ISO 14461-2:2005

Overview

ISO 14461-2:2005 (IDF 169-2:2005) specifies a routine quality‑control procedure for evaluating colony‑count results in microbiological testing of milk and milk products. It applies to colony‑count methods using subsequent 10‑fold dilution steps with one or two parallel Petri plates per dilution. The standard provides tabulated acceptance limits (Tables 1 and 2) and a decision flowchart to detect unacceptable variability in parallel plates or dilution series, helping laboratories assess the reliability of their colony counts.

Key topics and requirements

• Scope: Routine evaluation of enumeration results from colony‑count methods with 10‑fold dilutions and one or two parallel plates per dilution step.
• Comparison to tabulated limits:
  • Use Table 1 to compare results from two parallel plates; exceedance indicates potential problems in plating/counting.
  • Use Table 2 to compare sums of counts across two subsequent 10‑fold dilutions; exceedance indicates potential dilution errors or inconsistent series.
• Statistical basis: The tabulated limits are calculated to correspond to a high confidence level (tables give limits with ~99 % probability per comparison).
• Routine frequency: Apply the procedure regularly. If only one plate per dilution is used, perform the test at a specified minimum frequency (for example, once per 100 sample units).
• Evaluation rule: Results outside specified limits should not occur more often than once in 100 cases; more frequent exceedances require scrutiny of the test procedure.
• Root causes emphasized: The standard highlights key factors affecting colony counts-sample homogeneity, dilution accuracy, and inoculation/counting technique-and directs users to investigate these when limits are exceeded.

Practical applications

• Quality assurance and method control in dairy microbiology laboratories performing plate counts.
• Routine laboratory performance monitoring to detect systematic errors in dilution steps, pipetting, plating techniques, or colony counting.
• Supporting corrective action and training by identifying whether variability arises from parallel plating or from dilution series.
• Integrating with lab quality management systems (GLP) and internal QC programs for microbial enumeration of milk, milk products and related samples.

Who should use this standard

• Microbiologists and laboratory analysts in dairy industry testing labs
• Quality managers and technical supervisors responsible for analytical reliability
• Laboratory auditors, accreditation assessors, and method validation teams

Related standards

• ISO 14461-1 / IDF 169-1 - Analyst performance assessment for colony counts (companion document)
• Other ISO/IDF methods for microbial enumeration and dairy testing

Keywords: ISO 14461-2, milk and milk products, colony counts, microbiological laboratories, quality control, dilution steps, parallel plates, colony-count methods.