MEASUREMENT UNCERTAINTY / 06
Document, report and review uncertainty
A useful budget explains what the numbers mean, where they apply and when they need updating.
Make the reasoning auditable
| Record | Include |
|---|---|
| Result and scope | Measurand, procedure, specimen, units and concentration range |
| Evidence | Data period, sample sizes, certificates and exclusions |
| Model | Contributions, distributions, sensitivity and correlation |
| Output | Standard uncertainty; expanded uncertainty or coverage interval |
| Limitations | Missing contributions, provisional assumptions and intended use |
| Ownership | Version, reviewer, approval and review triggers |
State coverage honestly
Expanded uncertainty is U = k × u when that representation is appropriate. A factor of 2 is often associated with approximately 95% coverage under suitable assumptions; it is not an automatic guarantee. Small effective degrees of freedom, asymmetry or nonlinearity may require a different treatment.
Example reporting statement
For this procedure and concentration, the result is 100 units with expanded uncertainty 5.2 units (k = 2; approximately 95% coverage under the stated model). The estimate covers the specified analytical process, not all biological or clinical interpretation uncertainty. Use consistent rounding and avoid implying precision unsupported by the estimate.
Choose a meaningful performance target
Relate the assessment to intended clinical use, relevant decision levels and the consequences of analytical variation. Explain the basis of any performance specification. A biological-variation target, an EQA limit and a manufacturer claim are not interchangeable simply because each is expressed as a percentage.
Review when the evidence changes
Set a planned review interval locally, and review after meaningful changes in calibration, reagent lot behaviour, instruments, procedure, precision or bias. A calendar tick alone does not demonstrate continuing validity. Compare the old and new assumptions as well as the final number.
ISO/TS 20914 covers estimating and expressing MU in medical laboratories; its public scope does not make routine inclusion on every patient report a universal requirement. Follow the applicable local reporting policy and licensed requirements.
Sources & further reading
- ISO/TS 20914:2019 — public scope and status
- JCGM 100:2008 — Guide to the Expression of Uncertainty in Measurement
Educational guidance, not a substitute for a locally approved procedure. Worked examples are synthetic. Reviewed against the sources indicated; licensed standards must be consulted for their full requirements.