MEASUREMENT UNCERTAINTY / 01
Measurement uncertainty (MU) in medical laboratories
Understand what a laboratory uncertainty estimate describes, define the measurand and find the guide for evaluating, interpreting or documenting it.
By Stephen MacDonald PhD FRCPath · Free introductory guide
Choose your measurement uncertainty question
Read the introduction below for the meaning and scope of MU, or follow the question you are working on.
How do I use routine IQC data?
Work through the top-down guide and its pooling example.
Should I use SD or standard error?
Separate variation in individual results from uncertainty in an estimated mean.
What does calibration contribute?
Follow assigned values, their uncertainty and possible overlap with routine performance data.
How do I document the estimate?
Record the model, supporting evidence, scope and review triggers.
What uncertainty describes
A result is an estimate, not a perfectly known quantity. Measurement uncertainty describes the dispersion of values that can reasonably be attributed to the measurand on the information available. It is not the observed error, a guarantee of accuracy, or a prediction of the patient’s future result.
Write a useful measurand statement
The measurand is the quantity intended to be measured. Identify the system or specimen, component, kind of quantity and relevant conditions. Specify the procedure when the quantity is operationally defined. Units describe how the value is expressed; naming the analyte alone is rarely enough.
| Record | Example to adapt |
|---|---|
| Quantity | Amount-of-substance concentration of glucose in plasma |
| Reported result | One routine result, or an explicitly defined average |
| Scope | Procedure, instruments, specimen types and measuring interval |
| Conditions | Relevant handling or preparation conditions; exclusions stated separately |
Match the estimate to its use
Ask whether variability changes with concentration, specimen type, instrument or procedure. A single percentage across an entire measuring interval needs supporting evidence. A concentration-dependent function or a small set of justified intervals may be more defensible.
Keep the boundary visible
Analytical MU is not the whole uncertainty of a clinical interpretation. Biological variation, specimen problems and uncertainty in a diagnostic threshold may matter, but are not automatically included in an analytical uncertainty estimate.
Before calculating
- Define whether the output is numerical or categorical.
- State which routine sources of variation the evidence covers.
- Specify the concentration or interval at which the estimate applies.
- Identify the clinical or operational question the estimate will support.
Sources & further reading
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.
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Explain an uncertainty estimate with confidence.
Build a one-page explanation of what was measured, what the uncertainty means and which evidence supports it. Short practical tasks help you turn the theory into something you can use.
Measurement uncertainty: explain one result.
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Get ready for an MU discussion
Use six short questions to find records, note where they are kept and list what you need help with. Write “not known” where information is missing. An experienced reviewer can then decide what is relevant. This preparation sheet does not choose a method or perform calculations.
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