Water testing services provide evidence about the microbiological, chemical and physical characteristics of particular samples. A complete service combines a defined purpose, representative sampling, suitable containers, method-specific transport, laboratory analysis and interpretation. Drinking-water investigations, Legionella monitoring, cooling-system chemistry and environmental sampling have different requirements. Microbiological tests answer different questions and should not be treated as interchangeable indicators of safety. Chemical and physical testing should reflect the source, intended use and suspected problem. HSE explains that Legionella testing frequency depends on the system and risk assessment; microbiological monitoring is not usually required for generally enclosed hot and cold water systems, though sampling may be needed when controls are uncertain or recommended conditions are not achieved. Sampling does not replace risk assessment, operational checks or maintenance. First-draw and post-flush samples can provide different information. Sample bottles, preservation, holding times and transport conditions must follow the relevant method. UKAS accreditation to ISO/IEC 17025 applies to defined activities, so the current scope should be checked for the relevant test, matrix and any required sampling or transport. DWI sets out specific accreditation requirements for regulatory private drinking-water programmes. Reports need correct sample identities, units, methods, reporting limits and interpretation criteria. Not detected is not proof of absolute absence, and a single acceptable sample is not a permanent guarantee for an entire system. Significant findings require a proportionate investigation and documented response, not merely repeat testing until an acceptable result appears. Costs and turnaround depend on scope and methods. Reviewed 7 October 2026 using HSE, DWI and UKAS guidance.