Every NABL-accredited calibration lab in India has read ISO/IEC 17025:2017 clause 7.7 at least once, usually right before an assessment. It requires labs to "monitor the validity of results" through proficiency testing, and it is not a suggestion. Miss it, and an assessor can flag a nonconformity that puts your accreditation itself at risk. Yet a genuine proficiency testing scheme in India is surprisingly hard to find outside NABL's own accreditation pathway for PT providers. This guide covers what proficiency testing actually is, why clause 7.7 makes it mandatory rather than optional, and how to choose a scheme that will actually hold up under assessment.

Reviewed by Mahemood Ahmad, Lab Manager — NABL Accredited Laboratory, Global Lab India

Key Takeaways

  • ISO/IEC 17025:2017 clause 7.7 makes proficiency testing mandatory for NABL-accredited calibration labs, not optional best practice — a missed or unsatisfactory PT round is a genuine risk to your accreditation.
  • Under ISO/IEC 17043:2023, a PT scheme circulates a characterised reference artefact between labs, then evaluates every participant's result statistically against an assigned value using En numbers and z-scores (ISO 13528:2022).
  • Commercial PT schemes for common calibration scopes (UTM, weighing balance, temperature sensors, dimensional gauges) are genuinely scarce in India — most search results lead back to NABL's own accreditation information, not an actual provider you can enrol with.

What Is Proficiency Testing, and Why Does NABL Require It?

Proficiency testing is how a lab proves its results aren't just internally consistent, but actually correct when checked against independent, external evidence. A characterised reference artefact circulates between multiple participating labs; each calibrates or tests it using its own normal method, then submits results to a PT provider for statistical comparison. NABL is itself accredited to run proficiency testing schemes under ISO/IEC 17043, and is a signatory to the APAC Mutual Recognition Arrangement for this exact program (NABL India, Proficiency Testing Providers page).

The requirement itself sits in ISO/IEC 17025:2017 clause 7.7, which obligates every accredited testing and calibration laboratory to monitor the validity of its results. Internal QC checks help, but they can't catch a systematic bias the whole lab has drifted into together. An external interlaboratory comparison can. That's the entire point of the clause, and it's why NABL assessors ask for PT participation records at nearly every surveillance visit.

How Does an ISO/IEC 17043 Proficiency Testing Scheme Actually Work?

A scheme run to ISO/IEC 17043:2023, the international standard governing proficiency testing providers, follows a fairly consistent structure regardless of what's being measured. The provider characterises a reference artefact or sample, establishes an assigned value for it, and circulates it to enrolled participants on a published round schedule (ISO/IEC 17043:2023). Each lab measures the artefact using its own routine procedure, exactly as it would for a paying customer, then submits the result with its claimed uncertainty.

Here's where the real evaluation happens. Results are scored statistically against the assigned value using the methods in ISO 13528:2022, most commonly as an En number or a z-score (ISOBudgets, Proficiency Testing and Interlaboratory Comparisons). An |En| at or below 1 is generally read as satisfactory; anything higher flags a result worth investigating before it reaches a customer. Every participant gets a confidential report, so no lab's performance is visible to any other lab in the round, only to that lab itself and the provider.

What Happens If a Calibration Lab Skips Proficiency Testing?

The immediate risk is a nonconformity at your next NABL surveillance assessment, and depending on severity, that can escalate to a suspension of scope. But the deeper risk is quieter: without external comparison, a lab has no real way to know if its entire reference chain has drifted. Internal QC only catches inconsistency within the lab's own results, not a shared bias every technician has learned to work around (ANAB, What Is ISO 17043?).

A calibration certificate without documented proficiency testing participation is missing the one piece of evidence that proves the lab's results agree with its peers, not just with itself — and that's precisely what an assessor, an auditor, or a demanding customer will ask for first.

How Do You Choose the Right Proficiency Testing Scheme?

Start with scope match. A scheme has to cover the exact measurand your lab is accredited for, at a comparable range and uncertainty, or participation doesn't actually satisfy clause 7.7 for that scope. Check the provider's own accreditation next: a PT provider accredited to ISO/IEC 17043 carries more weight with your assessor than an informal comparison arrangement between labs, however well-intentioned.

Round frequency and logistics matter more than most labs expect going in. A scheme with a published, predictable calendar lets you plan participation across your whole scope instead of scrambling before an assessment. And for labs anywhere outside a handful of major cities, artefact circulation logistics quietly decide whether a scheme is workable at all — an international scheme with weeks-long shipping delays is a very different commitment than one designed for Indian geography.

A scheme that circulates internationally but takes six weeks to reach an Indian lab isn't really a scheme you can use, no matter how well it's run. Logistics decide whether a PT program fits your accreditation calendar or quietly falls off it.

Explore our proficiency testing scheme program for the full list of instrument categories currently open for enrolment, including UTM force calibration and weighing balance rounds.

What Instrument Categories Does Global Lab's PT Program Cover?

Global Lab runs interlaboratory comparison rounds across the categories Indian calibration labs most commonly need to demonstrate for NABL scope: force (Universal Testing Machine, Compression Testing Machine), temperature (RTD and thermocouple, with and without indicator), dimensional (vernier caliper, micrometer, profile projector, slip gauge, thread gauge), mass (weighing balance, standard mass), pressure (pressure gauge), and thermal equipment (hot air oven, muffle furnace, curing tanks).

CategoryExample InstrumentsEvaluation
ForceUTM, Compression Testing MachineEn / z-score, ISO 13528
TemperatureRTD, ThermocoupleEn / z-score, ISO 13528
DimensionalVernier Caliper, Micrometer, Profile ProjectorEn / z-score, ISO 13528
Mass & PressureWeighing Balance, Standard Mass, Pressure GaugeEn / z-score, ISO 13528

Each scheme follows the same evidence trail your assessor expects: a characterised artefact, a documented round, and a confidential performance report with En and z-score results ready to drop directly into your accreditation file.

How Do You Enrol in a Proficiency Testing Round?

Enrolment runs through our Live PT Program portal: pick the scheme matching your instrument category, confirm the round schedule, and we handle artefact circulation and logistics from there. Results and performance reports come back through the same portal, ready to file alongside your NABL documentation.

Send us your scope on WhatsApp or by phone to check the next round date for your category, no obligation attached.

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Frequently Asked Questions

Is proficiency testing mandatory for NABL accreditation?
Yes. ISO/IEC 17025:2017 clause 7.7 requires accredited labs to monitor the validity of their results, and proficiency testing is the standard way NABL expects that to be demonstrated. An unsatisfactory or missing PT record is a common finding at surveillance assessments.
What is an En number in proficiency testing?
The En number compares your submitted result and uncertainty against the scheme's assigned value and its uncertainty, per ISO 13528:2022. An |En| of 1 or below is read as satisfactory; a higher value flags a result that needs investigating before it's trusted again.
How often should a calibration lab participate in proficiency testing?
There's no single fixed interval mandated by the standard itself; NABL expects coverage across your accredited scope on a documented, risk-based schedule. Most labs plan around scheduled rounds so every major measurement category gets tested at least once across an accreditation cycle.
Does my lab's PT performance get shared with other participants?
No. Every participant's identity is coded, and only your own lab sees your specific performance report. Group statistics are shared anonymously across the round so you can see how your result sits relative to the field, without anyone identifying you.
What if my lab gets an unsatisfactory PT result?
An unsatisfactory result triggers a documented investigation, not an automatic accreditation problem. Assessors generally want to see the root cause identified and a corrective action taken, which is exactly why a confidential, well-run PT scheme is more useful to a lab than a passing grade alone.

Proficiency testing isn't a compliance checkbox you clear once and forget. It's the only external evidence that your lab's results actually hold up outside your own four walls. Browse the full proficiency testing scheme program, or enrol in the next round for your instrument category today.

Sources:
ISO, "ISO/IEC 17043:2023 — Conformity assessment — General requirements for the competence of proficiency testing providers," retrieved 2026-07-20, iso.org
NABL India, "Proficiency Testing Providers (PTP) as per ISO/IEC 17043," retrieved 2026-07-20, nabl-india.org
ANAB, "What Is ISO 17043?," retrieved 2026-07-20, blog.ansi.org
ISOBudgets, "Proficiency Testing and Interlaboratory Comparisons for ISO/IEC 17025," retrieved 2026-07-20, isobudgets.com