Testing of Disinfectants
The efficiency of disinfectants can be determined with the help of several tests. These are:
§ Phenol coefficient (Rideal–Walker) test
§ Chick Martin test
§ Capacity (Kelsey–Sykes) test
§ In-use (Kelsey–Maurer) test
Rideal and Walker designed the phenol coefficient test to compare the performance of a disinfectant with that of phe-nol for the ability to kill Salmonella typhi. Phenol coefficient is determined by dilution of the disinfectant in question which sterilizes the suspension of S. typhi in a given time divided by the dilution of phenol which sterilizes the suspension in the same time.
In this test, a series of dilutions of phenol and the experi-mental disinfectant are inoculated with the test bacteria S. typhiand S. aureus, then placed in a 20°C or 37°C water bath.These inoculated disinfectant tubes are then subcultured on a regular fresh medium at 5 minute intervals, and the subcul-tures are incubated for two or more days. The highest dilutions that kill the bacteria after a 10 minute exposure, but not after 5 minutes, are used to calculate the phenol coefficient. The reciprocal of the appropriate test disinfectant dilution is divided by that for phenol to obtain the coefficient. Suppose that the phenol dilution was 1/90 and maximum effective dilu-tion for disinfectant X tested was 1/450, then the phenol coef-ficient of X would be 5.
Chick Martin test
It is a modification of Rideal–Walker test, in which the disinfectant acts in the presence of organic contaminants (e.g., dried yeast, feces, etc.) to simulate the natural conditions.
The capacity (Kelsey–Sykes) test determines the appropriate use of dilutions of the disinfectants. It measures the capacity of a disin-fectant to retain its activity when repeatedly used microbiologi-cally. The disinfectant is assessed by its ability to kill bacteria by demonstrating growth or no growth on recovery culture media but not by comparison with phenol. The test is performed under both clean and dirty conditions, hence shows the effectiveness of a disinfectant in the presence of organic material.
The “in-use” (Kelsey–Maurer) test is a test that determines whether the chosen disinfectant is effective, in actual use, in hospital practice and also for the period of its use. The effectiveness of the disinfectant is determined by its ability to inactivate a known number of standard strain of a pathogenic staphylococci on a given surface within a certain given time. In-use test allows a more accurate determination of effective-ness of a disinfectant compared to phenol coefficient test.