What the standard actually requires
ISO/IEC 17025:2017 clause 7.7.2 says a laboratory must monitor its performance by comparison with the results of other laboratories, where available and appropriate. That monitoring has to be planned and reviewed. It includes proficiency testing, other interlaboratory comparisons, or both. The clause does not say "once a year" or "once every four years."
ILAC policy then tells accreditation bodies to require proficiency testing (or an equivalent comparison) as part of accreditation. The detail — how often, and how much of the scope must be covered — lives in each body's own PT policy.
What accreditation bodies generally expect
Published policies we have reviewed follow a similar shape:
- At least one PT or interlaboratory comparison in a typical year, with some bodies asking for more when the scope is large.
- Coverage of every major area or sub-discipline on the scope over a four-year cycle.
- A written PT plan that an assessor can follow, updated after scope changes and after each result.
- For initial accreditation, recent participation in at least part of the proposed scope.
A2LA is a clear public example. Its R103 — General Requirements: Proficiency Testing for ISO/IEC 17025 Laboratories (listed on A2LA's public documents page) asks laboratories to keep a documented plan that covers the applicable major sub-disciplines on the scope over a four-year period. A2LA also notes on its FAQ that authorized sub-disciplines are to be covered in a four-year period. Annual minimums in R103 depend on how many of those areas are on the scope, so do not treat "two PTs a year" as a universal A2LA rule.
Other bodies (ANAB, PJLA, IAS, and others) publish their own policies, often with a similar four-year coverage idea. Requirements change. Check the current PT policy from the body that accredits you before you lock a plan.
Building a four-year PT plan
- List the major areas on your scope. Group them the way your accreditation body does — for example mass, scales and balances, volume, dimensional, temperature, pressure, flow, and electrical.
- Pick representative programs. One well-chosen PT can stand for similar items and ranges. A 200 g analytical balance PT, for example, is a common way to cover analytical balance calibration.
- Put higher-risk work earlier. High-volume calibrations, tight uncertainties, new technicians, and any past unsatisfactory result belong near the start of the cycle.
- Add extra participants when it matters. Several technicians can measure the same kit. That shows consistency across staff without buying a second program.
- Review the plan every year. Update it after scope changes, staff changes, and each PT result.
En values and how pass/fail works
Calibration proficiency tests are usually scored with the En number (normalized error): the difference between your result and the provider's reference value, divided by the root sum of squares of both expanded uncertainties.
- An absolute En below 1 is satisfactory — a pass. The difference is covered by the combined uncertainty.
- An |En| of 1 or more is unsatisfactory. Either the measurement is off, or the uncertainty is too small to cover it.
- Each test point is scored on its own. One failed point does not automatically fail the rest of the report.
For scale and force (load cell) PTs, participants report the scale's calibration uncertainty with each reading. That is the Ulab in the En calculation. See how En numbers are calculated for the formula and a worked example.
At Lake Balance, participants get pass/fail the same day their results are reviewed. Point-by-point En values and the report PDF are released once payment is received and the kit's return shipment shows in transit with the carrier.
How a Lake Balance PT fits the plan
Lake Balance is an ISO/IEC 17043 accredited PT provider (PJLA certificate L24-898). Most programs are $695 plus $100 per extra participant, plus shipping. The kit ships the same day payment is received. Browse the PT program library or read the ISO/IEC 17025 PT requirements guide for how clause 7.7.2 fits a four-year plan.