Standardization of the reaction temperature for the determination of enzyme activity.

In view of the world-wide efforts to optimize and standardize the determinations of enzyme activities in clinical chemistry, the agreement upon a uniform reaction temperature is of essential importance. Both on the basis of practical technical considerations and for physical chemical reasons, 25°C is the best temperature for standard measurements. The following considerations make the lowest possible temperature desirable: a minimal difference between room temperature and the measurement temperature reduces temperature variations in the cuvette, and allows measurements to be made near the temperature of calibration of pipettes and other volumetric vessels. Substrates are often not soluble enough to allow optimal concentrations for reactions at higher temperatures. The inactivation of proteins under measurement conditions distinctly increases with temperature. Problems with allosteric enzymes also make a low temperature deskable. The understandable desire for the greatest possible measured effect in the shortest time (that is, high reaction rates) presses for the highest possible reaction temperature. This dilemma makes 25°C appear an optimal measurement temperature, especially since the 37°C which is often chosen for emotional reasons no more corresponds to "physiological" conditions than the other parameters in the cuvette. Attempts are made repeatedly to find correlations between values measured at different temperatures by the use of correction factors, especially when automatic analyzers are used. Reasons for caution with respect to this practice arc given, and the consequences for the further development of clinical chemistry are discussed.