Overview
EN ISO 21187:2021 (ISO 21187:2021) provides guidance for establishing and verifying a conversion relationship between results from an alternative method and an anchor (reference) method for the quantitative determination of the microbiological quality of milk. The conversion relationship can be used to:
- convert results from an alternative method to the anchor-method basis, or
- convert results/limits expressed on an anchor basis into units of an alternative method.
This CEN/ISO/IDF standard updates the earlier edition and is intended for laboratories, method developers, dairy producers and regulators involved in milk microbiology, method validation and routine quality control.
Key Topics
- Principles and organizational set-up: roles, responsibilities and general approach to establishing conversion relationships.
- Factors influencing conversion: environmental (animal species, seasonal variation, bulk storage), sampling and pre-treatment, test sample preservation, and analytical factors (instrument make/model, chemicals, high somatic cell counts).
- Test sample design: guidance on calculating the number and range of test samples and ensuring representativeness and proper sub‑sampling, storage and transport.
- Analysis and calculations: statistical methods for deriving a conversion relationship (linear regression on grouped data as per the revised formula), removal/identification of outliers, and calculation procedures.
- Verification and monitoring: recommended frequency of verification, statistical tests for significance, and procedures to confirm that the conversion remains valid over time.
- Reporting: required information for test reports documenting the conversion relationship and verification results.
- Informative annexes: practical examples and tools - sample-size calculations for regression (Annex A), outlier identification and example conversions (Annex B), and verification significance calculations (Annex C).
Applications
- Accredited testing laboratories converting routine method outputs to regulatory/reference units.
- Dairy industry quality control teams implementing or transferring rapid/alternative microbiological methods for on‑farm or factory testing.
- Regulatory authorities accepting alternative methods while ensuring comparability to anchor method limits.
- Method developers and validation specialists establishing performance equivalence and producing validation documentation.
Practical outcomes include reliable method transfer, harmonized reporting of microbial counts, and defensible conversion of limits between methods for compliance and risk management.
Related Standards
- Anchored reference methods for milk microbiology (national or ISO methods for specific organism counts or total viable counts).
- ISO/IDF series on milk sampling and microbiological methods.
Keywords: EN ISO 21187:2021, ISO 21187:2021, milk microbiological quality, conversion relationship, alternative method, anchor method, verification, quantitative determination.
