G. Suhren et M. Luitz, EVALUATION OF MICROBIAL INHIBITOR TESTS WITH INDICATOR IN MICROTITRE PLATES BY PHOTOMETRIC MEASUREMENTS, Milchwissenschaft, 50(8), 1995, pp. 467-470
Routine testing with the widespread use of microbial inhibitor tests w
ith indicator in microtitre plates is generally evaluated visually in
2-3 steps (negative, questionable, positive). The objectives in develo
ping a photometric evaluation were: To have the possibility of an obje
ctive evaluation in order to increase the repeatability of test result
s both within and between laboratories, and to fulfill the necessity o
f an evaluation method which allows more subtle differentiation of res
ults in validation studies and during the development/modificiation of
inhibitor tests. Four negative and 4 positive (4 mu g penicillin/kg)
controls were tested on each test plate. As control/reference samples,
1 batch of lyophilized milk was used for the whole examination period
in order to allow a comparison of data of all test plates. After the
incubation each well of the plates was read by an ELISA Reader (550 nm
/690 nm measurement/reference wavelength). In order to obtain a standa
rdized scale, the difference between the mean absorption values of neg
ative and positive controls was set as 100 % and the absorption values
of the wells of the corresponding test plates calculated in 10 % step
s, with respect to the control values (''relativated absorption in %''
). Due to the frequency distribution of relativated absorption values
within visually evaluated steps, samples classified either as negative
or positive are separated whereas some overlapping within the visuall
y evaluated step questionable exists. Examples of the application of t
he method of relativated absorption within a validation programme (rep
eatability between and shelf life of test kit batches), and during met
hodological development (choice of suitable beta-lactamase preparation
s and activities for the identification of beta-lactam antibiotics) ar
e given.