms. This is of minor practical significance due in part to the low rate of E. coli-positive drinking water samples, reflected in the low absolute risk increase at 1 month after a non-E. coli total coliform test (1.6%).