Proficiency Testing: The Analytical Lab Metric Your Raw Material QC Program Is Probably Ignoring

Inter-laboratory variation studies on dietary supplement raw materials consistently document a 20–35% spread in quantitative results when identical samples are analyzed across different analytical testing laboratories. For curcuminoid content in turmeric extract — one of the most commercially standardized botanicals in the industry — published ring trials have shown mean reported values diverging by 25% or more between participating labs. Same sample. Different numbers. Your COA represents a single data point from one of those labs. Whether it’s on the accurate end of that distribution is a question most raw material QC programs never think to ask.

That’s exactly what proficiency testing (PT) programs are designed to answer.

What Proficiency Testing Is — and Why It’s Different from ISO 17025 Accreditation

ISO 17025 accreditation is a structural credential. It confirms that an analytical testing laboratory has documented procedures, qualified personnel, calibrated instruments, and an audited quality management system. What it doesn’t guarantee is analytical accuracy on any specific matrix or analyte. An accredited lab can still systematically underreport lead if its sample digestion is suboptimal for high-fat botanical matrices, or overstate marker compound concentrations if its reference standards lack proper traceability.

Proficiency testing is the performance credential. In a PT program, a coordinating body distributes blind samples to enrolled laboratories. Each lab analyzes the sample using its standard in-house methods, submits results without knowing the assigned value, and receives a z-score comparing its performance against the consensus of all participants.

The math is straightforward. A z-score of 0 means the lab’s result matches the consensus exactly. ISO 13528 — the international standard governing statistical design and operations of PT schemes — classifies a z-score between −2 and +2 as satisfactory, between ±2 and ±3 as questionable, and beyond ±3 as unsatisfactory. An unsatisfactory result means the lab produced a value more than three standard deviations from what the collective of participating laboratories found. That’s not analytical noise. That’s a method problem.

ISO/IEC 17025:2017, clause 7.7.1, explicitly requires accredited laboratories to “monitor their performance by comparisons with results of other laboratories,” typically through participation in PT programs. But here’s the gap most buyers miss: PT enrollment scope is rarely printed on an accreditation certificate. A lab can maintain ISO 17025 accreditation while participating in just one or two PT rounds per year, covering analytes that may not overlap at all with your specific raw materials. The certificate tells you the quality system exists. It doesn’t tell you how the lab performs on botanical heavy metals or multi-residue pesticide screens.

What z-Scores Actually Tell You About Your Testing Partner

Ask your current testing laboratory for their PT participation records and z-score summaries for the past 24 months. If the response is vague or delayed, that tells you something.

Analytical testing laboratories that take PT seriously keep these records organized by analyte category: heavy metals by ICP-MS, pesticide residues by GC-MS/LC-MS/MS, mycotoxins, microbiological enumeration, and botanical identity or potency markers. For each category, they should be able to show which PT programs they participate in, when the most recent round was completed, and whether results consistently fell within the ±2 satisfactory band under ISO 13528 criteria.

You’re not looking for a perfect z = 0 across every round. You’re looking for a pattern of control. A lab that consistently scores between −0.6 and +0.6 for arsenic across six consecutive PT rounds has demonstrated that its ICP-MS calibration, sample preparation, and method execution are tracking accurately against the broader analytical community. A lab with two consecutive z-scores of +2.7 for lead in a botanical matrix has a documented accuracy problem — one you can evaluate before ever shipping a sample.

Published inter-laboratory data from AOCS and FAPAS collaborative studies for trace elements in botanical matrices show coefficients of variation (CVs) ranging from 12% to over 40% across participating labs, depending on the analyte and matrix complexity. For lead in multi-ingredient herbal materials, CVs above 25% are common in lower-quality PT populations. Laboratories that self-select into rigorous external PT programs typically perform within 8–10% CV for ICP-MS analytes. The discipline of participating in PT is itself a filter for methodological seriousness.

Five Analyte Categories Where PT Enrollment Should Be Non-Negotiable

Not every PT program is equally relevant for botanical raw material testing. These are the categories where analytical accuracy directly affects your product safety, label compliance, and regulatory exposure.

Heavy Metals (ICP-MS, USP <232>/<233>) Lead, arsenic, cadmium, and mercury have defined USP oral daily intake limits that leave narrow margins. An ICP-MS method running 15% high on arsenic will produce compliant COAs for batches that actually exceed permissible limits. PT programs from FAPAS (trace elements in botanical matrices) and LGC Standards are widely used by contract analytical testing labs serving the supplement industry. If your lab’s accreditation scope includes USP <233> ICP-MS, there is no excuse for absence from at least two PT rounds per year.

Pesticide Residues (Multi-Residue Screening) Multi-residue pesticide methods require validation across 200–400 compounds with matrix-matched calibration. Inter-lab recoveries for organophosphates and carbamates in complex botanical matrices can span 45% to 130% across laboratories that don’t benchmark externally. AOAC International’s proficiency testing program covers pesticide residues in food and dietary supplement matrices and represents the reference standard against which serious contract analytical testing laboratories measure themselves.

Mycotoxins (Aflatoxins B1/B2/G1/G2, Ochratoxin A, Deoxynivalenol) Immunoaffinity column cleanup variability and LC-MS/MS method differences create significant inter-lab spreads for mycotoxin quantification. FDA’s action level for total aflatoxins in food-grade botanicals is 20 ppb. A method that runs 30% low on aflatoxin B1 will consistently pass materials that should fail — and those decisions are being made based on the numbers on your COA.

Microbiology (USP <61>/<62>, Pathogen Screening) Microbiological PT programs test whether a lab’s culture media formulation, incubation conditions, and analyst technique generate counts consistent with certified reference materials. ATCC proficiency panels and NSF International’s microbiological PT program are used by most contract laboratories serving dietary supplement manufacturers. For PCR-based pathogen detection under USP <2021>, PT performance on confirmed-positive samples is particularly important — false negatives are method failures, not sampling anomalies.

Botanical Potency Markers (Spectrophotometry, HPLC) For quantitative assays — curcuminoids in turmeric, withanolides in ashwagandha, hypericin in St. John’s Wort, vinpocetine in periwinkle extract — inter-lab variation is the least standardized category in the industry. Published ring trials for curcuminoid quantification have documented CVs exceeding 30% across laboratories. USP reference standard interlaboratory exercises and AOAC First Action method validations provide the most relevant external benchmarks in this space, and a lab participating in them is one that cares about being right, not just being fast.

How to Ask for PT Records Without Making It Awkward

This doesn’t need to be a confrontational audit conversation. Analytical testing laboratories that participate actively in PT programs are usually proud of the results — it’s a differentiator that should be on their website. A direct question during supplier qualification works fine:

“Could you share your proficiency testing participation summary for the last 24 months — specifically covering ICP-MS heavy metals, pesticide multi-residue, and microbiology? We’d like to see the PT provider, round dates, analytes tested, and whether results were satisfactory under ISO 13528 criteria.”

FAPAS, LGC, and AOAC issue formal certificates for each completed round, showing the z-score distribution and the lab’s individual result against the assigned value. These documents are shareable. There’s no legitimate confidentiality reason to withhold them from a client asking as part of supplier qualification.

If the lab shows isolated unsatisfactory results — and over 24 months, even strong labs occasionally have an off round — the follow-up is: “What corrective action was taken?” A lab that responds to a poor z-score with root cause analysis, method recalibration, and demonstrated improvement in the subsequent round is one that operates with integrity. One that has no answer, or that treats the question as unwelcome, is one to reconsider.

Our testing is performed through ISO 17025–accredited facilities that participate in multiple external PT programs annually, covering heavy metals, pesticide residues, mycotoxins, and microbiology across botanical matrices. When clients ask for PT summary documentation, we provide it — because the CoA should be defensible, not just a piece of paper.

Adding PT Enrollment to Your Supplier Qualification Checklist

Under 21 CFR Part 111, dietary supplement manufacturers are required to confirm that incoming raw material testing is conducted by qualified means. While the regulation doesn’t explicitly name proficiency testing enrollment as a requirement, FDA investigators have cited inadequate laboratory qualification in Form 483 observations when manufacturers accepted COAs from labs with no documented external accuracy verification. The argument “they’re ISO 17025 accredited” has not always been sufficient when the accreditation scope didn’t cover the relevant analytes or the lab had no external performance benchmarks.

PT participation records — combined with ISO 17025 accreditation scope, method validation summaries, and reference standard traceability documentation — give you a complete picture of what a contract analytical testing laboratory is actually capable of. Accreditation answers the question “does this lab have a quality system?” PT answers “does this lab produce accurate results on samples that look like mine?”

Before your next incoming shipment of ashwagandha root powder, elderberry juice concentrate, or lion’s mane fruiting body extract lands at your dock, add PT enrollment data to the supplier qualification packet you request from your testing partner. It’s one document request. And it’s the one that tells you whether every other document they’ve sent you can be trusted.

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

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