Proficiency Testing Programs: What They Reveal About Your Analytical Testing Lab That an ISO Certificate Doesn't

When a supplier hands you an ISO 17025 certificate stapled to a CoA, the temptation is to check the box and move on. Certificate present, data attached — what more could you reasonably want? Quite a bit, actually. An accreditation tells you a lab was assessed on a specific date and met baseline competency requirements at that moment in time. It tells you almost nothing about how that lab performs on your matrix, analyzing your analytes, when samples arrive without warning during a routine production cycle.

Proficiency testing (PT) programs are where that gap closes. They’re the mechanism that separates labs claiming analytical competence from those that have demonstrated it, repeatedly, against a field of peers. If you’re sourcing botanical raw materials or contracted testing for finished herbal products through a Midwest supply chain and you’re not requesting PT participation records from your contract analytical testing lab — you’re making quality decisions on incomplete information.

What Proficiency Testing Actually Measures

PT programs work by sending identical, homogeneous blind samples to multiple participating laboratories simultaneously. Each lab analyzes the material without knowing the assigned target value, then reports results back to the program coordinator. The coordinator compares all submissions statistically and issues each participant a performance score governed by ISO 13528, Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison.

The primary metric is the z-score:

z = (x − X̂) / σ̂

Where x is the lab’s reported result, X̂ is the assigned value (typically the robust consensus median from all participants), and σ̂ is the standard deviation for proficiency assessment. Interpretation is consistent across all PT programs and analytes:

• |z| ≤ 2.0 — Satisfactory. The result falls within the expected performance range of peer laboratories.
• 2.0 < |z| < 3.0 — Questionable. A warning signal that should trigger internal investigation of bias or precision issues.
• |z| ≥ 3.0 — Unsatisfactory. An action signal under ISO 13528. The lab must conduct root cause analysis and implement corrective action before applying that method to client samples again.

That last point matters for buyers. An unsatisfactory z-score on a relevant analyte-matrix combination isn’t just a data point — it’s a documented quality event requiring a documented response. A lab that participates in PT and maintains that corrective action log is more trustworthy than one that never participates at all, because at least the former has a functioning feedback loop.

What z-scores don’t cover: turnaround time, sample chain-of-custody, report formatting, or communication. Those matter to your operations team. But a lab posting consecutive unsatisfactory scores on, say, lead in botanical matrices has a method performance problem that no certificate will paper over.

The Particular Challenge of Herbal and Botanical Matrices

Most discussions of PT performance reference pharmaceutical or simple food matrices. The complication with herbal supplement ingredients is that botanical matrices are genuinely more difficult. Natural variability in plant composition — driven by geographic origin, harvest season, drying conditions, and extraction parameters — amplifies the challenge of hitting a consensus target that dozens of labs can agree on.

Published inter-laboratory studies on botanical extracts have documented relative standard deviations (RSDs) of 15–35% between labs analyzing the same analyte in the same prepared material. ISO 5725-2 guidelines indicate that reproducibility RSDs above roughly 16% at the 1 mg/kg level suggest the method is under-validated for that matrix. In botanicals — curcuminoids in turmeric oleoresin, withanolides in ashwagandha, ginsenoside ratios in panax ginseng root — achieving consistent inter-lab agreement requires highly specific method validation work that not every analytical testing lab has completed.

This is why asking which PT programs a lab participates in matters as much as asking whether they participate. Satisfactory z-scores on a lead standard in drinking water, or ascorbic acid in a vitamin C tablet, do not demonstrate the same capability as satisfactory scores on lead in a concentrated botanical extract. The extraction chemistry, matrix interferences, and calibration strategy are fundamentally different problems.

For ICP-MS heavy metals work specifically — which is the backbone of USP <232>/<233> elemental impurity testing — labs must demonstrate matrix-matched calibration, internal standard recovery within specified windows, and method detection limits appropriate to USP oral daily dose limits. A PT round in botanical matrices will expose whether those controls are actually working.

PT Programs Worth Asking About by Name

Not every PT provider offers rounds in matrices relevant to dietary supplement ingredients. The programs with meaningful coverage include:

AOAC International Proficiency Testing Program — AOAC runs quarterly rounds covering heavy metals, fat- and water-soluble vitamins, dietary fiber, and select botanical constituent analytes. For products relying on AOAC Official Methods (OMA) for label claim testing under 21 CFR Part 111, AOAC PT participation is the natural audit trail for method performance. If your analytical testing lab cites AOAC methods in their SOPs, they should be participating in AOAC PT rounds.

FAPAS (FERA Proficiency and Analytical Science) — Operated by FERA Science Ltd. in the UK with global laboratory participation, FAPAS runs more than 100 proficiency schemes annually. Rounds particularly relevant to botanical ingredient buyers include heavy metals in herbs and spices, mycotoxins in plant materials, pesticide residues in botanicals, and authenticity markers. FAPAS z-score reports are internationally recognized under ILAC MRA arrangements, making them transferable when Canadian or EU customers request third-party substantiation.

NSF International Proficiency Testing — NSF’s PT programs are designed specifically for dietary supplement testing laboratories and cover microbiological counts (including those governed by USP <61>/<62>), heavy metals aligned to USP <232>/<233>, and selected label-claim active constituents. Because NSF is also a GMP certification body for supplement manufacturers, their PT programs carry particular credibility when audit documentation is requested by a retail partner or contract manufacturer customer.

USDA AMS National Science Laboratory — Primarily agricultural commodities, but their heavy metal and pesticide residue rounds in plant matrices overlap substantially with raw herbal ingredient testing needs, particularly for imported botanicals where agricultural chemical profiles differ from domestic crops.

A credible analytical testing laboratory should provide, on request: a list of current PT program enrollments, the most recent round z-scores for relevant analyte-matrix combinations, and corrective action records for any questionable or unsatisfactory results. If that documentation isn’t readily available, ask why. The answer tells you something.

ISO/IEC 17025:2017 Clause 7.7 — What the Standard Actually Demands

ISO/IEC 17025:2017 Clause 7.7 (“Ensuring the validity of results”) requires laboratories to plan and participate in interlaboratory comparisons, including PT programs, where available. The key phrase is “where available.” For heavy metals, microbiological counts, and common constituent assays in botanical matrices, PT programs are unambiguously available — which means a laboratory declining to participate needs documented justification.

US accreditation bodies — A2LA and Perry Johnson Laboratories (PJL) being the most relevant for supplement testing labs — audit Clause 7.7 compliance at every reassessment cycle. A pattern of non-participation, or a string of unsatisfactory z-scores without documented corrective action, will generate nonconformities. Serious or recurring ones can result in scope reductions: specific analyte-matrix combinations are removed from the accredited scope until method performance is re-established through PT data or witnessed test comparisons.

For buyers, the practical implication is this: if a lab’s ISO 17025 accredited scope has narrowed since your last review — if analytes or matrices have disappeared from the scope document — proficiency testing performance is the first place to look for the underlying reason.

Four Questions to Ask Before You Ship Your Next Sample

Getting this information doesn’t require a formal supplier audit. A lab confident in its performance will answer these without hesitation:

1. Which PT programs do you participate in for ICP-MS heavy metals in botanical matrices? There should be a specific program name — AOAC, FAPAS, NSF — not a generic reference to internal QC standards.

2. Can you share z-score results from your last three rounds for [specific analyte] in a botanical or plant-based matrix? Three rounds across satisfactory results demonstrates consistency, not a single fortunate data point.

3. When did you last receive a questionable or unsatisfactory z-score, and what was the corrective action? A lab claiming a perfect record across years of participation is probably either under-enrolled or not being fully transparent. Corrective action documentation is the mark of a mature quality system.

4. Is your ICP-MS or microbiology scope accredited specifically for botanical or complex organic matrices, or only for simpler matrices like water or nutritional powder? This distinction changes the relevance of the accreditation scope entirely for herbal ingredient buyers.

At our Countryside, IL receiving hub, every sample we accept for analytical testing is routed to Qalitex Laboratories in California — ISO 17025 accredited with active PT enrollment across botanical and nutritional supplement matrices. We maintain PT z-score records on file and make them available to clients on request, because that’s a reasonable expectation for any analytical testing lab handling commercial raw material decisions.

The Takeaway for Midwest Supplement Brands

A CoA is only as reliable as the analytical testing lab that generated the underlying data. ISO 17025 accreditation is the price of admission — the minimum credential that tells you a lab has been assessed against a recognized competency standard. Proficiency testing participation, with documented and satisfactory z-scores on analyte-matrix combinations relevant to your product, is what confirms the lab performs when the conditions are real and the results are blind.

Before your next purchase order for botanical raw materials clears receiving, send a two-line email to your testing lab and ask for their most recent PT z-scores. If the answer comes back clearly and quickly, you’ve found a lab operating with confidence in its data. If it takes three follow-ups and arrives without matrix-specific context, you have your answer too — just not the one you were hoping for.

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Written by Nour Abochama, VP Operations, Qalitex | Quality Consultant, Ayah Labs. Learn more about our team

Ship your sample to our Chicago facility — get a Qalitex CoA in 5–7 days. Contact us

Related from our network

• Qalitex Laboratories — ISO 17025 Accredited Supplement & Botanical Testing — Full-scope analytical testing for dietary supplements, botanicals, and nutraceuticals under ISO 17025 accreditation with active proficiency testing participation.