Uncertainty of measurement: what it is and what it should be.

Uncertainty of Measurement (UM) is defined [ISO15189 (3.17)] as “a parameter associated with the result of a measurand that characterises the dispersion of values.”2 In the clinical laboratory, similar information has long been available in the form of the Standard Deviation (SD) as a measure of imprecision. Thus the current discussion is about an old concept that is routinely measured and available from all accredited laboratories. What is therefore surprising is that UM is causing such anxiety and interest when it is so familiar to laboratory professionals. Perhaps some are concerned that highlighting UM issues may be interpreted by clinicians as a confession of previously undisclosed “laboratory error”. Although UM may not be a new idea for the laboratory, the present emphasis on the statistics tends to overlook the far greater problem of over-interpretation of results by clinicians. There are several areas where uncertainty in laboratory results can cause clinical problems. Examples are given below, as well as suggestions as to how guidelines on UM could be extended to provide useful information to clinical users.