A Failed PT Round Is an Investigation, Not a Score

What counts as a proficiency testing failure?#

Each scheme draws the line in a different place, and the table puts CLIA’s proficiency testing rules beside TNI’s and ISO/IEC 17025’s.

If CAP accredits you, read CAP’s proficiency testing requirements alongside CLIA’s. For environmental labs, we cover TNI’s pass criteria in detail.

All three schemes require a written investigation after a failure.

Why is passing the next round not the end of it?#

Passing the next round doesn’t explain the failed one. The Q4 pass tells you the method worked in Q4. It says nothing about why Q3 failed, or whether anything you reported between the two rounds was wrong.

Eurachem’s guidance on poor PT performance is direct about this. A lab “should acknowledge it, carry out an investigation and document a review of possible causes, even if it decides not to take any specific action.”

That last option is open to an ISO/IEC 17025 lab, which sets its own criteria for when to act. CLIA and TNI leave it closed. Every CLIA specialty section puts it plainly for any unacceptable analyte or unsatisfactory event: “Remedial action must be taken and documented.” TNI asks for the root cause and a corrective action. In those labs, the record always shows an action. For a clerical cause, it can be as small as fixing the transcription step.

We would rather read two short paragraphs on the miss and the response than a clean next round with nothing behind it. In a 17025 lab, the response can be “no action, here is why.” In a CLIA or TNI lab, it names the remedial action.

What should the investigation look at, in what order?#

Start with the cheapest explanation and work toward the most expensive one. First, check whether the PT report explains the result, and confirm it shows what you submitted.

Then look for clerical errors. Eurachem calls finding them “an important first step of an investigation,” and its first example shows why. A medical lab’s tumor-marker PT results were off by 20 percent while its daily controls were fine. An IT system update had introduced a wrong unit conversion factor for PT results. Patient results used a different unit, so none were affected.

Check the PT item next, then the run and the method. In a second example, a food lab undercounted Listeria because the analyst used an out-of-date procedure, and the lab’s 5 Whys ended at the workload of the person in charge of document control. Look at the scheme last. An assigned value with too much uncertainty can score a lab unsatisfactory for reasons outside the lab, and that finding goes in the record too.

For the Q3 analyte in that binder, check these ten points:

What do assessors cite after a PT failure?#

Assessors cite the missing record, and they can cite it years after the event. A CMS Form 2567 for an Illinois laboratory, from a survey completed April 14, 2026, reads like the binder from the opening.

The surveyor quoted the lab’s own policy, which required a root cause analysis and documented corrective actions. Then the surveyor wrote the finding under tag D5221: “Review of CAP PT records revealed a lack of corrective action documentation for one of one unacceptable PT performance.”

The event was in 2023, and the citation came in 2026. Under tag D6091, the same form found that only testing personnel had reviewed two of five PT events. In 2025, ASCLS wrote that “PT performance and review is a supervisory duty that often gets pushed to the back burner.”

Two things changed in 2026. Neither one changes what goes in the record. In July, CAP told laboratories that some international PT shipments would arrive late or not at all. Under CLIA, a missed event still needs a documented check of accuracy. In October, CMS proposed to end the six-month reinstatement wait after PT sanctions. It called the wait “more punitive than motivational” and left the documentation duties as they are.

Which reported results are affected, and do you tell the customer?#

What you found decides which reported results are in question. If the cause was a typo on the PT submission, the results you reported weren’t touched, and the record should say why. If the cause was in the method, the instrument or the procedure version, every result run under that cause is in question.

TNI asks labs to notify customers in writing if results may have been affected, under a policy that sets the time frame. ISO/IEC 17025 asks for an evaluation of significance, “including an impact analysis on previous results,” and a decision on acceptability. Then, “where necessary, the customer is notified and work is recalled.”

CLIA has no PT lookback rule. What you owe depends on what you found. Under 493.1445(e)(4), the director makes sure an approved corrective action plan is followed, and the specialty sections keep the remedial action on file for two years. When the investigation finds errors in reported patient results, 493.1291(k) says to “Promptly notify the authorized person ordering the test” and “Issue corrected reports promptly.”

Put the list of results you checked on the same corrective action record as the cause, so the assessor reads one record.

Write down which results you checked, even when the answer is none.

Would your last PT failure pass this check?#

Pull your last unacceptable PT result. The record should show a written review of possible causes, the decision and the reported results you checked. Under CLIA or TNI, it should also name the remedial action. A supervisor or director should sign and date it. If only the analyst signed, that’s the D6091 finding from the Illinois form.

Here’s an example half page for a clerical cause, modeled on Eurachem’s unit-conversion case:

• Result: tumor marker, Q3 event, unacceptable, 20 percent off the assigned value.
• Causes reviewed: daily controls in range, calibration and reagent lots unchanged, and the retained item agrees once the units are converted correctly.
• Cause: a system update changed the unit conversion factor for PT results only.
• Results checked: patient results are reported in a different unit, so none were affected.
• Remedial action: the factor corrected, and a second person checks units before each PT submission.
• Check: the Q4 event went through the corrected step and passed.
• Signed: the laboratory director, with the date.

If a page like this is missing, start it now, while the people who ran the sample still remember the day.

How does a LIMS help with a PT investigation?#

A LIMS helps most when the PT run is on the record before the score arrives.