What is Measurement Uncertainty?
Measurement uncertainty is a value that is associated with the quantitative result, that describes the variability of the measurement (applied to a sample or a group of samples). All analyses, especially at the levels that a laboratory measures at, have a certain degree of variability; this variability is expressed as measurement uncertainty (MU), often seen as a "±" value on the Certificate of Analysis. So for example, a concentration reported at 10ug/L and an MU of 1ug/L means the concentration in this sample is between 9 and 11ug/L.
This variability is influenced by a number of factors, including sample collection procedures, laboratory sub-sampling (representativeness), sample heterogeneity, extraction and instrument analysis. The highest influence on data variability comes from field sampling. However, keep in mind that measurement uncertainty reported by the laboratory represents variability due to laboratory testing only.
On a standard Certificate of Analysis from Bureau Veritas Laboratories, the MU may be reported (through customer request), following a review and assessment of the data by the QA group. Note that depending what the concentration of the analyte is, the MU value may actually be at or below the RDL, in which case you may see a "±RDL" or "±<RDL" value reported.