Reference values concept

We underline these results and the implied concepts quoting from a comprehensive review on this subject (Simon 1983). We remember indeed that, ever since it was experimentally possible to determine atomic distances in molecules and crystals, efforts have been made to draw conclusions about the nature of the chemical bonding, and to compare interatomic distances (dimensions) in the compounds with those in the chemical elements. Distances between atoms in an element can be measured with high precision. As such, however, they cannot be simply used in predicting interatomic distances in the compounds. In a rational procedure, reference values (atomic radii) have to be extracted from the individual (interatomic distances) measured values. Various functions have been suggested for this purpose. In the specific case of the metals it has been pointed out that interatomic distances depend primarily on the number of ligands and on the number of valence electrons of the atoms (Pearson 1972). 

Throughout this paper we use the term model. It will refer to concepts of varying degrees of sophistication and specification. A model can be a notion developed by the chemist to classify an observation, it can be an explicit procedure for the calculation of a value for a physico-chemical concept, or, it can refer to a mather matical equation for the prediction of an observation. We intentionally do not distinguish between these different uses in order to stress the point that the development of a model to further understanding is quite a common approach in science. 

Solberg, H.E. (1987) International Federation of Clinical Chemistry (IFCC), Scientific Committee, Clinical Section, Expert Panel on Theory of Reference Values. Approved recommendation (1985) on the theory of reference values. Part 1. The concept of reference values. Journal of Clinical Chemistry and Clinical Biochemistry, 25, 335-42. 

Virtually all chemical reactions in soils are studied as isothermal, isobaric processes. It is for this reason that the measurement of the chemical potentials of soil components involves the prior designation of a set of Standard States that are characterized by selected values of T and P and specific conditions on the phases of matter. Unlike the situation for T and P, however, there is no strictly Ihermodynamic method for determining absolute values of the chemical potential of a substance. The reason for this is that p represents an intrinsic chemical property that, by its very conception, cannot be identified with a universal scale, such as the Kelvin scale for T, which exists regardless of the chemical nature of a substance having the property. Moreover, p cannot usefully be accorded a reference value of zero in the complete absence of a substance, as is the applied pressure, because there is no thermodynamic method for measuring p by virtue of the creation of matter. 

The local canonical curvatures can be compared to a reference curvature parameter b [156,199]. For each point r of the molecular surface G(a) a number X = x(r,b) is defined as the number of local canonical curvatures [the number of eigenvalues of the local Hessian matrix H(r) that are less than this reference value b. The special case of b=0 allows one to relate this cla.ssification of points to the concept of ordinary convexity. If b=0, then p is the number of negative eigenvalues, also called the index of critical point r. As mentioned previously, in this special case the values 0, 1, or 2 for p(r,0) indicate that at the point r the molecular surface G(a) is locally concave, saddle-type, or convex, respectively [199]. 

To prevent the ambiguities inherent in the term normal values, the concept of reference values was introduced and implemented in the 1980s, At first glance, the substitution of reference values for normal values appears to be unimportant, but on closer scrutiny this change in nomenclature is an important step toward establishing a scientific basis for clinical interpretation of laboratory data. ... 

The IFCC also defines other terms related to the concept of reference values reference population, reference sample group, reference distribution, reference limit, and reference interval. Some of these terms are introduced in later sections of this chapter. 

Thus in the context of reference values, we need a more modest concept of health. Past experience has taught us that health is a relative concept It is possible to be ill in one respect and well in another what is considered healthy in a developing country may be judged to be rather unhealthy in Western Europe and North America, and so on. 

When producing reference values, we should ask ourselves, why do we need these values How are we going to use them To what extent does the intended purpose of the project determine how we should identify health In short, we need a goal-oriented concept of health. 

These practical problems have led to the search for simpler and less expensive approaches. The indirect method has become rather popular.It is based on the observation that most analysis results produced in the clinical laboratory seem to be normal, Figure 16-1 shows one example from the author s laboratory. As can be seen, the values of the serum sodium concentration have a distribution with a preponderant central peak and a shape not too far from that of the Gaussian distribution. The underlying assumption of the indirect method is that this peak is composed mainly of normal values. The advocates of the method therefore claim that it is possible to estimate the normal interval If we extract the distribution of normal values from this part of the distribution. Normal limits determined by the indirect method on the basis of the distribution shown in Figure 16-1 would, however, obviously be biased compared with the shown health-associated reference limits. (Note that the term normal is here used intentionally to distinguish between the concepts of normal values and reference values.)... 

The question of which diseases and risk factors to consider for exclusion of individuals is difficult (see the discussion on the concept of health earlier in this chapter). The answer lies partly in the intended purpose of establishing reference values the project must be goal oriented. 

Either philosophy is, however, concordant with the concept of reference values, provided that the conditions under which reference values are produced are clearly stated. 

Multicenter production of reference values is gaining acceptance, both as a theoretical concept and as a practical approach. A Spanish study introduced a cooperative model, simulating a virtual laboratory for 15 biochemical quantities. A recent project in the Nordic countries (NORIP ) was aimed at the production of common reference intervals for 25 analytes. 

International Federation of Clinical Chemistry, Expert Panel on Theory of Reference Values. Approved recommendation on the theory of reference values. Part 1. The concept of reference values. J CUn Chem Clin Biochem 1987 25 337-42 Part 2. Selection of individuals for the production of reference values. J Clin Chem Clin Biochem 1987 25 639-44 Part 3. Preparation of individuals and collection of specimens for the production of reference values. J Clin Chem Clin Biochem 1988 26 593-8 Part 4. Control of analytical variation in the production transfer and application of reference values. Eur J CUn Chem Clin. Biochem 1991 29 531-5 Part 5. Statistical treatment of collected reference values Determination of reference limits. J Clin Chem... 

Sunderman FW. Current concepts of normal values, reference values, and discrimination values in clinical chemistry [Editorial]. Clin Chem 1975 21 1873-7. 

It is difficult to separate these two concepts entirely. The accuracy of a determination is defined as the concordance between it and the true or most probable value of the quantity measured (V3), and the specificity of a determination may be defined as the ability of the method to determine solely the compound it purports to measure. Since the definition of accuracy includes the concept of the true or most probable value, this introduces the need for reference values against which to assess the accuracy of the determination. 

Terrestrial wildlife movements are such that site-specific tools are more efficiently used to refine exposure estimate. In this case, site-specific exposure estimates are used and compared with safe thresholds for toxicity, termed toxicity reference values (TRVs). Toxicity reference values for wildlife have been developed for energetic compounds. This chapter presents a brief overview of the processes used to establish these tools for ERA for explosives and related soil contaminants that are frequently of potential ecological concern at the affected military sites. This chapter also provides recommendations for use of these values in the ERA process. Investigations addressing the importance and extent of habitat disturbance as a component of the ERA process on explosives-contaminated ranges are reviewed in Chapter 11. General bioaccumulation principles and applications of the bioaccumulation factor and bioconcentration factor (BAF and BCF, respectively) concepts that are often employed in the ERA process to determine bioaccumulation potential of MC for terrestrial receptors are reviewed in Chapter 10. 

The volume is rather a basic concept of a region, as it appears in the derivation of all kinds of equations of continuity. For this reason, we introduce quantities that refer to the unit of volume. We will use the superscript X in order to indicate that this is X per volume. Remember that we are using C for the molar heat capacity (at constant volume), Cv for the specific heat capacity, C for a reduced heat capacity, i.e., divided by a reference value, and now C for the heat capacity density. The quantities X are basically X densities, for example, the mass density or the molar density ... 

---