Last Updated on September 25, 2025 by Melissa Lazaro

ISO/IEC 17043 : How to Conduct Homogeneity and Stability Testing ?

Let’s be real—this is one of the most misunderstood areas of ISO/IEC 17043 compliance.

If you’re a PT provider or quality manager preparing for accreditation (or maintaining it), you’ve probably asked:
“What exactly do I need to do for homogeneity and stability testing?”

You’re not alone. I’ve worked with many labs that nail the logistics of PT scheme design, but hit a wall when it comes to proving their items are homogeneous and stable. And it’s not because they don’t care—it’s because they’re not sure where to begin.

This article walks you through exactly what homogeneity and stability testing is, how to do it right, and how to keep your records clean and audit-ready.

Why Homogeneity and Stability Testing Matters

The foundation of a credible PT scheme

If your PT items aren’t consistent and reliable, then the entire scheme becomes questionable.
Homogeneity ensures that all participants receive equivalent samples. Stability ensures those samples don’t change over time or during transport.

Assessors will expect clear, statistical proof of both.

Understanding ISO/IEC 17043 Requirements

What the standard actually says

ISO/IEC 17043:2023 (specifically Clause 7.5.1) requires PT providers to:

• Confirm the homogeneity of PT items before distribution.
• Assess stability under conditions of storage and transport.
• Use appropriate statistical techniques.
• Document all findings and decisions.

And unless you can justify otherwise, you’re expected to conduct both.

How to Conduct Homogeneity Testing

Step 1: Design your sampling strategy

Randomly select items from the PT batch. A common practice is to test at least 10 units or 10% of the total batch, whichever is greater.

Don’t just grab the first items packed. Use random numbers or pull from various points in the batch to reduce bias.

Step 2: Choose your test method

Your method should be:

• Relevant to the PT scheme
• Repeatable
• Ideally validated or ISO/IEC 17025-accredited

You’ll typically test each unit in duplicate or triplicate.

Step 3: Perform statistical analysis

Most labs use:

• One-way ANOVA
• Standard deviation comparisons
• Or in some cases, HorRat values

You’re looking for variation between items to be statistically insignificant.

Example from the field:

One provider I worked with tested 12 PT items for a water quality scheme using triplicate measurements and one-way ANOVA. Their results showed no significant difference (p > 0.05), which they documented as proof of homogeneity.

How to Conduct Stability Testing

Step 1: Simulate real-world conditions

Stability testing should mimic actual transport, storage, and participant use conditions.
If your participants receive items within 5 days and use them within 10, your testing period should cover at least 15 days.

Step 2: Select time points for testing

Most stability studies test at:

• Day 0 (baseline)
• Midpoint (after shipping simulation)
• End of shelf-life or use window

Your timing depends on the nature of the PT item—biological samples may require tighter windows than stable chemicals.

Step 3: Measure and compare against baseline

Test items at each time point using the same method.
You’re looking for changes beyond acceptable limits—whether that’s ±5% of initial values, changes in appearance, or loss of analyte integrity.

Example from the field:

A microbiology PT provider tested a powdered milk sample at Day 0, Day 7 (simulating shipping), and Day 21. Colony counts were statistically similar across all time points. They used this data to show stability over the use period.

How to Document Testing for Audit Success

Keep it structured and complete

Your documentation should clearly show:

• Sampling plan (how you selected items)
• Measurement method
• Test results and raw data
• Statistical calculations
• Conclusion (pass/fail)
• Sign-off by authorized personnel

What assessors want to see

Not just numbers—they want to see that your process is repeatable, statistically sound, and aligned with your risk-based thinking.

Consider creating a standard Homogeneity & Stability Report template. This keeps your records consistent and easy to review.

What to Do When a PT Item Fails Testing

Establish clear decision rules

If an item fails homogeneity or stability testing, you need to have a plan. That includes:

• Rejecting the batch
• Investigating the root cause
• Reprocessing or repackaging
• Communicating with participants, if needed

Real-world example:

A provider running a chemical PT scheme discovered excessive variation in homogeneity testing. They revised their mixing process, re-produced the batch, and documented the change in their quality records. The assessor commended their traceability and response.

Pro Tips for Better Testing and Documentation

Test early with pilot batches

Don’t wait until your full PT production run. Use a small test batch to check homogeneity and stability first. It saves time, material, and risk.

Use blinded analysts for measurements

If you can, have someone perform tests without knowing which item came from where. This minimizes unconscious bias.

Track and sign off every step

Create a physical or digital checklist for every PT scheme that includes sampling, test dates, data review, and approvals.

Simulate the worst-case scenario

Want to impress assessors? Test your items under extreme conditions (temperature, delay, handling). That shows you’ve thought about real-world risks.

Document in real time

Don’t backfill logs later. Record your sampling, testing, and analysis as it happens. It adds credibility and eliminates gaps.

Common Mistakes to Avoid

Only testing one or two items

A single test item doesn’t prove anything. You need a statistically meaningful sample—especially if your batch is large.

Skipping transport simulation

PT items may sit in hot trucks or cold warehouses. If your stability testing doesn’t account for that, your risk exposure goes up.

No statistical analysis

Visual checks or averages aren’t enough. You need proper calculations to back your conclusion.

Using an unfit method for testing

Don’t assume a method used in your regular lab work is acceptable. You must demonstrate it’s suitable for checking homogeneity or stability.

Failing to document rationale for exclusions

If you decide not to test stability (e.g., for solid metal artifacts), you still need to justify it in writing—ideally with evidence or historical data.

Frequently Asked Questions

Do I need to conduct stability testing for every PT item?

Not always. If your item is inherently stable and you can demonstrate that (e.g., sealed glass vials of pure salts), you may justify omitting stability testing. But that justification must be documented.

Can I outsource homogeneity or stability testing?

Yes. Many PT providers use external ISO/IEC 17025-accredited labs. But the responsibility for the quality of the PT scheme—and interpretation of results—still lies with you.

How much variation is acceptable?

It depends on the measurand. But as a rule, if variation affects a participant’s ability to be evaluated fairly, it’s too much. Your statistical method should help define that threshold.

Conclusion

Homogeneity and stability testing are not just paperwork requirements—they’re the foundation of a reliable PT program. Done properly, they protect the integrity of your scheme and boost your credibility with both participants and assessors.

And here’s the good news: once you build a repeatable process and templates, this becomes just another quality control checkpoint—not a stress point.

If you’re still unsure where to start, I’ve created a ready-to-use Homogeneity & Stability Testing Checklist and Report Template you can download. Or, if you want help reviewing your system, feel free to reach out.

No stress, no guesswork—just confidence in your PT scheme.