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Letters |
Servei de Bioquímica Clínica, Ciutat Sanitària i, Universitària de Bellvitge, 08907 L’Hospitalet de Llobregat, Catalonia, Spain, Fax 34-93-260-7546, E-mail xfa{at}csub.scs.es
To the Editor:
Random and systematic errors can act together to produce an error of measurement (total error) and generate a doubt (uncertainty) about the true value of the measured quantity.
The international metrological organizations, keeping in mind these facts, have developed the concept of uncertainty of measurement. This concept has become an important issue in general metrology, and by extension, its importance is increasing in clinical laboratory sciences. It is thus important to clarify the concept and to identify the practical difficulties in the use of uncertainty of patients’ results.
Uncertainty of measurement (hereafter referred to as uncertainty) is a parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand (i.e., the measured quantity) (1); in other words, uncertainty is numerical information that complements a result of measurement, indicating the magnitude of the doubt about this result. Uncertainty is described by means of one of the following three parameters (2):
)
desired. If k = 2, then 1 - 
0.95; if
k = 2.6, then 1 - 0.99. The international scientific and standardization bodies recommend that the uncertainty of patients’ results obtained in clinical laboratories should be known (3)(4)(5); the rationale for this recommendation is that full interpretation of the value of a quantity obtained by measurement also requires evaluation of the doubt attached to its value. The common opinion of these bodies is that clinical laboratories should supply information about the uncertainty of their results of measurement when applicable; ideally, this information should be attached to the patients’ results as shown in this example:
S–Alanine aminotransferase; cat.c. = (1.15 ± 0.23) µkat/L, where 1.15 µkat/L is the result given by the system of measurement, and 0.23 µkat/L is the expanded uncertainty multiplied by 2 as coverage factor. (According to IFCC and IUPAC, S is serum, and cat.c. is the catalytic concentration.)
Institutional guidelines for estimating uncertainty of measurement, containing examples in fields of application other than clinical laboratory sciences, have been published (2)(6)(7)(8). An excellent review of uncertainty (and traceability) in clinical chemistry was published recently (9).
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 "type A standard uncertainty", or estimated by use of an assumed probability distribution based on experience or other available information, termed "type B standard uncertainty".
In general, in clinical laboratory sciences the most relevant elements that can contribute to uncertainty for a given system of measurement are:
Estimation of the combined uncertainty, expressed as a variance, is the sum of the values, all expressed as variances, corresponding to several of the above elements. Perhaps variances corresponding to these elements can be easily estimated in some clinical laboratories, but for others their evaluation is certainly not easy, as may be derived from the following points:
Bearing in mind these points, the following questions arise:
Although some of the most relevant elements contributing to uncertainty can potentially be evaluated in clinical laboratories, the effort required to undertake such an endeavor might be so great that it will be difficult to bring into general use the uncertainty of patients’ results.
References
The following articles in journals at HighWire Press have cited this article:
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  W. Dimech, B. Francis, J. Kox, G. Roberts, and for the Serology Uncertainty of Measurement Workin Calculating Uncertainty of Measurement for Serology Assays by Use of Precision and Bias Clin. Chem., March 1, 2006; 52(3): 526 - 529. [Abstract] [Full Text] [PDF]
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N. Monge-Azemar and X. Fuentes-Arderiu More on Premetrologic Variation Clin. Chem., December 1, 2004; 50(12): 2459 - 2460. [Full Text] [PDF]
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X. Fuentes-Arderiu and B. Gonzalez-de-la-Presa Interchangeability of Estimates of Day-to-Day Imprecision between Commercial Control Materials and Serum Pools Clin. Chem., March 1, 2002; 48(3): 573 - 574. [Full Text] [PDF]
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