A calibration laboratory can pass every internal audit, maintain impeccable documentation, follow all its SOPs perfectly — and still produce measurements that no other laboratory in the world would agree with. Internal quality controls catch procedural errors and documentation gaps. They cannot catch the one failure mode that only external comparison reveals: systematic measurement bias that runs through the entire laboratory’s work, affecting every calibration it produces, without any internal indicator that something is wrong.

This is the gap that proficiency testing fills.

Proficiency testing (PT) — also called interlaboratory comparison (ILC) — is the process by which a calibration laboratory compares its measurement results against those of other laboratories, using the same or similar reference items measured under defined conditions. It is the only objective, external validation that a laboratory’s measurements are not just internally consistent but actually correct when evaluated against a global reference.

For Philippine calibration laboratories, proficiency testing is not optional. ISO/IEC 17025:2017 Section 7.7.2 requires laboratories to monitor their performance by comparing results with other laboratories through participation in PT and/or ILC programmes. The Philippine Accreditation Bureau (PAB), as the national accreditation body and signatory to the ILAC Mutual Recognition Arrangement, requires demonstration of satisfactory PT performance as part of the accreditation process. As noted in the PAB assessment process: laboratories must demonstrate satisfactory performance in proficiency testing programmes — calibrating a reference object alongside other accredited laboratories worldwide, with results compared by an independent PT provider.

Yet for many Philippine calibration laboratories — particularly those newer to the PAB accreditation process, or operating in-house calibration programmes without prior laboratory accreditation experience — proficiency testing remains one of the least understood and least implemented requirements of the ISO/IEC 17025 framework. This article provides the most comprehensive explanation of proficiency testing available for Philippine calibration laboratories and quality professionals: what it is, why it matters, what ISO/IEC 17025 and PAB require, how it works technically, how to find and participate in PT programmes, how to interpret results, and what to do when results are unsatisfactory.

The One-Sentence Explanation of Why PT MattersProficiency testing is the only way a calibration laboratory can know that its measurements agree with the rest of the world — not just with itself.Internal audits verify that you follow your procedures. PT verifies that your procedures are producing the right answers.

1. What Proficiency Testing Is — and How It Differs from Interlaboratory Comparison

The terms proficiency testing and interlaboratory comparison are closely related but technically distinct, and ISO/IEC 17025:2017 uses both — requiring laboratories to understand the difference.

Interlaboratory Comparison (ILC): The Broad Category

An interlaboratory comparison is any organised set of measurements performed on the same or equivalent items by two or more laboratories under specified conditions. ILC is the broad category: any time two or more labs measure the same thing and compare their results, that is an ILC. ILC results can be used for many purposes — verifying measurement agreement, validating new methods, establishing reference values, or assessing laboratory performance.

Under ISO/IEC 17025, ILC includes all organised comparisons, and the standard requires laboratories to participate in ILC programmes appropriate for their scope. The specific type of ILC most commonly required for accreditation purposes is proficiency testing.

Proficiency Testing (PT): The Specific Form

Proficiency testing is a specific type of interlaboratory comparison that: is organised and managed by an independent third-party proficiency testing provider (not by one of the participating laboratories); involves multiple participating laboratories measuring the same or equivalent reference items; evaluates participants’ performance against pre-established acceptance criteria using statistical methods; and produces individual performance scores for each participant that allow accreditation bodies to assess whether each laboratory’s measurements are within acceptable bounds.

The key distinctions: PT is managed independently, evaluated statistically against defined criteria, and scored. All PT is ILC, but not all ILC is formal PT. ISO/IEC 17043:2023 (published in 2023, updated from the 2010 version) is the current international standard governing PT providers.

The Performance Metrics: z-Scores and En Numbers

PT providers use standardised statistical performance metrics to evaluate each participant’s result against the assigned reference value (the ‘true’ value for the PT exercise). The two most commonly used metrics are:

The z-Score

The z-score measures how far a laboratory’s result deviates from the assigned value, normalised by the standard deviation for proficiency assessment (σ_p) set by the PT provider. The formula is:

z = (x − X) / σ_p

Where x is the laboratory’s result and X is the assigned reference value.

Performance is assessed as:

• |z| ≤ 2: Satisfactory — the laboratory’s result is within an acceptable range of the reference value
• 2 < |z| < 3: Questionable — the result is marginal; the laboratory should investigate and may be required to demonstrate improvement
• |z| ≥ 3: Unsatisfactory — the laboratory’s result differs significantly from the reference value; mandatory corrective action is required under ISO 13528:2022 and ILAC P9:01/2024

The En Number (Normalised Error)

The En number is used specifically for calibration laboratory PT exercises where measurement uncertainty must be accounted for in the performance evaluation. Unlike the z-score, which uses the PT provider’s assigned standard deviation, the En number incorporates both the laboratory’s own stated measurement uncertainty and the reference value’s uncertainty:

En = (x − X) / √(U_lab² + U_ref²)

Where U_lab is the expanded uncertainty claimed by the participating laboratory and U_ref is the expanded uncertainty of the reference value.

Performance is assessed as:

• |En| ≤ 1: Satisfactory — the laboratory’s result, accounting for both its own uncertainty and the reference uncertainty, is consistent with the reference value
• |En| > 1: Unsatisfactory — the laboratory’s result is inconsistent with the reference value even accounting for all stated uncertainties; investigation and corrective action required

The En number is more appropriate than the z-score for calibration laboratory PT evaluation because it accounts for the quality of the laboratory’s uncertainty claims — a laboratory that claims unrealistically low measurement uncertainty will show a worse En performance than one that states honest, realistic uncertainty, even if their absolute measurement results are similar.

2. What ISO/IEC 17025:2017 and PAB Require

The proficiency testing requirements embedded in ISO/IEC 17025:2017 represent a significant strengthening from the 2005 version of the standard. Understanding exactly what the current standard requires — and how PAB interprets and enforces those requirements for Philippine laboratories — is essential for any laboratory managing its accreditation.

ISO/IEC 17025:2017 Section 7.7.2: The PT Requirement

ISO/IEC 17025:2017 Section 7.7.2 states: ‘Laboratories shall monitor their performance by comparing their results with those of other laboratories, where available and applicable. This monitoring shall be planned and reviewed and shall include, but not be limited to, one or both of the following: participation in proficiency testing; participation in interlaboratory comparisons other than proficiency testing.’

This represents a change from the 2005 version, where proficiency testing participation was recommended but not required (Section 5.9.1b used the language ‘may include’). In the 2017 standard, the requirement is clear: laboratories shall monitor their performance through comparison with other laboratories, where available and applicable. The ‘where available and applicable’ language provides some flexibility for measurement areas where no suitable PT scheme exists — but this flexibility must be justified, not assumed.

ILAC P9:01/2024: The Policy Framework

ILAC Policy P9 (updated in January 2024) provides the framework that all ILAC MRA signatory accreditation bodies — including PAB — apply for proficiency testing requirements. ILAC P9 governs how accreditation bodies require and assess PT participation across all 121 ILAC MRA signatory economies. It establishes the minimum frequency of PT participation required by accreditation bodies and the criteria for evaluating PT results as part of accreditation surveillance.

Under ILAC P9, accreditation bodies are required to verify that accredited laboratories participate in appropriate PT schemes at appropriate frequencies, and that any unsatisfactory PT results are followed by documented corrective action. PAB, as an ILAC MRA signatory, applies ILAC P9 in its accreditation requirements for Philippine calibration laboratories.

PAB Requirements for Philippine Calibration Laboratories

The Philippine Accreditation Bureau requires demonstration of satisfactory proficiency testing performance as part of the PAB accreditation process. Key PAB PT requirements include:

• At least one PT programme covering part of the laboratory’s accredited scope must be completed satisfactorily before initial PAB accreditation can be granted
• After accreditation, the remaining scope must be covered by satisfactory PT participation within four years
• Ongoing participation in PT schemes at frequencies appropriate to the measurement types and the PT schemes available — PAB surveillance assessors will review PT participation records during surveillance assessments
• Unsatisfactory PT results (z-scores ≥ 3 or En numbers > 1) must be documented with a root cause analysis and corrective action plan, with evidence that the corrective actions were effective
• Where no formal PT scheme is available for a specific measurement type in the laboratory’s scope, the laboratory must document this and demonstrate alternative means of validating its results (such as bilateral interlaboratory comparison with another accredited laboratory)

The Proficiency Testing Gap That Philippine Labs Most Commonly MissMany Philippine calibration laboratories — particularly those that obtained PAB accreditation before the 2017 revision of ISO/IEC 17025 — have not updated their PT participation programmes to meet the strengthened requirements of the 2017 standard and ILAC P9:01/2024.Under the 2005 standard, PT participation was recommended but not strictly required. Under the 2017 standard, it is a mandatory requirement where programmes are available. PAB assessors applying the 2017 standard and ILAC P9 will specifically verify that the laboratory has a documented PT plan and records of participation — a gap that was less rigorously assessed under the previous standard.If your laboratory is accredited to ISO/IEC 17025:2005 and has not yet conducted a gap assessment against the 2017 standard, PT participation requirements should be among the first gaps you investigate.

3. Why Proficiency Testing Matters Beyond Regulatory Compliance

For Philippine calibration laboratories, proficiency testing is important for regulatory compliance — but its value extends significantly beyond satisfying PAB accreditation requirements. Understanding the deeper value of PT motivates genuine engagement with the process rather than minimum-compliance participation.

PT Catches What Internal Controls Cannot

Internal quality controls — repeatability checks, use of check standards, internal audits, management reviews — are essential for operational calibration quality. But they all share a fundamental limitation: they can only detect variation and error relative to the laboratory’s own internal references. They cannot detect a systematic bias that runs consistently through all of the laboratory’s measurements and its reference standards simultaneously.

A common scenario: a calibration laboratory’s reference thermometer has a calibration certificate from a PAB-accredited laboratory that shows a stated correction. The laboratory applies this correction consistently in all its calibrations. Internal checks show excellent repeatability. All procedures are followed correctly. But if the original correction on the reference thermometer’s certificate was itself affected by a systematic error in the accredited laboratory’s reference standard — an error that has since been identified and corrected — the Philippine laboratory’s reference thermometer may be carrying a systematic bias that no internal check will detect. Only a PT exercise that compares this laboratory’s results to those of other laboratories using independently traceable references will reveal the discrepancy.

PT Validates Your Uncertainty Claims

The En number performance metric specifically tests whether a laboratory’s claimed measurement uncertainty is credible and realistic. A laboratory that claims unrealistically small measurement uncertainty — perhaps through an incomplete uncertainty evaluation that omits significant uncertainty sources — will produce En numbers greater than 1 even when its absolute measurement accuracy is adequate, because the numerator (the difference from reference value) exceeds what the stated uncertainty can account for.

Conversely, a laboratory with well-evaluated, realistic uncertainty claims and good measurement accuracy will produce En numbers ≤ 1 consistently — demonstrating that its uncertainty statements are credible predictors of its actual measurement performance. In this way, PT results provide an ongoing external validation of the quality of the uncertainty evaluation programme described in our previous articles on measurement uncertainty.

PT Builds Client and Regulatory Confidence

For Philippine calibration laboratories serving pharmaceutical, food, and industrial clients — particularly multinational clients who may conduct supplier qualification audits — the ability to present consistent satisfactory PT results (z-scores within ±2 or En ≤ 1) across the laboratory’s measurement scope provides concrete, third-party evidence of measurement competence that is far more convincing than internal quality records alone.

A laboratory that can show a client a track record of satisfactory participation in international PT schemes is demonstrating that its measurements have been validated against global standards — not just against its own internal references. This is the commercial value of PT beyond accreditation compliance.

4. Finding PT Schemes: Where Philippine Laboratories Can Participate

The most common practical challenge Philippine calibration laboratories face in meeting PT requirements is identifying and enrolling in appropriate PT schemes for their measurement scope. Here is a systematic guide to finding PT opportunities relevant to Philippine laboratory measurement types.

APLAC Proficiency Testing Programmes

The Asia Pacific Laboratory Accreditation Cooperation (APLAC) — the regional accreditation cooperation body of which PAB is a member — organises and facilitates proficiency testing programmes across the Asia-Pacific region. APLAC PT programmes cover a range of measurement types relevant to Philippine calibration laboratories, including temperature, dimensional, mass, pressure, electrical, and chemical measurement. APLAC PT programmes are specifically designed for Asia-Pacific laboratories and are directly recognised by PAB in the Philippine accreditation context.

APLAC PT programmes are typically organised on an annual or biennial basis. Philippine laboratories can obtain information on current and upcoming APLAC PT programmes through the PAB and through the APLAC website. Participation costs vary by programme but are generally moderate — APLAC programmes are designed to be accessible to accredited laboratories throughout the Asia-Pacific region, including those in developing economies.

BIPM Key and Supplementary Comparisons

The International Bureau of Weights and Measures (BIPM) organises international measurement comparisons — the highest level of metrological comparison — through the BIPM Key Comparison Database (KCDB). These comparisons are primarily conducted between national metrology institutes (NMIs) rather than individual calibration laboratories, and participation from individual Philippine commercial laboratories is typically through the DOST-ITDI National Metrology Laboratory (NML) rather than directly.

The significance of BIPM comparisons for Philippine calibration laboratories is indirect: the traceability of PAB-accredited calibration certificates in the Philippines ultimately rests on the BIPM comparisons conducted by DOST-ITDI NML, which establish the consistency of Philippine national measurement standards with international standards. When a Philippine laboratory’s measurements are traceable to DOST-ITDI NML, and DOST-ITDI NML participates in BIPM comparisons, the Philippine laboratory’s measurements are implicitly connected to the global measurement system.

International PT Providers

A growing number of international PT providers offer schemes that are accessible to laboratories worldwide, including the Philippines. Several of the most well-established providers relevant to Philippine calibration laboratory measurement types include:

• EURAMET (European Association of National Metrology Institutes): Organises interlaboratory comparisons among European NMIs and associated laboratories; some schemes are open to non-European participation
• EPTIS (European Proficiency Testing Information System): A database of proficiency testing providers and schemes across all measurement areas — useful for finding PT schemes by measurement type, even for non-European laboratories
• A2LA WorkPlace Training / NVLAP (USA): Some US-based PT schemes accept international participants; particularly relevant for dimensional and electrical measurement types
• NATA (National Association of Testing Authorities, Australia): Australia’s accreditation body organises PT schemes for a range of measurement types; geographically closer to the Philippines than European providers and often more accessible for Asia-Pacific laboratories
• DOSH Proficiency Testing Provider (Malaysia) and other ASEAN-region providers: Regional providers with schemes specifically designed for Southeast Asian calibration conditions

Bilateral ILC with Other PAB-Accredited Philippine Laboratories

Where a formal PT scheme is not available for a specific measurement type in the laboratory’s scope, a bilateral interlaboratory comparison with another PAB-accredited Philippine laboratory calibrating the same measurement type can serve as an alternative. Both laboratories measure the same reference item under defined conditions, and the results are compared and evaluated statistically.

A bilateral ILC is not equivalent to a formal PT programme in the full accreditation sense — it involves only two participants and is not independently evaluated — but it is recognised by PAB as an alternative where formal PT is unavailable. The bilateral ILC must be documented formally, with a written agreement, defined measurement conditions, statistical evaluation of results, and records maintained for accreditation review.

Identifying potential bilateral ILC partners among other PAB-accredited Philippine calibration laboratories can be done through the PAB directory or through professional networks such as the Quality and Metrology Society (Qualimets) of the Philippines.

5. How to Participate in a PT Scheme: The Step-by-Step Process

For a Philippine calibration laboratory enrolling in a PT scheme for the first time, the participation process has defined steps that must be followed correctly to ensure that results are valid and that participation can be cited in PAB accreditation documentation.

Step 1: Identify an Appropriate PT Scheme

Select a PT scheme appropriate for your measurement scope — the measurement type, measurement range, and measurement uncertainty level of the scheme must be relevant to your laboratory’s calibration activities. For temperature calibration in the pharmaceutical cold chain range (+2°C to +8°C), for example, an appropriate PT scheme would involve circulating reference thermometers to be calibrated at multiple temperature points in the cold chain range, with results evaluated using En numbers against reference values established by NMIs.

Verify that the PT provider is accredited to ISO/IEC 17043 by an ILAC MRA signatory accreditation body — only accreditation to ISO/IEC 17043 ensures that the PT provider meets the international standard for PT management, and results from an accredited PT scheme carry more weight with PAB than results from a non-accredited scheme.

Step 2: Enrol and Receive the PT Sample or Artifact

Contact the PT provider, complete the registration process (which typically requires providing your laboratory’s accreditation details, scope of measurement, and claimed measurement uncertainty), and pay the participation fee. The PT provider will then send you the PT sample, artifact, or reference item to be calibrated.

For temperature calibration PT schemes, this may be a calibrated reference thermometer or data logger that is circulated among participating laboratories in sequence. For dimensional schemes, it may be a gauge block or ring gauge. Each laboratory calibrates the item according to its normal procedures and returns results and the item to the PT provider within the specified timeframe.

Step 3: Perform the Calibration Under Normal Conditions

This is the most important principle of PT participation: calibrate the PT artifact exactly as you would calibrate any client instrument under your normal operating procedures. Do not use special procedures, extra care, or staff who do not normally perform this type of calibration. The purpose of PT is to evaluate your laboratory’s normal performance — not your best possible performance under ideal conditions.

A laboratory that applies special handling to its PT artifact will produce results that overestimate its normal measurement quality. When PAB assessors or clients later compare the PT results to actual client calibrations, any systematic discrepancy will raise questions about whether the PT participation was representative.

Step 4: Report Results to the PT Provider

Report your calibration result in the format specified by the PT provider — including the measured value, your expanded measurement uncertainty, the coverage factor used, and any relevant metadata required by the PT scheme. The measurement uncertainty you report must be the actual uncertainty of your calibration of the PT artifact — not a generic uncertainty figure copied from your uncertainty budget without regard to the specific conditions of the PT calibration.

Step 5: Receive and Interpret the PT Report

After all participating laboratories have submitted their results, the PT provider performs the statistical evaluation and issues the PT report. The report shows: the assigned reference value and its uncertainty, each participant’s result (typically identified by a code rather than by name, to maintain confidentiality), the calculated performance score (z-score or En number) for each participant, and the overall distribution of results across all participants.

Your PT report will show your laboratory’s result alongside the reference value and the performance metric. If your score is satisfactory (z ≤ 2 or En ≤ 1), document the result in your PT records and file the report with your accreditation documentation. If your score is questionable or unsatisfactory, proceed to the corrective action process described in the next section.

6. Responding to Unsatisfactory PT Results: What to Do When Your Score Is Poor

An unsatisfactory PT result — a z-score ≥ 3 or an En number > 1 — is not a catastrophe, but it requires a systematic, documented response. Most accreditation bodies, including PAB, do not revoke accreditation for a single unsatisfactory PT result, provided the laboratory responds appropriately. The response to an unsatisfactory result is as important as the result itself in demonstrating a laboratory’s commitment to measurement quality.

Step 1: Do Not Panic — and Do Not Ignore It

The first response to an unsatisfactory PT result should be neither panic nor dismissal. Both are common reactions — and both are inappropriate. An unsatisfactory result is valuable information: it tells you that something in your measurement system is producing results that are inconsistent with the broader measurement community. The goal is to find out what that something is and correct it.

Do not file the poor PT report away and hope PAB assessors do not notice it during the next surveillance assessment. PAB assessors specifically review PT participation records and results, and a poor result with no documented response is significantly more problematic than a poor result with a well-documented corrective action programme.

Step 2: Conduct a Root Cause Analysis

A formal root cause analysis is required for all unsatisfactory PT results. The analysis should examine every possible source of the discrepancy:

• Reference standard: Is your reference standard’s calibration current and correct? Could there be an error in the correction applied from the reference standard’s certificate? Has the reference standard drifted between its last calibration and the PT exercise?
• Measurement uncertainty: Is your stated measurement uncertainty accurate and complete? Have you omitted significant uncertainty sources? Is the En number > 1 because your true measurement uncertainty is larger than you claimed — rather than because your measurement accuracy is actually poor?
• Calibration procedure: Was the PT artifact calibrated according to your normal SOP? Were there any deviations from the normal procedure? Were the environmental conditions in the calibration area appropriate?
• Equipment: Is your calibration bath or dry-block calibrator performing correctly? Has its temperature uniformity been recently verified?
• Personnel: Was the calibration performed by your most experienced technician? Is there a difference in results between technicians that might indicate a technique issue?

Step 3: Implement and Document Corrective Actions

Based on the root cause analysis, implement specific corrective actions targeted at the identified root cause:

• If the root cause is an error in the reference standard correction: recalibrate the reference standard with the PAB-accredited provider, verify the correction, and recheck the PT artifact if possible
• If the root cause is an incomplete uncertainty evaluation: revise the uncertainty budget to include the omitted components, verify the revised uncertainty is realistic, and document the revision
• If the root cause is a procedure issue: revise the relevant SOP, train all technicians on the revision, and demonstrate that the revised procedure produces results consistent with the reference value
• If the root cause cannot be definitively identified: document the investigation thoroughly, implement multiple corrective actions targeting the most likely causes, and plan to participate in a follow-up PT exercise as soon as possible to verify improvement

Step 4: Verify Effectiveness and Report to PAB

After corrective actions are implemented, verify their effectiveness — ideally through participation in a subsequent PT exercise or a bilateral ILC that confirms the laboratory’s results are now consistent with the reference value. If a follow-up PT exercise is not immediately available, document the internal verification activities conducted to assess whether the corrective actions addressed the root cause.

The complete corrective action documentation — root cause analysis, corrective actions implemented, evidence of effectiveness, and the follow-up verification — must be submitted to PAB if required by the accreditation assessment schedule, and maintained in the laboratory’s quality records for review during the next PAB surveillance assessment.

7. Building a PT Participation Plan: Practical Guidance for Philippine Labs

A documented PT participation plan is required by ISO/IEC 17025:2017 Section 7.7.2 and will be reviewed by PAB assessors. Here is practical guidance for building and maintaining an effective plan for a Philippine calibration laboratory.

Step 1: Map Your Scope to Available PT Schemes

Create a matrix that maps each measurement type in your laboratory’s accredited scope to available PT schemes. For each measurement type, identify: the PT providers who offer appropriate schemes, the frequency of available schemes (annual, biennial), the cost of participation, and whether the scheme uses z-scores or En numbers for evaluation. Where no suitable scheme is found, document this and identify the bilateral ILC alternative.

Step 2: Schedule PT Participation

Based on the scheme availability map, prepare a multi-year PT participation schedule that ensures all measurement types in your scope are covered within the PAB-required timeframe (at minimum, at least one satisfactory PT exercise per accreditation scope within the re-accreditation cycle). For high-volume measurement types (such as temperature calibration for a laboratory serving the pharmaceutical cold chain sector), annual PT participation is best practice.

The PT schedule should be documented formally — a table showing the PT scheme, the provider, the planned participation date, and the status (planned, enrolled, completed, result received) — and maintained as part of the laboratory’s quality records.

Step 3: Budget for PT Participation

PT participation has a cost — participation fees, shipping of PT artifacts, and staff time for calibration and result reporting. For Philippine calibration laboratories, international PT scheme participation fees typically range from USD 50 to USD 500 per scheme (approximately ₱3,000 to ₱29,000 at current exchange rates), with additional costs for international shipping of PT artifacts if required.

APLAC PT schemes are typically more affordable for Asia-Pacific participants than European or North American schemes, and some are subsidised for developing economy participants. Building PT participation costs into the laboratory’s annual quality budget ensures that PT is treated as a routine operational expense rather than an exceptional cost requiring special approval.

Metrologie Solutions Philippines: Calibration Support for PT ParticipationParticipating successfully in PT schemes requires well-calibrated reference standards, accurate uncertainty evaluation, and consistent calibration procedures — all elements we have covered in previous articles in this series.Metrologie Solutions Philippines provides PAB-accredited temperature calibration services for reference thermometers and data loggers, measurement uncertainty training for calibration technicians, and consulting support for Philippine calibration laboratories building or improving their ISO/IEC 17025 quality systems and PT participation programmes.Contact us at metrologiesolutions.com to discuss how we can support your laboratory’s PT compliance.

8. Frequently Asked Questions: Proficiency Testing for Philippine Labs

Is proficiency testing required for all Philippine PAB-accredited calibration laboratories?

Yes. ISO/IEC 17025:2017 Section 7.7.2 requires all accredited calibration laboratories to monitor their performance through comparison with other laboratories, and PAB requires demonstration of satisfactory PT participation as part of the accreditation and re-accreditation process. The scope of PT participation required covers all measurement types in the laboratory’s accredited scope, with at least one satisfactory PT exercise per scope area required before initial accreditation is granted and ongoing participation required to maintain accredited status. Where no formal PT scheme exists for a specific scope area, a documented bilateral ILC is the recognised alternative.

How often does a PAB-accredited Philippine laboratory need to participate in PT?

PAB requires at minimum one satisfactory PT exercise covering each area of the laboratory’s accredited scope during each accreditation cycle (typically three to five years between full re-accreditation assessments). For laboratories with broad scopes, this means participating in multiple PT schemes across different measurement types during each accreditation cycle. Best practice — and increasingly the expectation for laboratories serving regulated industries such as pharmaceutical and food — is annual PT participation for each major measurement type.

What happens if we cannot find a PT scheme for our measurement type?

ISO/IEC 17025:2017 uses the language ‘where available and applicable’ when describing PT requirements, acknowledging that formal PT schemes do not exist for every measurement type. If no appropriate PT scheme is available, document this fact (with evidence of your search for available schemes) and implement alternative means of performance verification — typically a bilateral ILC with another PAB-accredited laboratory calibrating the same measurement type. The bilateral ILC must be formally managed with a written agreement, defined conditions, statistical evaluation, and documented results. Contact PAB directly for guidance on accepted alternatives for specific measurement types where PT schemes are genuinely unavailable.

Can we use our internal check standards instead of participating in external PT?

No. Internal check standards and internal quality controls serve an important role in day-to-day measurement quality management, but they are not a substitute for external PT participation. Internal checks can only verify consistency within the laboratory’s own reference system — they cannot detect systematic biases that run through the entire laboratory, including its reference standards. ISO/IEC 17025 specifically requires external comparison with other laboratories for exactly this reason.

What is the difference between a z-score and an En number, and which applies to temperature calibration?

The z-score compares a laboratory’s result to the assigned value using the PT provider’s defined standard deviation for proficiency assessment, without regard to the laboratory’s claimed measurement uncertainty. The En number divides the deviation from the reference value by the combined measurement uncertainty of both the laboratory’s result and the reference value. For temperature calibration PT where laboratories are expected to state their measurement uncertainty and where that uncertainty claim is a meaningful part of the assessment, the En number is the more appropriate metric. A laboratory claiming very small measurement uncertainty will have its uncertainty claim tested by the PT — if the claimed uncertainty cannot account for the deviation from the reference value, the En number will exceed 1 even if the absolute deviation is small.

Conclusion: Proficiency Testing Is the External Mirror That Every Laboratory Needs

A calibration laboratory’s internal quality system — its procedures, its records, its calibrated reference standards, its trained personnel — tells it how well it is following its own standards. Proficiency testing tells it whether those standards are producing results that are right.

For Philippine calibration laboratories seeking or maintaining PAB accreditation, proficiency testing is a mandatory regulatory requirement. But beyond compliance, it is the most valuable tool available for catching the one category of error that internal controls cannot detect: systematic measurement bias that affects the entire laboratory’s output simultaneously.

The laboratory that engages genuinely with PT — planning participation across its full scope, calibrating PT artifacts under normal conditions rather than special conditions, responding thoroughly to unsatisfactory results, and using PT performance data to drive uncertainty budget improvements — is building a measurement quality programme that deserves the confidence of its clients, regulators, and the international accreditation community.

The laboratory that treats PT as a box to tick — participating minimally, using special procedures for PT artifacts, and setting unsatisfactory results aside without investigation — is building compliance theatre rather than measurement quality. The difference shows up, eventually, in calibration results that clients and auditors begin to question.

Metrologie Solutions Philippines is committed to supporting Philippine calibration laboratories in building genuine measurement quality — through PAB-accredited reference standard calibration that provides the traceability foundation of every measurement, measurement uncertainty training that produces honest and defensible uncertainty claims, and consulting support for laboratories building and improving their ISO/IEC 17025 quality systems and PT participation programmes.

Ready to Build Your PT Participation Programme?Contact Metrologie Solutions Philippines for guidance on identifying appropriate PT schemes for your measurement scope, building a PT participation plan that meets PAB requirements, and ensuring your calibration programme has the measurement uncertainty quality needed to achieve satisfactory En numbers in PT evaluation.Website: metrologiesolutions.com   |   Services: Calibration · ISO/IEC 17025 Consulting · Measurement Uncertainty Training · Thermal Mapping

About Metrologie Solutions PhilippinesMetrologie Solutions Philippines is a PAB-accredited calibration laboratory and the Philippines’ leading provider of thermal mapping, calibration, and metrology training. We help calibration laboratories, pharmaceutical companies, food manufacturers, and healthcare facilities understand and meet their proficiency testing and interlaboratory comparison obligations under ISO/IEC 17025:2017 and PAB accreditation requirements. Our calibration services support the traceability chain that underpins compliant PT participation.Website: metrologiesolutions.com   |   Services: Calibration · Thermal Mapping · Training · ISO/IEC 17025 Consulting