Quantities and units for metabolic processes as a function of time (Recommendations 1992)

In order to standardize time-related reports in clinical chemistry, basic concepts like system, component and kind-of-quantity are recalled. Changes of a quantity, and of chemical and physical processes involved in these changes are refined. Starting from the general format recommended by IUPAC and IFCC, including system, component and kind-of-quantity, recommendations are formulated. Specifications, especially time-related information, may be added in parentheses after each part of the format. Interrelated specifications are given after the kind-of-quantity. This paper provides examples of various specifications which show that all these elements are necessary for reporting time-related data.


Introduction
Time-related measurements are performed in a variety of clinical chemistry investigations, for example endocrine stimulation or suppression tests, in nutritional or therapeutic drug-loading test, in monitoring physiological and pathological processes and the excretion or secretion of components, and in measuring catalytic activities of an enzyme in a system. The basis ofthe understanding of the test results lies in biochemical, physiological, pharmacological and pathological studies.
Because of the differences between tests and their applications, results have until now been presented in a variety of ways in clinical chemistry.
This document aims to: (1) Analyse the elements of the specifications for timerelated measurements in the clinical laboratory.
(2) Recommend a format that will unify, simplify, clarify, and, hence, improve the reporting and understanding oftime-related quantities measured in the clinical laboratory.
2. Basic concepts, their terms and definitions 2.1. The concepts defining a quantity comprise system, component, kind-of-quantity, numerical value and unit [1].
A quantity, Q, is a measurable, real property, physical or chemical, of a specified system. Example: substance concentration of triglycerides in the blood plasma of a named patient at a given date.
A system is a term that may be applied to any arbitrarily chosen, but stated part ofthe universe irrespective ofform or size. Examples: a specified patient, a tube of plasma. A component is a stated part of the system.
A kind-of-quantity is the abstract concept of the property common to a number of real phenomena (quantities); an example is substance concentration.
Numerical value is the number that gives the magnitude of the measured quantity when multiplied by the unit.
Unit is a chosen reference quantity which may be used for comparison of quantities of the same kind. An example is mole (symbol: mol). It is customary to consider that the quantity, Q, is expressed by the product of a numerical value, {Q}, and an appropriate unit; [Q], also called the value of the quantity:

Q= {Q} [Q].
When speaking about the differential of a quantity, dQ, it is the differential of its value that is being described.
2.2. Change of a quantity is the increment of the value of Q with time. The change may be expressed either infinitesimally at time by the differential dQ or dQ(t), or in practice it may be expressed by a finite increment over the time interval (tl; t2), that is Q(t2) Q(tl) which may be written A Q or A Q(tl ;t): AQ Q(tl;t) Q(t2)-Q(tl). Examples are mass change, Am; amount-of-substance change (short form: substance change), An; volume change, lk V; substance concentration change, Zkc.
Note: Net change of a quantity in a system is the algebraic sum of the changes of the quantity affected by different processes: dlnet Z dQi or in practice it may be expressed by a finite time interval (t;t). AQ Examples: Mass fractional change, dm(t)/m(t); amountof-substance fractional change, dn(t)/n(t) (short form: substance fractional change); amount-of-substance fractional change of concentration, dc(t)/c(t) (short form: substance fractional change of concentration).
2.4. Change ratio of a quantity may be expressed infinitesimally at time by a ratio of differentials dQl(t)/dQ(t), where the kind-of-quantities are the same but for different components in the same system. In practice, the ratio for a finite time interval, is: Note: Change ratio has the dimension one.
2.5. Rate of change of a quantity is defined by the time derivative dQ/dt of the value of the quantity. The differential quotient may also be called derivative (or instantaneous) rate of change.
Examples: rate of change of mass, dm/dt (short form: mass rate); rate of change of amount-of-substance, dn/dt (short form: substance rate); rate of change of amount-ofsubstance concentration, dc/dt (short form: substance concentration rate).
The change in a quantity is often measured as the difference between the values at the two ends of a calendar time interval. Then, the mean rate of change is defined as: [(Q(t) Q(t)]/(t;tl) AQ/At. Examples: mass rate ratio (dml/dt)/(dm/dt); amount-ofsubstance rate ratio (dna/dt)/(dn/dt) (short form: substance rate ratio); amount-of-substance concentration rate ratio (dcl/dt)/(dc2/dt) (short form: substance concentration rate ratio).

Processes and related quantities
A process is a phenomenon by which change takes place in a system. In physiological systems, a process may be chemical, physical or both.
The same format of presentation should be used for suppression or stimulation tests.
where A is the area of the surface. It is sometimes called flux, but the term flux is used in some disciplines for the kinds-of-quantity defined in section 3.2.1. The concept may be expressed by areic mass rate, areic substance rate, or areic volume rate [4].

Combined processes
The combination of two or more processes (chemical and/or physical) is usual in physiological systems. In that case, a net change may be calculated (see section 2.2). That result may also be reported as an amount-ofsubstance in the urine sample: Urine-ammonium; amount-of-substance (8:00;16:00) 21 bmol.
Such a presentation of a result is adequate for a comprehensive report, provided the purpose is to assess the patient's status at the moment in time when the sample was obtained.
That result may also be reported as a substance ratio: Example: Plasma-coagulation, tissue factor induced; time (procedure) 33 s. 4.4.3. When time occurs in the denominator of a derived kind-of-quantity, the word rate should be a part of of the kind-of-quantity name.