Understanding ISO/IEC 17025:2017 Clause 7.3.2 - Sampling Methods

Sampling is one of the most critical steps in the testing process, especially in food laboratories where safety, quality, and compliance depends on the accuracy of test results. Clause 7.3.2 of ISO/IEC 17025:2017 focuses on the content and structure of the sampling method used by laboratories when they are responsible for sampling food materials, or products. The clause ensures that laboratories maintain consistency, reliability, and transparency in how samples are selected, collected, and prepared for testing or calibration.

Clause 7.3.2 specifically states that the sampling method must describe three essential components: a) the selection of samples or sites, b) the sampling plan, and c) the preparation and treatment of the sample(s) to yield the item required for testing or calibration. This structured approach ensures that all relevant variables are considered, controlled, and documented, providing a solid foundation for producing valid and defensible results.

These three elements are explained in detail with examples:-

1. Selection of Samples or Sites

The first requirement is that the sampling method must clearly define how samples or sites are selected. This is important because the sample is representative of the entire lot or batch. In food testing, variability is common due to processing, packaging, or storage conditions. A random, stratified, or systematic selection process can help ensure that all parts of the batch are equally represented.

For example, consider a laboratory that is contracted to test a shipment of 100 bags of basmati rice for pesticide residues. To ensure a representative sample, the lab may use stratified random sampling—selecting 10 bags from different sections of the storage area (e.g., front, middle, back) and drawing samples from each using a sampling probe. This process ensures that the sample reflects the quality of the entire batch rather than just a single bag.

2. The Sampling Plan

The sampling plan provides the blueprint for how the sampling should be conducted. It includes detailed instructions/procedure about:

· The number of samples to be collected

· Tools and containers to be used

· Safety and hygiene precautions

· Environmental controls (e.g., temperature during sampling)

· Documentation and labeling requirements

· Applicable references or standards

A well-constructed sampling plan ensures repeatability and reduces the likelihood of bias or contamination. For example, a lab might need to test raw milk from multiple farm locations for microbiological contamination. The sampling plan would specify that samples be collected from each farm's milk storage tank during morning and evening shifts, using sterilized stainless-steel dippers into pre-labeled sterile containers, and transported to the lab in insulated cool boxes maintained between 2°C and 8°C.

Following such a plan helps ensure consistency across all sampling personnel and conditions, maintaining the integrity of the test results and making the lab audit-ready at all times.

3. Preparation and Treatment of Samples

Once the sample has been collected, the next step is to prepare it for analysis. Clause 7.3.2 emphasizes that the sampling method must describe how the sample will be treated to yield a testable item. This is particularly important because improper handling, mixing, or treatment can significantly affect the accuracy of the results.

For example, suppose a lab is testing spice powders for aflatoxin contamination. After sample collection, the protocol might require that the sample be ground to a fine powder, homogenized, and then divided into sub-samples. One portion would be used for immediate testing, while the rest is sealed and stored for potential reanalysis. Such careful preparation ensures that the sample being tested is uniform and that results are truly reflective of the original batch.

Note on Clause 7.4

It is important to note that the Clause 7.3.2 also refers to Clause 7.4, which outlines how samples should be handled once they are received in the laboratory. While Clause 7.3.2 focuses on field sampling and preparation, Clause 7.4 ensures that samples maintain their integrity from receipt through testing. Both clauses work together to safeguard the chain of custody and reliability of results.

ISO/IEC 17025 Clause 7.3.2 is about standardizing how laboratories perform sampling when they are responsible for collecting materials for testing or calibration. In food testing labs, where consumer health and regulatory compliance are at stake, and adherence to this clause ensures that all samples are collected, handled, and prepared in a scientifically valid and traceable manner. By clearly defining the selection process, establishing detailed sampling plans, and controlling how samples are treated before testing, labs can maintain the integrity of their results, meet accreditation requirements, and build trust with their clients and regulatory bodies alike.