Researchers in Vietnam and Switzerland have developed the first large-scale method for validating the microbial reporter-based test for measuring arsenic concentrations in natural water resources. Their test is based on a modified bacterium (Escherichia coli), engineered to bioluminesce on induction by arsenic ions. The team has developed specific protocols for overcoming interference from iron and has now confirmed the viability of the test against results obtained using atomic absorption spectroscopy.