A comparison of the quantitative suspension tests for the assessment of disinfectants.

The germicidal effect of two disinfectant standards has been determined by three quantitative suspension tests: the Dutch Standard-Suspension Test, the French AFNOR test, and the in vitro test. Particularly in case of Pseudomonas aeruginosa the germicidal-effect values found according to the three methods differ in a significant way (Tables 1 and 2). The only factors that could explain these differences, since they are not the same in the three techniques, are the preparation of the bacterial suspension and the diluent of the disinfectant solution. Through this the composition of the reaction mixture is different: in the in vitro test only 0.0085% NaC1 and 0.001% Tryptone are contained, in the AFNOR test the same constituants are ten times more concentrated and in the Standard-Suspension Test 0.882% NaC1 and 0.0306% albumin are present. If in the same trial these substances have been incorporated in the final concentration above-mentioned, then germicidal-effect values equal to those of the original tests have been obtained (Table 3). The disinfectant diluent and the bacterial suspension fluid have to be the main factors responsible for the differences in activity found by the three testing methods. In our opinion the in vitro test gives the clearest picture of the anti-bacterial properties of a disinfectant since in this test the less addition of extraneous matter is encountered.