What Proficiency Testing Programs Actually Reveal About an ISO 17025 Lab

A supplement brand came to us not long ago after receiving two very different test results from two ISO 17025 accredited laboratories. Same product. Same lot number. Same label claim of 300 mg magnesium per serving. One lab reported 342 mg. The other reported 289 mg. That’s a 53 mg swing — 18% — on a mineral with a straightforward, well-established ICP-MS method.

Both labs held current ISO 17025 certificates on their websites. Both passed their most recent accreditation audit. And yet one result would have confirmed label compliance while the other would have triggered a reformulation conversation, potential regulatory exposure, and a product hold.

This happens more often than the industry acknowledges. And the reason it happens has almost everything to do with something most brands never think to ask about: proficiency testing.

What ISO 17025 Accreditation Actually Certifies — And What It Doesn’t

ISO/IEC 17025 is the international standard for testing and calibration laboratory competence. First published in 1999 and last updated in 2017, it specifies requirements for a lab’s management system, technical competence, and the validity of its results. When an accreditation body like A2LA, ANAB, or PJLA grants ISO 17025 accreditation, they’re certifying that the lab has documented procedures, qualified personnel, calibrated equipment, and a functioning quality management system — and that an auditor has verified all of that on-site.

What a certificate cannot tell you is whether that lab’s results are accurate for your specific analytes at this moment in time. Accreditation is, fundamentally, a snapshot of process quality. It’s audited periodically — usually every one to two years for full assessments, with annual surveillance visits in between. A lot can drift in the intervals between audits: reagent lots change, instruments age, analysts turn over, and subtle method deviations accumulate without triggering an internal alarm.

Proficiency testing (PT) is what fills that gap. It’s the only external, real-time performance check that tells you how a lab’s reported numbers compare to a consensus reference value on an actual test sample.

How Proficiency Testing Works and How to Read the Results

Proficiency testing is defined under ISO/IEC 17043:2010 as the evaluation of participant performance against pre-established criteria by means of inter-laboratory comparisons. A PT provider — AOAC INTERNATIONAL, FAPAS, LGC Standards, or any number of accredited schemes — prepares a blind reference sample with a known (but withheld) concentration of the target analyte. They send it to participating labs. Each lab runs its standard method and reports a result. The provider then calculates z-scores based on the spread of all results.

The z-score formula is: z = (x − X̄) / σ̂

Where x is the lab’s reported value, X̄ is the consensus reference value, and σ̂ is the target standard deviation for that analyte. The interpretation is standardized across virtually all PT schemes:

• |z| ≤ 2.0 = satisfactory performance
• 2.0 < |z| ≤ 3.0 = questionable (a warning signal requiring investigation)
• |z| > 3.0 = unsatisfactory (corrective action required under ISO 17025:2017 clause 7.7.2)

ISO 17025:2017 clause 7.7.2 explicitly requires accredited labs to monitor their performance through PT participation or inter-laboratory comparisons. Accreditation bodies typically mandate at least one PT round per year for each test method within a lab’s accreditation scope. But “at least one” is a floor, not a benchmark — rigorous labs participate in two to four rounds annually per method family, often across multiple PT providers.

Here’s what most clients miss: a lab can hold a valid ISO 17025 certificate while simultaneously having questionable z-scores sitting in its most recent PT report. The accreditation doesn’t automatically get revoked between audits. The certificate on the website doesn’t update in real time.

What the Published Data Shows About Inter-Lab Variability

The variability documented among ISO 17025 accredited labs on identical samples is larger than most clients expect — and the published evidence for this is straightforward.

Inter-laboratory studies on vitamin D quantification in dietary supplements, run through AOAC INTERNATIONAL, have documented inter-laboratory RSDs ranging from 7.2% to 14.8% across participating labs, all using validated methods. For a product claiming 1,000 IU of vitamin D3 per serving, that spread represents a realistic range of approximately 850 to 1,148 IU between labs. If your label claim sits at the lower end of that range, the lab you choose isn’t a neutral variable — it’s a compliance variable.

Microbial testing tells a similar story. For aerobic plate counts run under USP <61>, inter-lab variation of 0.5 to 1.0 log CFU/g is considered normal in external quality assurance schemes. A result of 1,000 CFU/g at one accredited lab could plausibly come back as 3,000 CFU/g at another, both within the expected range of method variation. For products with a specification limit of 1,000 CFU/g, that variability isn’t a statistical footnote — it’s a pass/fail question.

Heavy metals testing via ICP-MS is generally the most precise of the common supplement assays, but PT data from FAPAS and LGC Standards has documented RSDs of 5–12% for lead and cadmium in complex botanical matrices. Turmeric, ashwagandha, and spirulina fall squarely in this category. The matrix matters enormously, and not every lab’s digestion and calibration approach handles it the same way.

None of this means ISO 17025 labs are unreliable as a class. The vast majority perform well on well-characterized analytes. What the data shows is that accreditation creates a quality floor, not a fixed accuracy point. Proficiency testing history tells you how far above that floor a given lab is actually performing.

Why Labs Rarely Volunteer This Information

PT participation records are part of a lab’s quality management documentation under ISO 17025:2017, but they’re almost never included in a standard quote package. There are a few reasons for that.

Some labs participate in PT programs that don’t cleanly map to their full commercial test menu. They might have strong z-scores for pesticide residues in botanical supplements but have never submitted to a PT round for heavy metals in protein powders or potency testing for fat-soluble vitamins. Presenting partial PT data raises questions the lab might not want to field in a sales conversation.

Others operate under confidentiality arrangements with PT providers, where individual lab identities are coded in the group report. Under ISO/IEC 17043:2010, labs aren’t required to disclose z-scores to clients — though nothing in the standard prevents voluntary disclosure.

And, frankly, if a lab has had a string of questionable results on a method, there’s no commercial incentive to lead with that. The accreditation certificate is still valid. The website still shows the accreditation body logo.

The result is a market where buyers select contract labs based primarily on turnaround time, price, and an ISO 17025 certificate — without ever accessing the one dataset that directly measures real-world accuracy.

Five Questions Worth Asking Before You Sign a Testing Contract

Requesting PT data doesn’t require a technical background to do well. These five questions are specific enough to be meaningful and clear enough that any client can ask them.

1. Which PT schemes do you participate in for this analyte? AOAC INTERNATIONAL, FAPAS, LGC Standards, and AAFCO are recognized PT providers in the supplement and food space. For cosmetics, look for participation in BIPEA or EPTIS-registered schemes. If a lab can’t name a PT provider for your method, ask why.

2. What were your z-scores for the last two PT rounds on this specific method? Ask for the numerical values, not just a pass/fail summary. A z-score of 1.9 and a z-score of 0.3 are both “satisfactory” — but they signal very different precision margins for an analyte near a compliance threshold.

3. What corrective actions were taken following any questionable results? A well-run lab treats a questionable z-score as a diagnostic event, not an embarrassment. If a lab has never had a questionable result and can’t describe a corrective action process, either their PT coverage is thin or they’re not being straightforward.

4. Is your PT participation aligned with your commercial test scope? If a lab offers 80 test parameters but has PT coverage for only 15, the remaining 65 are running on internal validation data alone. That’s not automatically a problem — some analytes simply don’t have available PT schemes — but you should know which category your testing falls into.

5. How does your PT data factor into your measurement uncertainty estimates? ISO 17025:2017 clause 7.6 requires labs to evaluate measurement uncertainty for each test. PT outcomes are one of the primary real-world inputs into that calculation. A lab that can walk you through how they use PT results to set their uncertainty estimate is a lab that treats the data as a working tool rather than a compliance checkbox.

What This Means for Your Compliance Program

If your testing program is tied to a regulatory threshold — 21 CFR Part 111 label claim compliance, a California Prop 65 action level, a USP compendial specification, or an FDA-regulated performance standard — the lab you choose is a material input to your compliance risk profile. Not just your paperwork, but your actual exposure.

At Qalitex, we participate in PT programs through AOAC INTERNATIONAL and LGC Standards across our supplement chemistry panel, and through external QA schemes for our microbiology methods. We track z-scores by method and analyst internally, and we make those records available to clients during onboarding and upon request at any time during an active testing relationship.

We do that not because it’s required, but because a client who understands how we validate our accuracy can make better decisions about when our results are fully actionable and when a retest or method discussion makes sense. That kind of transparency also keeps our own team honest — there’s no better motivation for maintaining precision than knowing clients have direct visibility into performance data.

ISO 17025 accreditation is the right starting point when evaluating a contract lab. Proficiency testing records are what tell you the lab is still performing at the level that earned them that accreditation in the first place.

Ask for the z-scores. Any lab worth your testing budget should hand them over without hesitation.

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Written by Nour Abochama, Vice President of Operations, Qalitex Laboratories. Learn more about our team

Talk to our team about your testing needs. Contact us

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