Overview
EN ISO 17994:2014 - Water quality: Requirements for the comparison of the relative recovery of microorganisms by two quantitative methods - specifies an evaluation procedure and the mathematical basis for comparing two quantitative microbiological methods. Published by ISO and adopted by CEN (EN ISO 17994:2014), the standard is intended for methods with established performance characteristics (see ISO/TR 13843) and covers methods based on colony counts or most probable number (MPN) estimates. It focuses on assessing the average relative performance (relative recovery) between two methods; it does not provide precision data or single-laboratory verification procedures.
Key topics
- Scope and principle: Comparison of paired results from two quantitative microbiological methods for the same target organisms or groups.
- Confirmed counts and presumptive counts: Use of confirmed counts (presumptive count × confirmation coefficient) and handling of MPN or colony counts.
- Study design essentials: Requirements for sample types, number of samples, and participating laboratories in comparison trials.
- Data processing and calculations: Preliminary data editing, calculation of basic relative differences (natural-log scale), and computation of confidence intervals (half-widths).
- Evaluation criteria: Two-sided and one-sided evaluations against predetermined stipulated limits (confidence interval bounds, commonly referenced examples: 2L = 10% for drinking water, 2L = 20% for environmental waters).
- Reporting: Minimum contents of a test report and informative annexes (flowchart, example evaluations, sample-size derivation).
Applications
- Validation and comparison of a new or trial method against a reference or standard method for microbiological analysis of water.
- Design and analysis of interlaboratory comparison trials where paired measurements are collected.
- Regulatory or contractual assessments where stakeholders need evidence that two methods are not quantitatively different within predefined limits.
- Method transfer exercises and demonstration of equivalent recovery for microbial targets (e.g., indicator bacteria, pathogens) in drinking, recreational, or environmental water testing.
Who should use it
- Accredited water testing laboratories and quality managers
- Method developers and validation teams in microbiology
- Regulatory agencies and public health authorities
- Water utilities and environmental monitoring programs
- Standards bodies and interlaboratory study coordinators
Related standards
- ISO/TR 13843 - Guidance on validation of microbiological methods (performance characterization)
- ISO 8199 - General guidance on enumeration of microorganisms by culture
Keywords: EN ISO 17994:2014, water quality, relative recovery, microorganisms, quantitative methods, comparison study, MPN, colony counts, ISO/TR 13843.
