Influence Quantities and Uncertainty of Measurement

Abstract

According to the International Organization for Standardization (ISO), an “influence quantity” is a “quantity that is not the measurand but that affects the result of the measurement” \[the measurand being the particular quantity subject to measurement\] (1). The effect of an influence quantity on the measurement process is an interference that generates a systematic error. In clinical chemistry, depending on the molecular entity responsible for an interference, influence quantities may be classified as endogenous or exogenous. Concentrations of bilirubin, hemoglobin, and triglyceride in plasma (or serum) are generally studied by the in vitro diagnostic industry in the development of measurement systems as potential endogenous influence quantities. However, depending on the criteria that the reagent manufacturer uses to decide whether the systematic error caused by an influence quantity is significant, influence quantities may be the most important component of the uncertainty of a patient’s result. Let me illustrate this statement using the measurement of urate substance concentration in plasma as an example. In this example, the substance concentration …