Measurement Uncertainty in Medical Laboratories — ISO 15189:2022 Guide

Measurement uncertainty (MU) is one of the most technically challenging requirements of ISO 15189:2022 for medical laboratories. Many labs struggle with it — but it’s non-negotiable for NABL accreditation.

Measurement uncertainty expresses the doubt that exists about any measurement result. No measurement is perfect — there is always some uncertainty arising from instrument imprecision, calibration errors, operator variability, and other sources. ISO 15189:2022 requires labs to calculate and report MU for all quantitative tests.

Why Does MU Matter Clinically?

MU tells clinicians how much confidence to place in a test result. A serum sodium reported as 138 mmol/L with an expanded uncertainty of ±3 mmol/L means the true value likely lies between 135 and 141 mmol/L — within or outside the reference range depending on where you look. This matters for clinical decisions.

The IQC-Based Approach to Calculating MU

For most clinical chemistry and haematology tests, the simplest acceptable approach to MU calculation uses internal QC data:

1. Collect at least 6 months of IQC data for each analyte at each level
2. Calculate the within-run imprecision (SD) from daily IQC runs
3. Calculate the between-run imprecision (SD) from consecutive day IQC data
4. Calculate the combined standard uncertainty: u = √(u_within² + u_between²)
5. Calculate the expanded uncertainty: U = 2 × u (for ~95% confidence)
6. Express as a percentage: U% = (U / mean) × 100

Reporting MU to Clinicians

MU must be available to clinicians on request. You are not required to print it on every report, but your laboratory must be able to provide it when asked and must use it in the interpretation of results near decision limits.