Standardization of colorimetric analysis—dogma disputed

A favoured concept is that all analyses should be standardized with the purest materials available. This ideal is unattainable as a significant number of problems remain unsolved and, to eliminate certain of these problems, secondary standards are often used. An alternative approach is to relate concentration directly to absorbance by a calibration relationship dependent on the molar extinction coefficient of a reaction product or reactant. The present study examined the dogma that a standard must be analysed in each batch of tests by comparing the analytical performance achieved in 4 manual colorimetric tests urea, glucose, phosphate and albumin—using daily primary or serum standards (the variable calibration mode) with that obtained using constant calibration relationships (the constant calibration mode). It was shown that the mode of standardization selected for each and every method must be chosen as an objective consequence of experimental study.