Pragmatic measurement uncertainty: Aligning IQC data duration with clinical reality in long-term monitoring

ISO 15189:2022 requires medical laboratories to estimate measurement uncertainty (MU), and ISO/TS 20914:2019 provides the practical guidance for doing so, typically through a top-down approach in which the within-laboratory reproducibility standard uncertainty (uRw) is derived from 6 to 12 months of internal quality control (IQC) data. This criterion is defined by the routine sources of variation captured, not by the window over which results are actually compared. For acute, single-use tests the two coincide; for analytes used in long-term monitoring - thyroglobulin, PSA, creatinine/eGFR, and many tumor markers and endocrine parameters - a result from 2021 may be compared with one from 2026, and the relevant uncertainty spans every reagent and calibrator lot, recalibration and maintenance event within that interval, whatever the analyte. Where a valid estimate of within-subject biological variation is available, the index of individuality identifies the analytes for which this shortfall matters most, interpretation then resting on the reference change value. This letter proposes aligning the duration of IQC data with the clinical application window of the test. Partitioning by reagent and QC lot, as recommended by CLSI C24 and ISO/TS 20914, keeps long-term estimates sound. This perspective extends the "Measurement Uncertainty for Practical Use" (MUPU) model to the temporal axis, advocating a shift from idealised short-term precision to pragmatic, clinically fit-for-purpose MU. A two-stage scheme is recommended: a rolling annual uRw for analytical trends and an aggregate long-term uRw for clinical reporting. Future guideline revisions should individualise the uRw duration according to the clinical use window of the test.