Good Laboratory Practice for PT Follow Up – 2023 Page 1 of 6 

Good Laboratory Practice (GLP)

for 

Interlaboratory Comparison, Intralaboratory Comparison, and Proficiency Testing 

Follow-up

1 Introduction 

Purpose 

The purpose of this procedure is to provide consistent evaluation of 

intralaboratory comparisons, interlaboratory comparisons, and proficiency tests

and may be used for Interim and Final results. For simplicity, the term 

proficiency test (PT) will be used in this procedure to cover all of these 

comparisons. This GLP provides an outline for monitoring performance by 

comparison with results of other laboratories that is planned and reviewed. The 

practice includes analyzing data from monitoring activities including 

proficiency testing, interlaboratory comparisons, internal surveillance, 

measurement assurance activities, and training or verification of training (e.g., 

Laboratory Auditing Program problems). The resulting analysis is used to 

monitor and control (where necessary), and if applicable, improve the

laboratory's activities. When results of the analysis of data from monitoring 

activities are found to be outside pre-defined criteria, appropriate action is 

documented on the laboratory Action Item forms with action taken as soon as 

possible to prevent incorrect results from being reported. 

Application 

The procedure is applicable for interlaboratory comparisons and proficiency 

tests used for method validation, competency assessment for personnel, and 

formal proficiency testing programs. 

Prerequisites: 

The laboratory must have the documented proficiency testing plan, 

analyses, and analysis reports. 

The laboratory measurement assurance data (e.g., control charts), 

calibration history of standards, and uncertainty budgets must be 

available when integrated analysis is conducted using the data from 

intralaboratory comparisons, interlaboratory comparisons, and 

proficiency tests. 

2 Methodology 

Summary
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This procedure provides an outline for consistent evaluation of intralaboratory 

comparisons, interlaboratory comparisons, and proficiency testing to comply 

with ISO/IEC 17025 and OWM Handbook 143, Program Handbook for 

laboratory recognition, including integrated evaluation with other applicable 

laboratory data and analyses. 

Procedure 

Assess Differences and Bias from Reference Values (Accuracy)

2.2.1.1 Evaluate Accuracy of the Laboratory Results 

Evaluate the accuracy of the reported measurement results. The 

following statistics are often provided as a measure of accuracy or 

inaccuracy in PT reports: normalized error, En, and bias, Z value. When 

the normalized error is greater than the absolute value of one, the 

laboratory must investigate the source of this bias or inaccuracy. When 

the bias or Z values show an offset, it should be evaluated against 

additional laboratory data. When bias is greater than the laboratory 

reported uncertainty or the Z values exceed a value of three and the 

uncertainty is less than the bias value, there is cause for investigation. 

2.2.1.2 Evaluate Additional Laboratory Data 

Evaluate the reported measurement results, final report analysis, and 

statistics from the previous item against the laboratory measurement 

assurance data. Additional statistics might include the stability or trends 

for historical mean values from calibrations, the current or changing 

mean values from control charts, and any bias values included in 

uncertainty budgets.

2.2.1.3 Questions to Consider When Evaluating Differences and Bias 

• What (if any) En failures were reported?

• Were any En values highlighted between 0.7 and 1?

• If En failures were reported, are the differences or bias

correlated with data that has been observed on control

charts or calibration history?

• Were any Z values between 2 and 3, or greater than 3

indicating a difference greater than the PT standard

deviation?

• If a bias was present, even if all statistics were passing, are

there any overriding reasons for differences from the

reference values? (E.g., are there any measurement errors

that need to be corrected?)

• If the laboratory includes bias in the uncertainty budgets,

do the bias values need to be updated or evaluated further?
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Assess Laboratory Uncertainties (Precision)

2.2.2.1 Questions to Consider When Evaluating Uncertainties 

• Were reported uncertainties consistent with those submitted

to accreditation or recognition bodies?

• Were reported uncertainties significantly higher or lower

than values submitted to accreditation or recognition bodies

(if so, why?)

• Are uncertainties consistent with the procedure defined in

the PT plan?

• Were reported uncertainties consistent with other

participant laboratories performing similar procedures?

Assess Uncertainties with respect to Decision Risk (Precision and 

Conformity Assessment)

2.2.3.1 Assess the Precision Assessment of Laboratory Results

Evaluate the reported measurement uncertainties against the required 

specifications, maximum permissible errors, or tolerances identified in 

the PT plan and for the class/tolerance specified for the PT item(s). The 

following statistics are often provided as a measure of precision 

assessment in PT reports: normalized precision, Pn. 

2.2.3.2 Evaluate Additional Laboratory Data 

Evaluate the reported measurement uncertainties, final report analysis, 

and statistics from the previous item against the laboratory uncertainty 

data and values submitted to the accreditation or recognition bodies as 

applicable. Additional uncertainties in the laboratory might include 

those for alternative calibration procedures or might be incomplete with 

respect to the Standard Operating Procedure called out in the PT Plan. 

If the laboratory uncertainty showed a Pn precision failure prior to 

participation in the PT, corrective actions should have already been 

identified and in process. 

2.2.3.3 Questions to Consider When Evaluating Uncertainties and 

Decision Risk 

• Do the documentary standards for this measurement

include requirements for uncertainty to be evaluated with

respect to decision rules?

• What are the applicable documented limits for decision

rules for this PT?

• Does the laboratory include references to the applicable

documentary standards on the certificate?
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• Note: if tolerances or maximum permissible errors are

listed on the calibration certificate(s), this is considered a

conformity assessment and additional statements compliant

with ISO/IEC 17025 must be listed. Are applicable

statements included on the certificates?

Assessment of Failures Not Specific to Measurement Results

When a PT plan specifically calls out additional components for evaluation, the 

final report should have additional observations, comments, or notes regarding 

actions. Examples of observations that might be evaluated or included in a PT 

plan or in the PT final report: 

• Certificate compliance with ISO/IEC 17025:2017;

• Completion of follow up calibration;

• Correct completion of amended certificates;

• Inclusion of uncertainty components for detailed

evaluation;

• Completion of the official data sheets;

• Completion of receipt or shipping forms;

• Cleaning or adjusting standards contrary to instructions;

• Poor shipping and handling of PT standards;

• Participation by staff and laboratories who were not

approved for participation;

• Laboratory Auditing Program (LAP) problem failures; and

• Undue delay of the PT.

Documented Analysis and Records

Document the analyses and answers to applicable questions in the PT Follow 

Up form (see Section 3) and appendices where needed. Identify where the 

records are stored as appropriate. 

A 4-year history assessment is a useful summary and provides a basis for 

examining trends and demonstrating compliance with PT participation 

requirements. The laboratory may also maintain a Proficiency Testing Log to 

ensure ongoing participation for all staff and especially laboratory Approved 

Signatories. 

Action Plans

Be sure to document all action items (whether corrective action, risk mitigation, 

or improvement actions) on the applicable laboratory forms.

Executive Summary and Impact 

Write an executive summary that summarizes the PT scope, range, and results 

with narrative that describes the impact of success or failures from each PT. 
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The executive summary may be used in the laboratory management review, as 

a lab best practice. Include any applicable action items that require(d) 

management approval to dedicate resources. 
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3 Proficiency Testing Follow-up Form 

Instructions: complete one PT Follow-up Form per PT and per laboratory (do not complete 

one form for each staff member unless the form is being used as part of the competency 

assessment for that staff member’s Laboratory Auditing Program (LAP) problems). This form 

may be used to summarize critical PT highlights that will be used in laboratory Management 

Reviews.

Laboratory

Date

Completed By

PT Measurement Parameter, Range, and Scope 

Description

PT Identification (OWM Code) and artifact ID

List of Participating Personnel (17025, Section 6.2)

Note Approved Signatory or In Training Status

Assessment Results and Evidence

2.2.7 Executive Summary and Program Impact.

PT Failure Summary. 

2.2.1 Difference, Bias, Offset Assessment. 

2.2.2 Uncertainty Analysis. 

2.2.3 Decision Rules and Conformity Assessment 

2.2.4 Non-Measurement Result Observations or 

Failures. 

2.2.5 Records. 

2.2.6 Analysis and Action Plan with Assigned 

Personnel and Deadlines.