Question
Phenol co-efficient
Answer
The phenol coefficient is a numerical measure of the germicidal (bactericidal) efficiency of a disinfectant relative to phenol, tested under standardized conditions — used historically to compare the potency of different disinfectants against a common reference standard.
Principle/method: the test disinfectant and phenol are each diluted to a range of concentrations and added to a standardized suspension of a test organism (classically Salmonella Typhi, sometimes Staphylococcus aureus) at a fixed temperature (typically 20°C or 37°C); at fixed time intervals, samples are subcultured to determine the highest dilution of each disinfectant that kills the test organism within a specified exposure time (commonly 10 minutes but not 5 minutes, by convention).
Calculation:
Interpretation: a phenol coefficient greater than 1 indicates the test disinfectant is more potent than phenol (i.e., effective at a higher dilution); a coefficient less than 1 indicates it is less potent than phenol.
Limitations: the test uses only pure bacterial suspensions under idealized laboratory conditions, so results do not necessarily reflect real-world performance in the presence of organic matter (blood, pus, faecal material), which can substantially reduce the activity of many disinfectants; it also does not test activity against spores, viruses, or fungi. Despite these limitations, the phenol coefficient remains a useful standardized comparative index for assessing and classifying the relative bactericidal strength of chemical disinfectants.