How to Cite this article: Uncertainty of measurement in Clinical Microbiology, Fuentes-Arderiu X, eJIFCC vol 13 no 4: http://www.ifcc.org/ejifcc/vol13no4/130401006.htm
Uncertainty of measurement in clinical microbiology
Xavier Fuentes-Arderiu Servei de Bioquímica Clínica Ciutat Sanitària i Universitària de Bellvitge 08907 L’Hospitalet de Llobregat CataloniaSpain Fax: 34 93 260 75 46 E-mail: email@example.com An important part of the activity in a clinical microbiology laboratory is the measurement of quantities related to concentrations of microorganisms, antibodies nucleic acids, etc. When measuring a microbiologic quantity random and systematic errors can act together on the result producing an error of measurement and generating a doubt —uncertainty—about the true value of the measured quantity. International scientific organizations, keeping in mind these facts, have developed the concept of uncertainty of measurement (1,2). The importance of this concept is increasing in all fields of health sciences (3-5). By this reason, it is important to clarify the concept and show the practical way to bring estimate the uncertainty of patients’results. Uncertainty of measurement is a parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measured quantity (1); in other words, uncertainty is a numerical information that complements a result of measurement, indicating the magnitude of the doubt about this result. Uncertainty is described by means of oneof the following three parameters (2): Standard uncertainty (u) is the standard deviation that denotes the uncertainty of the result of a single measurement. Combined standard uncertainty (uc) is the standard deviation that denotes the uncertainty of the result obtained from other results of measurement. It is obtained by combining the standard uncertainties of all individual measurements accordingto the law of propagation of uncertainty.
Expanded uncertainty (U) is the statistic defining the interval within which the value of the measured quantity is believed to lie with a particular level of confidence. It is obtained by multiplying the combined standard uncertainty by a coverage factor, k, the choice of which is based on the level of confidence (1-a) desired. If k = 2, then 1-a »0,95; if k = 2,6, then 1-a » 0,99. The international scientific and standardization bodies recommend that the uncertainty of patients’ results obtained in clinical laboratories should be known (3-5); the rationale for this recommendation is that full interpretation of the value of a quantity obtained by measurement requires also evaluation of the doubt attached to its value. The common opinion ofthese bodies is that clinical laboratories should supply information about the uncertainty of their results of measurement, when applicable Depending on the field of application, uncertainty is attributable to different sets of elements. Each element of uncertainty, expressed as a standard deviation, may be estimated from_the probability distribution of values with repeated measurements, termed typeA standard uncertainty, or estimated by using assumed probability distribution based on experience or other available information, termed type B standard uncertainty (2,6). In general, in clinical microbiology the most relevant elements that can contribute to uncertainty for a given measurement procedure are: incomplete definition of the particular quantity under measurement (specially for antigenand antibodies), • pre-analytical variation, • uncertainty related to calibration processes, • inappropriate calibration function used by an analyzer, • interferences, • imprecision, • rounding of results.
Vol 12 No 4
All these sources of uncertainty do not apply in all cases; for each measurement procedure is necessary to identify which of these sources...