This document provides guidelines for testing laboratories wanting to use rapid test methods on water samples that may be contaminated following a nuclear or radiological emergency incident. In an emergency situation, consideration should be given to: — taking into account the specific context for the tests to be performed, e.g. a potentially high level of contamination; — using or adjusting, when possible, radioactivity test methods implemented during routine situations to obtain a result rapidly or, for tests not performed routinely, applying specific rapid test methods previously validated by the laboratory, e.g. for 89Sr determination; — preparing the test laboratory to measure a large number of potentially contaminated samples. The aim of this document is to ensure decision makers have reliable results needed to take actions quickly and minimize the radiation dose to the public. Measurements are performed in order to minimize the risk to the public by checking the quality of water supplies. For emergency situations, test results are often compared to operational intervention levels. NOTE Operational intervention levels (OILs) are derived from IAEA Safety Standards[8] or national authorities[9]. A key element of rapid analysis can be the use of routine methods but with a reduced turnaround time. The goal of these rapid measurements is often to check for unusual radioactivity levels in the test sample, to identify the radionuclides present and their activity concentration levels and to establish compliance of the water with intervention levels[10][11][12]. It should be noted that in such circumstances, validation parameters evaluated for routine use (e.g. reproducibility, precision, etc.) may not be applicable to the modified rapid method. However, due to the circumstances arising after an emergency, the modified method may still be fit-for-purpose although uncertainties associated with the test results need to be evaluated and may increase from routine analyses. The first steps of the analytical approach are usually screening methods based on gross alpha and gross beta test methods (adaptation of ISO 10704 and ISO 11704) and gamma spectrometry (adaptation of ISO 20042, ISO 10703 and ISO 19581). Then, if required[13], test method standards for specific radionuclides (see Clause 2) are adapted and applied (for example, 90Sr measurement according to ISO 13160) as proposed in Annex A. This document refers to published ISO documents. When appropriate, this document also refers to national standards or other publicly available documents. Screening techniques that can be carried out directly in the field are not part of this document.