Making Informed Decisions: A Practical Approach to Selecting Water Microbiology Tests

Neil Leat, Head of Microbiology, Watercare Laboratory Services

The evaluation of waterborne pathogen risks often leads to requests for microbiological testing. However, the limitations of microbiology tests are not always properly considered. This can lead to the selection of inappropriate tests, which may misallocate resources and create inaccurate perceptions of the risk. 

This paper presents a set of questions that serve as a practical tool for assessing the appropriateness and effectiveness of microbiology tests. While these questions are not necessarily new, they are not always applied. By ensuring that these questions are addressed, water quality professionals can make better-informed decisions when selecting tests, ultimately improving water resource management and public health protection. The questions are structured around the following concepts:

1. What are quantitative Health Outcome Targets, and how do they influence the selection of microbiology tests? This question considers Health Outcome Targets based on diarrhoeal disease frequencies (e.g., 1 infection per 10,000 individuals per year) and emphasizes the importance of establishing such targets before selecting microbiology tests.

2. How to evaluate logical relationships between microbiology tests and Health Outcome Targets. The discussion emphasizes the need to ensure that every link in the relationship is valid. If not, tests might be conducted that don’t meaningfully reduce risk. Inconsistencies should not be overlooked.

3. How to evaluate the performance characteristics of microbiology tests. Even if a test is logically related to a Health Outcome Target its performance characteristics may not be adequate? The discussion provides a summary of the properties of microbiology tests that should be considered. These include the feasibility of collecting representative samples, the efficiency with which target microorganisms are detected, the alignment of turnaround times with public health requirements and the implications of method uncertainty. 

The paper provides a discussion of each concept, offering examples and practical guidance for water quality professionals seeking to implement microbiology tests. A concise overview of the concepts and questions is also presented in the quick- reference guide in Appendix 1. Readers are encouraged to assess the value of the tests they conduct against this reference.